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Notes: The Art of Stargazing

Month 5: June - July 2013

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The Art of Stargazing – Month 5

Contents – The Art of Stargazing (Month 5)

What You Will Learn This Month .................................................................................................. 4

Science of Astronomy – Stars, Part III ........................................................................................... 5

Review ........................................................................................................................................ 5

Evolution of Sun-Like Stars........................................................................................................ 6

Evolution of Very Low Mass Stars ........................................................................................... 11

The Evolution of Massive Stars ................................................................................................ 12

Tour of the Night Sky – Lyra, Ophiuchus, Serpens, Scorpius ...................................................... 17

Overview ................................................................................................................................... 17

The Constellation Lyra ............................................................................................................. 17

The Constellations Ophiuchus and Serpens .............................................................................. 18

The Constellation Scorpius ....................................................................................................... 20

Observing Techniques –Telescope Mounts .................................................................................. 22

Overview ................................................................................................................................... 22

Alt-Azimuth Mounts ................................................................................................................. 23

Equatorial Mounts ..................................................................................................................... 25

Go-To and Push-To Mounts ..................................................................................................... 27

Some Recommended Mounts ................................................................................................... 29

Solar System Observing – Retrograde Motion; Moon Craters ..................................................... 30

Retrograde Motion of the Planets ............................................................................................. 30

Lunar Impact Craters – Big, Small, Old, New .......................................................................... 32

The Deep Sky This Month ............................................................................................................ 38

Overview ................................................................................................................................... 38

The Ring Nebula (M57) ............................................................................................................ 38

M56 ........................................................................................................................................... 40

Epsilon Lyrae ............................................................................................................................ 40

70 Ophiuchi ............................................................................................................................... 42

IC 4665...................................................................................................................................... 43

M5 ............................................................................................................................................. 44

M10 and M12 ............................................................................................................................ 46

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Antares ...................................................................................................................................... 47

M4 ............................................................................................................................................. 48

Rho Ophiuchi ............................................................................................................................ 50

M6 and M7 ................................................................................................................................ 52

What You Have Learned This Month ........................................................................................... 53

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What You Will Learn This Month

Welcome to Month 5 of The Art of Stargazing!

This month, we take a quick look at how stars move from the middle-age on the main sequence

through the final stages of their lives as red giants or supergiants, and finally to white dwarfs or

neutron stars… or even black holes! Big stars live fast, die young, and go out with a big

explosion. Mid-sized stars just gently fade away. Still others, the slow-burning and low mass

red stars, burn their fuel so frugally they may last for a trillion (yes, a trillion) years. Armed with

this knowledge of stellar evolution, you will come to understand the life stage of pretty much any

star in the night sky. Some of the brightest and best known stars, as it turns out, are big stars on

the way to their doom.

In the sky tours this month, it’s off to the leading edge of the prominent constellations of

northern summer. You visit the small and famous constellation Lyra, the Lyre and the big and

famous southern constellation Scorpius, one of the few in the night sky to unmistakably resemble

its namesake as a giant scorpion. You also visit the large constellation Ophiuchus, which itself

bisects the constellation Serpens, the Serpent, the only constellation in the heavens split into two

parts.

As for deep-sky tours, we get a good variety this month… open clusters, globular clusters, some

excellent double and multiple stars, and a planetary nebula. But no galaxies this month… the

night sky is turning back to the starry plane of the Milky Way which obscures our view of other

galaxies.

Last month you got a look at the pros and cons of the most commonly available telescope

designs. This month we look at telescope mounts, which are just as important as the optical part

of a telescope at providing pleasing views of the night sky. You’ll get an overview of the two

main types of mounts, you’ll learn how to easily evaluate a telescope mount, and you’ll get a

look at the best mounts on the market today for visual observers and for potential

astrophotographers.

Finally, a look at the solar system. You meet the seemingly strange way that planets seem to

move backwards across the sky for a few weeks, an effect which completely baffled ancient

stargazers. And you get a quick overview of lunar craters… what they are, how they are appear,

the main types to look for, and how to judge the relative age of craters on the Moon at a glance.

Let’s get started…

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Science of Astronomy – Stars, Part III

Review

Let’s dip into a little science again before we wander out to look at the stars. This month, we

look into the end stages of stars. Many of the brightest stars are in this state in one form or other,

so this lesson will help us make more sense of what we can see.

First, a quick recap on stars. Here’s what we know so far…

Stars form in large clouds of interstellar gas that collapse under the influence of gravity.

As small pockets of these large clouds collapse, they heat up, start glowing, and

eventually get hot enough to start turning hydrogen gas into helium gas in a central core

through the process of nuclear fusion. At this point, the gas cloud becomes a star

When hydrogen turns into helium, a small bit of mass (m) is turned into a huge amount of

energy according to Einstein’s famous equation E=mc2

(where c is the speed of light, a

very large number)

The big gas clouds in which stars form often give birth to hundreds of stars which set the

remaining gas aglow as a nebula; eventually, the nebula dissipates and a cluster of new

stars remains

The temperature and lifetime of a star depends on its mass. More massive stars burn

faster and live shorter lives, and also have hotter cores and surfaces than less massive

stars

The hottest stars glow with a primarily blue light; cooler stars glow with white, yellow,

orange, or reddish light (in order of decreasing temperature)

Astronomers discovered a simple relationship between the brightness and temperature of

many stars. They found that hotter, bluer stars were brighter than cooler, whiter stars,

which in turn are brighter than cooler, redder stars. Astronomers plot the stars according

to brightness and color onto an HR diagram (see below).

Many stars on the HR diagram fall along a line called the “main sequence. Stars along

the main sequence burn hydrogen in their cores and are in their youth to middle age.

Stars on the HR diagram that do not lie on the main sequence are older stars that are

moving towards their end stage of life. These stars are the topic of this month’s lesson…

Here’s what it’s all about…

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An example of the HR diagram, showing the main sequence, as well as other more evolved stars.

As a star burns through its hydrogen, its size and brightness remain more or less the same.

That’s because gravity, which tries to squeeze the star and make it smaller and hotter, is balanced

by the outward pressure of the intense light and hot gas near the core. So the star remains in a

state of equilibrium. If the fuel stopped burning in the core, there would be less light and heat to

push back against the pull of gravity, and the star would shrink. Conversely, without gravity to

hold the whole affair together, a star would push itself apart and cast its material off into space.

Evolution of Sun-Like Stars

Armed with this knowledge, we can understand the basics of what happens to stars as they run

out of hydrogen fuel and stop releasing energy through nuclear fusion.

Let’s start with a Sun-sized star, a star with about 0.3x to about 8x the mass of our Sun. When

the hydrogen runs out, energy production slows down and the helium-rich core and its

surrounding regions can’t push back against gravity and they begin to collapse under their own

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weight. The star still contains mostly hydrogen in the outer layers, but there’s no way for the star

to mix this hydrogen into the core. As the core collapses, it gets squeezed and grows much

hotter until a thin shell of hydrogen around a non-burning helium core re-ignites and starts fusing

into helium again. This hot shell fuses hydrogen faster than when the star was on the main

sequence, so the star brightens considerably. The outer layers of the star also get in on the

action. The hot-burning hydrogen shell heats up and expands the star’s outer layers, and as they

expand they cool from 6,000-8,000 K to about 3,500-4,000 K. The star becomes redder and

much larger, about 100x its main sequence radius. The star’s mass remains the same, so the

swollen outer regions are large but tenuous, with a density not much greater than the interstellar

medium itself. In some cases, a considerable amount of mass is lost as the extreme outer layers

escape from the star into interstellar space.

So we started with a Sun-like star on the main sequence, and now we have a much larger, cooler,

redder, and brighter star. On the HR diagram, the star moves upward (brighter) and to the right

(cooler). It has become a red giant.

As you might guess, because the star has cooled, its spectral type changes from type A, F, or G

to type K or M. But they are not the same as K or M main sequence stars. They are much

brighter. To distinguish red-giant stars from K and M-type main sequence stars, astronomers add

a Roman numeral to the star’s type. Main sequence stars always have a ‘V” in their type, for

example G2V or M4V and so forth. A red giant has IV or III in its spectral type. The star

Arcturus, which you know well by now, is a nearby red giant star of type K2III. Gamma Crucis

in the Southern Cross is a red giant of type M3III. As a star evolves from the main sequence to a

red giant, it briefly passes through a stage as a “sub giant” with designation “IV”. The image

below shows you how the star moves off the main sequence to the red-giant stage.

When our Sun becomes a red giant in about 5 billion years, the outer layers of our star will swell

so large that Mercury and Venus will surely be incinerated, but Earth will likely be spared. But

the huge extra energy output from the Sun will make it far too hot for life on our planet. The

oceans and atmosphere will boil away and our world will be scorched and barren.

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Cross section of a main sequence star and a red giant. The image is not to scale: a red giant

expands by 100x in radius compared to its main sequence radius.

A Sun-like star evolve off the main sequence to become a red giant star as it burns hydrogen in a

hot shell around an inert helium core

But that’s not the end… things keep happening. As the hot hydrogen shell burns near the core,

which it does for hundreds of millions of years, it adds more helium to the inert non-burning

core. This core grows denser and shrinks, its temperature increases from about 15 million K to

100 million K. Then something remarkable happens. The helium itself starts to burn, fusing into

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heavier elements like carbon and oxygen and releasing enough energy to push back against

gravity. This happens suddenly (in a matter of hours) so it’s sometimes called the helium flash.

The extra heat from the helium burning causes the core to expand and cool slightly, so the energy

output actually drops. The star dims somewhat and contracts to become slightly less red and less

giant, and it moves a little to the lower left of the HR diagram (see below). This stage lasts for a

few tens of millions of years.

The “helium flash” causes a red giant star to cool and shrink slightly

All good things must end, and so it is with red giants. The helium fuel runs out and the core,

made mostly of carbon and oxygen now, contracts again. But this time, it does not get hot

enough to start burning carbon into heavier elements because the star does not have enough mass

to squeeze the core and make it any hotter. The hot carbon core becomes so dense that the

electrons themselves push back against the force of gravity, and the carbon core, which is about

the size of the Earth, stops collapsing without igniting. A very hot, thin shell of helium and an

outer shell of hydrogen continue to burn around the carbon core for just a million years or so,

and this causes the star to brighten again and swell and cool. So it becomes an even brighter red

giant and moves to the upper right in the HR diagram (see below).

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When helium stops burning in the core, a red giant cools and swells again and moves to the

upper right in the HR diagram

At this stage, the extra energy from the hydrogen and helium shells drives the outer layers of the

star into space where they escape the star’s gravity and float away forever. The energy from the

core of the star briefly sets this escaping gas aglow as what we call a planetary nebula. You’ve

met a few such objects already, such as the Cat’s Eye in Draco, and you will meet another this

month. A planetary nebula lasts for just 50,000 years before it dissipates into interstellar space.

In 5 billion years, the Sun will swell into a red giant and boil away the Earth’s oceans and air

The hot and dense carbon-oxygen core remains behind as a white dwarf. It holds much of the

mass of the original star, but is only the size of the Earth and so has an amazingly high density.

A teaspoon of white dwarf matter has a mass of 5,000 kg, as much as a school bus! A new white

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dwarf star is also hot, with a temperature of 60,000-100,000 K or more. The heat is left over

from the days of hydrogen and helium burning. At this temperature, the star appears blue to

bluish-white. But because it generates no more energy, a white dwarf is quite faint and occupies

the lower-left corner of the HR diagram. White dwarfs slowly radiate energy into space over

billions of years. In time, they will redden and grow fainter like a dying ember in a campfire.

Whereas a red giant briefly becomes a thousand times brighter than its days on the main

sequence, a white dwarf is many hundreds of times fainter than a main sequence star. For this

reason, they are hard to see and none are even remotely visible to the unaided eye.

A planetary nebula. The central blue star is the exposed core of a red giant casting off its outer

layers and setting them aglow. This core will settle as a white dwarf star.

Now you know the fate, from birth to death, of a Sun-like star and other stars with about 0.3 to 8

solar masses. Some 95% of all stars fall into this category. You also know why there are red

giants and white dwarfs on the HR diagram. We are making progress.

Evolution of Very Low Mass Stars

Before we get into the spectacular lifecycle of very massive stars, let’s look quickly at very small

stars with about 1/10 of a solar mass. These M-type main sequence stars are small and cool, but

unlike their larger brethren they can mix in hydrogen from the outer layers down to the core. So

they have access to all the hydrogen in the star, which works out to nearly as much as in the core

of a mid-sized star like the Sun. Since these small stars burn hydrogen so frugally, they have

extremely long life spans of trillions to tens of trillions of years (not a typo). They never become

red giants, and they will just slowly turn all their hydrogen into helium, slowly radiating away

their heat (assuming the universe lasts that long)!

Slightly larger stars, from 0.1 to 0.3 solar masses, will undergo some degree of hydrogen shell

burning around a compressed but inert helium core and do become red giants. But they are not

massive enough to cause the “helium flash”, so they don’t go through the extra phase of

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brightening of Sun-like stars. They just slowly fade away into white dwarfs made mostly of

helium.

The Evolution of Massive Stars

Main sequence stars with more than 8 solar masses make up a tiny fraction of all stars, but they

make up a large percentage of the stars we can see in the night sky simply because they are so

bright. They also go through quite spectacular end stages. The details are complex, but here’s a

summary to help you understand the basics.

The massive O and B type stars on the upper-left part of the main sequence also burn hydrogen

in their cores, but they do so at a much faster rate than smaller stars. Why? Because gravity

squeezes the insides of a massive O-type stars to a much higher temperature than a smaller star,

and the higher temperature accelerates the rate of hydrogen fusion in the core. A typical O-type

star might have a mass 60 times as great as our Sun, but a brightness more than 1,000,000 times

as great. The lifetimes of such stars are just a few million to a few tens of millions of years at

most compared to the 10-billion-year lifetime for our Sun, or a 10-trillion-year lifetime of a small

frugal M-type main sequence star.

Big stars turn hydrogen into helium with a slightly different process that uses carbon, nitrogen,

and oxygen catalysts. The process is called the CNO cycle. We will skip the details here. But

in time the hydrogen runs out and the core of a big star contracts and heats up enough to start

fusing helium into carbon in the core, and hydrogen into helium in a shell around the core. The

increased temperature causes the outer layers of the star to expand, and the hot blue main

sequence star becomes a large red star, but larger and brighter than a red giant. So it’s called a

red supergiant. However, unlike the case with Sun-like stars, the massive star will not increase

its overall brightness during this phase because the new fusion processes in the core do not create

more energy than during the hydrogen burning phase. Of course, the star was immensely bright

to begin with. On the HR diagram, the star moves directly to the right.

A red supergiant is an M-type star, but it is distinguished from M-type main sequence stars by

adding the Roman numeral “I” after the star type. They are further divided into, in decreasing

order of brightness, Ia, Iab, and Ib. The bright red star Betelgeuse in Orion is a type M1.5Iab

type star, for example. It was once a type-O main sequence star.

Red supergiants are, well, super giant. If a red giant star like Arcturus or Aldebaran were at the

center of our solar system, its photosphere would extend to the orbit of Venus or Earth. A red

supergiant, on the other hand would extend to the orbit of Jupiter or Saturn. See the image below

for a comparison.

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Evolution of massive main sequence stars into red supergiants

Relative size of red giant stars to red supergiant stars. The Sun (Sol) is at lower left

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In mid-sized stars, once the helium runs out, that’s the end of the road. But massive stars can

squeeze the core to temperatures exceeding 600 million K and very high pressures. So fusion

continues as carbon burns into heavier elements like oxygen, neon, magnesium, and silicon.

When the core runs out of fuel for one element, it burns heavier elements while fusion of the

lighter elements moves to thin shells around the core, giving the interior of the star a structure

something like an onion. As new elements begin burning, the star zigzags back and forth across

the HR diagram. The exterior of the red supergiant star remains unaffected and more or less

retains its appearance during this final shell-burning stage.

Shells of nuclear fusion in an end-stage red supergiant star

When the core of the big star burns heavier elements until it contains mostly iron and nickel, the

game is over. That’s because iron, unlike the lighter elements, cannot undergo nuclear fusion

and release energy. So there’s no more energy to hold up the star. The core collapses suddenly

and becomes very dense like a white dwarf. For mid-sized stars, the white dwarf is the end stage

because the electrons have enough pressure to withstand the force of gravity. But if the core of

the big star has a mass greater than 1.4 solar masses, it will continue to collapse, in just a few

seconds, and turn all the protons and electrons in the core into neutrons. The pressure from the

neutrons is sufficient to withstand further collapse. What remains is a small dense remnant of

the core, about the size of a small city, but with the mass of 2 or 3 Suns. This remnant is called a

neutron star. It has a density such that a piece of neutron star as small as a grain of salt has the

mass of a 747 jetliner.

During collapse, the outer layers of the star remain unchanged. But when the collapse stops, a

shock wave rebounds outwards and blows the star into space in one of the most violent

explosions in the universe: a supernova. This explosion releases a huge amount of energy. The

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star brightens from one million times as bright as our sun to ten to one hundred billion times as

bright in just a few hours. That’s almost as much energy as all the stars in a galaxy. After

brightening, the supernova slowly decreases in brightness over several weeks. It leaves behind

the neutron star and a shell of hot expanding gas that will become visible as a nebula in the

coming decades and centuries. This nebula is called a supernova remnant. You will see a few of

these remnants with your telescope later in the course.

Star in its final months as a red supergiant (right) and after exploding as a supernova (left)

During this violent physical event, there are particles and energy flying everywhere… atoms,

gamma rays, neutrons, everything. Many neutrons get captured by atomic nuclei in what’s

called the r-process. This is how most of the elements heavier than iron get made, right up to

and including uranium, the heaviest natural element. This material is blasted back into the

interstellar medium, where in hundreds of millions of years it may collapse to form new stars and

planets. In a sense, this is the way old stars recycle themselves into new stars. All the atoms in

the universe heavier than hydrogen and helium were created inside stars, and elements like iron,

nickel, gold, lead, and uranium were all formed during supernovae explosions. The iron in your

blood, the gold in your jewelry, and the copper in your plumbing was all made in a supernova

explosion billions of years ago.

A few final words…

The type of supernova just described is called a Type II supernova. Another kind of supernova,

Type Ia, arises from a completely different process. We will come to this type of supernova later

in the course.

The most massive stars, with more than 80 solar masses, burn their heavy elements so quickly, in

a matter of years, that the outer layers do not have time to expand. So they become Type II

supernovae without ever becoming red giants.

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In a few cases, the core of a massive star is so heavy even neutrons cannot withstand its collapse.

The dense remnant continues to collapse, essentially down to a small point with a mass of a few

solar masses or more. The gravity of such a small dense object is so large, even light cannot

escape. Such a dense remnant of a supernova explosion is called a black hole. For the very

largest stars, the supernova may be so violent and the destruction so complete that not even a

black hole will remain.

Supernovae are comparatively rare events. In our Milky Way galaxy, a supernova occurs once

every 50 years on average. Some may happen on the other side of our galaxy and not be visible

from Earth. The last supernova in our galaxy visible from our planet occurred in the year 1604.

The last supernova visible without optical aid occurred in the Large Magellanic Cloud, a nearby

dwarf galaxy, in 1987. Several supernovae occur each year in distant galaxies, but they are

objects for mid-sized or larger telescopes. All the red supergiant stars in our sky, and all the blue

supergiant main sequence stars, will all become supernovae over the next few million years.

Such stars include Betelgeuse, Spica, Rigel and most of the bright stars in the Orion region, and

the star Antares which you will meet later this month.

A summary of the lifecycle of stars from protostar to main sequence to end stage.

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Tour of the Night Sky – Lyra, Ophiuchus, Serpens, Scorpius

Overview

Finally, the stars of northern summer arrive! Even a casual glance at the eastern sky in late June

and early July will show many more bright stars than the relatively dim constellations of spring.

That’s because the night side of Earth is turning back towards the plane of the Milky Way, and

that’s where most of the bright stars are located. Look especially in the southeast, towards the

very bright orange star Antares, and you will see dozens of blue-white O and B-type stars that lie

on a line of sight towards the center of our galaxy. Antares itself is one of the largest stars in the

sky, a type-M supergiant well on its way to blowing up as a supernova in the next few million

years.

In dark sky, around Antares and to the east, you may see glimpses of clouds of unresolved stars

lined with dark knots of obscuring gas and dust. Turn your binoculars toward these clouds and

you will see them resolve into thousands of individual stars. Next month we will get a better

look towards the center of the Milky Way, the “starriest” part of the sky. This month we take an

advance tour of the northern summer stars.

The Constellation Lyra

Let’s move from north to south along the meridian this month. Face northeast and look high in

the sky for the bright whitish-blue star Vega. You can’t miss it.

At magnitude +0.03, Vega is the 5th

-brightest star in the sky and the 2nd

brightest star north of the

celestial equator after Arcturus. Vega is a type A0V main sequence star, and is perhaps the most

rigorously observed star in the heavens except for the Sun. It was the first star for which a

spectrum was obtained and the first star to be photographed. More recently, astronomers have

discovered the presence of a disk of dust and debris around Vega. There may be at least one

Jupiter-sized planet obscured in this disk. Vega lies about 26 light years away.

Because of the precession of the Earth’s axis, Vega was once the north pole star about 12,000

B.C. and it will come within 4o of the north celestial pole again in 13,700 A.D. The star is about

500 million years old, some 1/10 as old as the Sun, and 40 times the Sun’s brightness. Since it’s

twice as massive as our Sun, it will last about 1/10 as long. So Vega is halfway through its

lifespan. It will take the route of red giant to planetary nebula to white dwarf.

Vega is the brightest star by far in the constellation Lyra, the Lyre. So it’s also called α Lyrae.

Lyra is a small constellation, but it’s one of the most recognizable in the night sky. The

constellation was one of the original 48 mapped out by Ptolemy. Lyra lies directly east of

Hercules. The constellation takes the shape of a small parallelogram formed by the blue-white B

and A-type stars delta (δ), zeta (ζ), beta (β), and gamma (γ), with blue-white Vega marking a

jewel in the head of this small musical instrument. Next to Vega lies ε (epsilon) Lyrae which is a

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widely-spaced double star, each component of which is a closely-spaced double. It’s often called

the “Double Double”.

Legend holds that the lyre belonged to Orpheus, who played music of such beauty, the god of the

underworld, Pluto, was persuaded to allow the deceased wife of Orpheus, Eurydice, to return to

the world of the living. However, Pluto demanded that Orpheus not look back until both reached

the light of the world. Orpheus looked back just before the couple emerged from the underworld,

and lost Eurydice forever. He wandered the world in sadness for the rest of his days, and the

gods placed his lyre in the sky to honor him and his beautiful music.

While small, Lyra is situated in a fine star field on the edge of the band of the Milky Way. It

holds a few lovely sights for small telescopes. More about this shortly...

The star Vega and the constellation Lyra (left), east of Hercules

The Constellations Ophiuchus and Serpens

Ophiuchus (“Oaf-ih-YOU-kus”), the Serpent Bearer is a large dim pentagaonal constellation

directly south of Hercules. The constellation is identified with the ancient healer Asclepius, who

according to legend, discovered the healing arts when he saw one snake lay healing herbs on the

head of another. Artful star atlases show Ophiuchus grasping a serpent, which is represented by

the constellation Serpens to the west. Serpens is split into two discontinuous sections, the head

(Serpens Caput) and the tail (Serpens Cauda).

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The brightest star in Ophiuchus is 1st magnitude Rasalhague, which means “Head of the Serpent

Bearer”. It’s right next to α Herculi, or Rasalgethi, which means “Head of the Kneeler”.

Rasalhague is a white A5III star, which means it’s evolved off the main sequence and started to

burn helium in its core. It will presumably evolve into a red giant.

Speaking of red giants, look along the southwestern edge of the constellation for the two closely

spaced stars Yed Prior and Yed Posterior. These are the “hands” of the Serpent Bearer that grasp

the head of the snake Serpens Caput. Yed Prior is an M0 III red giant star. Yed Posterior is a K-

type red giant. The stars appear to form a wide double star, but they are unrelated. Prior lies

about 170 light years away, while Posterior is about 100 light years away.

Look again to the east side of Ophiuchus to find a small group of stars that looks like a mad little

bull charging east towards the Milky Way. This horned beast is a striking sight, and, frankly, a

little unsettling. This asterism is called Taurus Poniatowski, or Poniatowski’s Bull. It was

named in 1777 by Abbe Poszobut after King Stanislaus Poniatowski of Poland. For a time, this

little group was considered a constellation, but it’s now part of Ophiuchus. While “Taurus P”

didn’t make the cut as a modern constellation, the name of this star group remains. And it’s a

pretty little group. In binoculars, the background is flecked with fainter 9th and 10th- magnitude

stars that straggle off the western edge of the Milky Way. The V-shaped head of the bull

consists of three stars: 67, 68, and 70 Ophiuchi. The two stars at the back end of this little beast

are γ and β Ophiuchi (Cebalrai).

Ophiuchus also falls along the ecliptic, the band of sky in which we find the Sun, Moon, and

planets. The 12 constellations along the ecliptic are called the zodiacal constellations. Ophiuchus

was not included because ancient astrologers considered it unlucky to include 13 constellations

in the zodiac.

While there are no bright stars in Ophiuchus, the constellation is chock-a-block with globular

and open star clusters, as well as dark nebulae in the southern extremes of the constellation near

the border with Scorpius. It holds some interesting double stars as well.

The constellation Serpens, the Snake, like Ophiuchus, is one of Ptolemy’s original 48. But it’s

the only constellation split into two disconnected parts. The head of the snake is called Serpens

Caput. It lies to the west of Ophiuchus. The tail, Serpens Cauda, lies to the east. Stars in the

head include α, β, γ, δ, ε, ι, κ, λ, μ, π, ρ, σ, τ, χ and ω Serpentis. Stars in the tail include ζ, η, θ, ν,

ξ, and ο Serpentis. The brightest star in the constellation is Unukalhai (“uh-NOO-kul-lye”),

Arabic for “Serpent’s Neck”. Older star maps list the star as Cor Serpentis, the “Heart of the

Serpent”. Call it what you will, but 3rd

-magnitude α Serpentis is a fine orange K2 III red giant

star that’s swollen to 15x the diameter of our Sun and 70x the brightness of our Sun.

Serpens has two excellent deep-sky objects. The globular cluster M5 in Serpens Caput is one of

the finest in the northern sky and rivals the splendor of the more famous M13 cluster in Hercules.

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We’ll look at it shortly. In the southern reaches of Serpens Cauda lies the emission nebula M16,

the Eagle Nebula, which you will meet briefly next month.

The constellations Ophiuchus and Serpens Caput and Serpens Cauda, south of Hercules

The Constellation Scorpius

Now to the remarkably beautiful constellation Scorpius, or Scorpio, a constellation represented

by a fierce celestial scorpion. These stars were beheld as a scorpion by Greek, Romans, and by

the ancient Babylonians more than 4,000 years ago.

And small wonder. This large constellation is one of the few that clearly resembles its

namesake. Rising head first above the southeastern horizon, Scorpius presents a long body and

dramatically curved tail tipped with two bright stars for a stinger. There are dozens of bright

blue-white O and B-type stars here, and a single red supergiant, Antares, that marks the heart of

the scorpion.

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You have heard the legend of Scorpius before, back in Month 1, when we met the constellation

Orion. In Greek legend, Orion was a mighty hunter but a bit of a dim-witted brute. One legend

tells of the hunter killing the Earth’s animals until too few remained. The goddess Artemis put

an end to Orion’s greed by sending the fearsome scorpion Scorpius to sting the hunter, killing

him instantly. Another version of the legend has Orion pursuing Artemis with romantic

inclinations before the virginal goddess unleashed the Scorpion. In both cases, Artemis regretted

the death of the hunter and asked Zeus to place Orion and Scorpius in the heavens at opposite

ends of the sky.

In late June through July, from the northern hemisphere, you will find Scorpius rising in the

southeast, its midpoint culminating at about 10 p.m. by mid July. From claws to stinger, the

constellation spans about 25o. From mid-northern latitudes and northwards, it’s impossible to

see the entire constellation since the lower regions of the tail never get above the horizon.

Scorpius and its next door neighbor, Sagittarius, are the southernmost constellations of the

zodiac. The pair is almost overhead for mid-latitudes in the southern hemisphere where they are

a jaw-dropping sight.

The claws in the westernmost part of the constellation are marked by the three stars Achrab (also

called Graffias), δ Scorpii (also called Dschubba), and π Scorpii (see map). Directly eastward

lies the fiery orange-red supergiant star Antares, α Scorpii. Because of its color, this star is often

mistaken for the planet Mars, which is where it got its name. Ant Ares is ancient Greek for

“compared to Ares”, and Ares is the Greek name for Mars, the god of War.

The winding body of the Scorpion consists of τ Sco, ε Sco, μ Sco, ζ Sco, η Sco, θ Sco, and κ Sco.

The stinger is marked by the intrinsically bright blue subgiant star Shaula (λ Sco) which is

Arabic for “stinger”, and Lesath or υ (upsilon) Scorpii. At a distance of 365 light years, the

magnitude 1.6 star Shaula is one of the closest stars expected to explode as a supernova in

several million years. Shaula and Lesath are not a true double star in that they do not rotate

around each other. But they are related, along with several other bright blue-white stars in

southern Scorpius, because they are part of the Scorpius OB1 Association, a loose group of stars

that had a common origin and now move in the same direction.

In this part of the sky, we are looking at the thick and distant star clouds towards the center of the

Milky Way galaxy. You can see these star clouds if you have dark sky. Some beginners think

these are earthly clouds, but they are not. Turn your binoculars towards them and they resolve

into tiny pinpoints of light.

Scorpius harbors many, many deep sky objects, especially star clusters. You could spend every

summer night just observing the region in and around Scorpius, and it would be a summer well

spent. The globular cluster M80 lies tangled in the claws, and the resplendent globular M4 is

right next to Antares. Dozens of open clusters, including the dazzling M6 and M7 mark the

region of the tail.

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With binoculars, make sure to take in the optical double star μ Scorpii, a pair of nearly equal

brightness. Then move to the stunning optical double ζ Scorpii. One of these stars is a blue-

white B-type supergiant. The other is an orange K-type giant star. The contrast in colors is

lovely. Just north of the pair lies a bejeweled patch of slightly fainter stars that look magnificent

when set against dark sky.

The constellation Scorpius, the Scorpion

Observing Techniques –Telescope Mounts

Overview

Last month you had a look at the pros and cons of the many types of telescopes available today.

This month, it’s time for a quick look at mounts, a key piece of equipment often overlooked by

first-time telescope users. When evaluating a telescope, you must take into account not just the

optics, but also the size, cost, stability, and type of the mounting system. In a way, a telescope

mount is as important as the telescope itself, because even the finest telescope isn’t of much use

if the image in the field of view bounces and shakes and makes it impossible for you to see any

detail.

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Many telescopes include a mount and a tripod with the optical tube. Most manufacturers try to

match their telescopes to a decent mount, at least for telescopes in the price range of $200 and

above. Some refractors and catadioptric telescopes are undermounted and a little shaky.

Dobsonians, by design, tend to have very stable mounts even in the lower-end of the price scale.

Smaller scopes and more advanced telescopes like ED refractors come with an optical tube with

mounting rings or plates which allows them to be attached to a mount which you buy separately.

In either case, if you’re evaluating a telescope and mount to purchase, you must evaluate the

stability of the mount. It’s not terribly difficult: look through the scope at high magnification—

even just out the window of the telescope shop-- and give the tube a good tap on the side. If the

mount takes longer than 5-7 seconds to damp out the vibrations, then it’s unsuitable. It’s that

simple.

Naturally, there’s a trade-off in cost and weight and stability. The two telescopes below

illustrate the point. The telescope on the left, a 4.5” Newtonian reflector, has a barely adequate

mount… you can tell by the relatively small size of the mount relative to the telescope. This

mount is not useless, but it will be quite shaky at moderately high magnification. On the right

you see a larger 8” Newtonian with a heavy and solid mount that will damp vibrations in just a

few seconds. The mount on the left, by itself, would cost about $150 and weight about 10 lbs.

The mount on the right costs about $1500 and weighs about 40 lbs.

Alt-Azimuth Mounts

Aside from stability, the other factor to consider is the operation of the mount. All telescope

mounts are one of two types: alt-azimuth and equatorial. An alt-azimuth mount moves the scope

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up-down (altitude) and left-right (azimuth). This type of motion is intuitive and very easy to use.

With these two motions, you can point a telescope to any object in the sky.

However, an “alt-az” does not follow the natural motion of the sky. As you recall, stars and

planets appear to move around the sky in circles centered about an imaginary line through the

north and south celestial poles. And they follow a path in the sky that’s a combination of altitude

and azimuth. So to keep an alt-az-mounted scope centered on a celestial object, you’ll have to

move the scope in both axes, which is bothersome for visual observing especially at high

magnification. Some more advanced and costly altazimuth mounts include motors on both axes

and a hand-held controller/computer which help keep an object centered in the field of view

automatically for long periods of time. For visual use, this is a great advantage.

Altazimuth mounts are, however, almost completely useless for astrophotography, especially if

you want to take images through a telescope. Even if the mount has motors that track the motion

of the sky, the field of view in the telescope appears to rotate, an effect called “field rotation”

that ruins images of longer than a few seconds. So if you want to take images through a

telescope, do not choose an altazimuth mount.

Common versions of altazimuth mounts include the “rocker box” mounts of Dobsonian

reflectors. With these mounts, you give them a push in one or both axes to point the telescope.

In many cases, the telescope is held in place by the mount’s friction.

A Dobsonian reflector on a “rocker box” altazimuth mount

Many Schmidt-Cassegrain telescopes come with solid “fork” altazimuth mounts that solidly hold

the telescope on each side. Others have a single-arm altaz mount that is less expensive but also

less stable (see below).

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Single-arm and dual arm (fork-type) altazimuth mount for SCT’s

Equatorial Mounts

A more involved mount, designed to track the motion of the stars by turning on a single axis, is

called an equatorial mount. When the “polar axis” of an equatorial mount is aligned to the

celestial pole, objects can be tracked with the movement of only the polar axis, which follows the

longitudinal (or right ascension) circles in the sky. Because only one axis needs to be moved,

equatorial mounts can be more easily motorized to track celestial objects. Of course, to get an

object in the field of view in the first place, the telescope still must be moved in both axes.

A high-end Schmidt-Cassegrain telescope on a German equatorial mount

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An equatorial mount takes a little getting used to, but it’s a powerful tool for moving a telescope

about the sky and it’s indispensable for astrophotography. There are many variations of

equatorial mounts; they all tend to be larger and heavier than alt-az designs. The most common is

the German equatorial mount, pictured below.

The “GEM” holds the telescope in a saddle and balances the weight of the telescope with a set of

counterweights. To help with alignment, many GEM mounts include a very small rotatable

telescope in the polar axis to help align the axis of the mount with the north celestial pole (NCP).

For visual use, aiming the polar axis at Polaris using the polar-alignment scope is good enough.

The mount may not track exactly but you can correct it every so often when the object you

observe moves out of view. Of course Polaris is not exactly at the NCP, so more precise

alignment of a GEM is a little trickier. That’s why polar scopes include a small reticle which

helps you locate the NCP relative to Polaris at any time of year.

A German equatorial mount. A polar alignment scope is just to the upper right of the “a”.

If you are primarily interested in visual observing, an equatorial mount is not necessary. A good

alt-az, especially a motor driven alt-az, may be all you ever need. But your interests may

change, so the extra money and learning involved in purchasing an equatorial mount can save

you the hassle of upgrading your equipment in a few years.

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The view through a polar alignment scope showing the position of the NCP relative to Polaris

Go-To and Push-To Mounts

Some telescopes come with motorized alt-azimuth or equatorial mounts with computerized

databases, hand-held computers, and motion sensors on each axis to let you automatically point

the scope around the sky at the push of a button. Once you enter the current date, time, and

location, the computer and mount can point and track thousands of celestial objects. Some “go

tos” include a GPS module so you don’t have to enter time and location. A few even let you

choose a guided tour of the best celestial sights, complete with a digital readout describing

information about each object. There is a learning curve with a go-to mount simply because you

have to learn to set it up and align it to the night sky, usually by aiming it at two or three bright

stars at the beginning of each observing session. But it’s not too hard.

Go to telescopes are a great convenience, and help you spend more time looking at objects and

less time finding them. Go-to mounts are a great help for beginners who are often frustrated by

finding faint celestial objects. And they are a wonderful tool for city-based astronomers, even

experienced astronomers, who struggle to find faint stars to guide them from object to object in

the murky urban skies.

Most major telescope brands have their own version of go-to mounts and controller/computers.

Well-tested and reliable incarnations of go-to controller systems include NexStar (Celestron),

SynScan (Skywatcher), Autostar (Meade), and Orion’s version which does not have a fancy

name.

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A Schmidt-Cassegrain telescope with an altazimuth mount, motors, and go-to computer

A Dobsonian reflector with a “push-to” mount and hand-held computer

Some telescopes, especially Dobsonians have altazimuth mounts equipped with sensors that

detect the movement of each axis, and a computer to help make sense of the movement. But

they do not have motors to move the telescope in either axis. The movement must be supplied

by the observer, who merely pushes the telescope towards a particular celestial object. These are

called “push-to” or “shove-to” mounts. As with a go-to mount, the user must supply time and

location data to a hand-held computer before the observing session. Once the computer knows

where and when, it provides a digital readout to the observer to help find the way to a selected

object. Because they don’t use motors, push-to mounts are low-cost alternative to go-to mounts

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for large telescopes like Dobsonians. Push-to telescopes include the Intelliscope series by Orion

and other variations.

A word about the wise use of go-to and shove-to mounts…

Go-to and shove-to telescopes are no substitute for learning your own way around the sky.

Beginning astronomers who use Go-To telescopes to hop from object to object like they’re

flipping channels on a television are bound to lose interest in astronomy in just a few nights. The

pleasure of amateur astronomy comes not from mindless sightseeing, but from gaining your own

understanding, using your imagination, and enjoying the occasional feeling of pride and

accomplishment when you find and understand a new sight in the night sky.

Some Recommended Mounts

As with telescopes, there is a dizzying array of mounts available. Some come with telescopes,

some are separate. Here are a few good bets to guide your choice.

Note: If you are choosing a mount separately, keep in mind they have a specified carrying

capacity. For example, the Orion Atlas equatorial mount is rated to carry 40 lbs. All mounts get

a little shaky when they are loaded up to capacity. You are better off loading the mount to less

than ¾ of the carrying capacity to achieve best performance. That includes the telescope,

eyepieces, cameras, everything.

Altazimuth

Any Dobsonian. By design, Dobsonian telescopes come with solid altazimuth mounts. Few

have motorized drives. Most are completely manual, which means no motors and no

computer/controllers. Some, like Orion’s excellent IntelliScope series come with a hand

controller and motion sensors.

Celestron NexStar and CPC Series. These telescopes were mentioned last months as good bets.

The NexStar is a one-arm alt-az mount with go-to. It’s a little shaky at higher magnifications but

is light and affordable. The two-arm CPC series is solid as a rock but heavier and more

expensive.

Meade. The LS and LT series SCT’s from Meade come with one-arm alt-az mounts that are

more solid than the Celestron NexStar. But they are more expensive. Some of these mounts

completely automate the process of alignment to the night sky and require no input from the

user. The higher-end LX90 and LX 200 two-arm mounts complete with the Celestron CPC

series

iOptron. Chinese-based iOptron has a long list of innovate alt-az mounts with go-to capability.

Some mounts come with a telescope, others require you to buy the optical tube separately. The

Mini-Tower and Mini-Tower Pro are solid high-end mounts with go-to capability and cost $1000

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and up. The Cube Pro is about $400. Quality was not as good as Celestron and Meade, but has

improved recently.

Vixen/Astrotech/Orion. These three manufacturers each sell a manual alt-az mount (no optical

tube) for less than $300. They are great for visual observing with small refractors or catadioptric

scopes.

Equatorial

Celestron. Celestron has a range of equatorial mounts, all with NexStar go-to capability. The

lower-end mount, aimed primarily at visual observers was the venerable CG-5, which was

replaced by the Advanced GT series, which has now been replaced by the Advanced VX. This is

a good mount for scope (including accessories) of less than 30 lbs (say an 8” SCT). The higher-

end CGEM Series, CGE, and CGE Pro are very expensive and intended for serious

astrophotrographers with largers scopes (up to 11” to 14”).

Meade. Meade once had a mid-grade equatorial mount called the LX75. It has been replaced by

the innovative LX80 series which can switch between alt-az and equatorial in the same mount.

Reviews of this new mount, which was released in 2012, have been mixed.

Orion/Skywatcher. These brands, both made by the Chinese company Synta, make two excellent

mounts for mid-sized scopes. The Sirius and Atlas made by Orion are the same as the HEQ5-Pro

and EQ6 made by Skywatcher. They go for about $1,100 and $1,600 respectively and are

distinguished by their carrying capacity. The former is rated for 30lbs and the latter for 40 lbs.

These are good mounts for serious visual observers and astrophotographers with small-to-mid-

sized scopes. Both come with go-to capability.

Solar System Observing – Retrograde Motion; Moon Craters

Retrograde Motion of the Planets

As you learned earlier in this course, the superior planets, the planets further from the Sun than

Earth, slowly move from west to east across the sky as they revolve around the Sun. Yes, they

also move east to west across the sky during each day because of the Earth’s daily rotation, but

over the months and years, they move west to east. Jupiter, for example, which moves around

the Sun once every 12 years, seems to move eastward by one zodiacal constellation each year.

After 12 years, it moves all the way around all 12 zodiacal constellations.

Except once a year, more or less, or every two years in the case of Mars, the steady eastward

motion of the superior planets is temporarily reversed. For several weeks, a planet seems to slow

down and then reverse its eastward motion and move westward, then slow down its westward

motion and move again east. The image below shows the path of Mars a few years ago from

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July 1 to March 1 the next year. You see from October 1 until early December, Mars turned

back westward before moving eastward again.

An example of the retrograde motion of Mars

This type of motion is called apparent retrograde motion. When a planet moves this way, it’s

said to be “in retrograde”. When a planet moves normally, that is to say eastward, this called

prograde motion.

This motion completely baffled ancient stargazers. They believed the Earth was the center of the

solar system, and there was no clean, intuitive way to explain the apparent retrograde motion if

the Earth is at the center of things. The astronomer Ptolemy in the 2nd

century A.D. introduced

the idea of epicycles, which were circular paths within circular paths on which the planets

moved. Even this ad hoc and physically absurd explanation could not exactly explain the details

of retrograde motion.

But once astronomers understood the Sun was at the center of the solar system and Earth was the

third planet from the Sun, retrograde motion made sense. Earth completes its orbit in a shorter

period of time than the superior planets (which have larger orbits and which move slower). So

Earth periodically overtakes these planets like a car on a racetrack. When this happens, the planet

being passed will first appear to stop its eastward drift, and then drift back toward the west.

Then, as Earth moves past the planet, it appears to resume its normal west-to-east motion relative

to the background stars. The image below traces out the situation at 7 different points in the

orbit to illustrate the idea.

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As you can see, planets retrograde when they are near opposition, that is they are opposite the

Sun as seen from Earth.

Slow moving Saturn, Uranus, and Neptune retrograde once per year, roughly, since the relative

motion is mostly caused by Earth’s motion. Jupiter, with its 12-year orbital period, retrogrades

once every 13 months because it moves a little faster. And speedier Mars, which orbits the Sun

every 2.1 years, retrogrades about once every 25 months since it takes the Earth longer to catch

up.

This month, Saturn is just completing retrograde motion near the star κ Virginis. It’s just

reaching its westernmost point in its retrograde loop, then will start moving eastward again by

mid July.

Lunar Impact Craters – Big, Small, Old, New

In Month 2 of “The Art of Stargazing”, you took a whirlwind tour of the major “seas”, or maria,

on the surface of the Moon. These large, smooth, dark areas were caused by volcanic activity

triggered internally or by large impacts on the Moon’s surface more than 3 billion years ago.

They are the dominant features of the lunar surface. But even the most humble pair of binoculars

reveals another feature… craters… that fleck the lunar surface by the thousands. This month,

we’ll have a quick look at the origins and evolution of lunar craters. This will help you

understand a little of what you see on the Moon’s surface and what it tells us about the history of

the Moon and the inner solar system.

Craters are more or less circular excavated holes made by impacts of small asteroids and comets

on the Moon’s surface. The circular shape results from material flying out in all directions as a

result of the explosion upon impact. The impactor itself does not have to be spherical. In fact,

few are. Craters are the most common surface features on our Moon as well as the moons of

many other planets, and planets themselves, especially Mercury.

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Lunar craters, small, big, and huge.

When an impactor such as a comet or an asteroid strikes the surface of the Moon, a shock wave

spreads out from the site of the impact. The shock wave fractures the rock and digs out a large

cavity, much larger than the impactor itself. The impact sprays material-- called ejecta-- out in

all directions. The impactor is shattered into small pieces and may melt or vaporize. Sometimes

the force of the impact is great enough to melt some of the nearby rock. If an impactor is large

enough, some of the material pushed toward the edge of the crater will fall back toward the

center and the rock beneath the crater will rebound, creating a central peak in the crater. The

edges of these larger craters also may fall inward, creating layers of terraces that step down into

the crater.

Most craters have the following components…

Floor – The bottom of a crater, either cup-shaped or flat, and usually lies below the level of the

surrounding surface.

Central peak – Peaks formed in the central area of the floor of a large crater, a crater that’s

usually a few tens of kilometers or more in diameter.

Wall – The interior sides of a crater, usually quite steep. Larger craters may have giant stair-like

terraces that are created by slumping of the walls pulled down by gravity.

Rim – The edge of the crater. It is elevated above the surrounding surface because it is pushed

up at the edge during impact.

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Ejecta – Rock material thrown out of the crater area during an impact. It is sprayed outward from

the crater's rim onto the planet's surface as debris

Rays – Bright streaks extending away from the crater radially, sometimes for hundreds of

kilometers. Rays are composed of ejecta material.

The large lunar crater Tycho spans 86 km

Craters with all of these features tend to be large and complex and many tens of kilometers wide.

These are called complex craters. The crater Tycho, above, is an example, as is the crater

Copernicus (see below).

Complex lunar crater Copernicus spans some 95 kilometers. Copernicus is a young crater with

an age of about 100 million years.

Smaller craters, which span about 10-20 km or less on the Moon, are too small to form terraces

and peaks and flat floors. They are simple bowl-shaped features with smooth, curved walls and

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well-defined circular rims. They often called simple craters. There are intermediate types of

craters as well that exhibit a combination of features of simple and complex craters.

Because they are small, simple craters are much harder to see with a small telescope without

very high magnification, and even then they appear tiny. So most of the craters we see on the

Moon with backyard telescopes are large complex craters.

The simple bowl-shaped lunar crater Moltke, about 7 km across

You can tell quite a bit about many craters at a glance, especially age. Most craters on the Moon

were formed during a phase some 3.5 billion years ago when the solar system was full of many

small bodies left over from its formation. Thousands of these chunks of debris collided with the

inner planets. No traces remain on Earth of these collisions because the Earth’s atmosphere,

rain, and flowing water have eroded them away. But the Moon has neither atmosphere nor

water, so thousands of craters remain as a result of the impact of these primordial chunks of ice

and rock, especially in the light-colored southern highlands of the Moon. Many of these craters

have been around so long, newer impact craters have formed over top of them. This is the

easiest way to get a rough idea of the age of a crater. Older craters themselves are worn down by

newer craters (see below). The newer craters themselves may be 2-3 billion years old since the

rate of crater formation has dwindled since then. But older lunar features, as a rule of thumb,

have been exposed to more impacting bodies over time than younger features. This is true for

craters and also maria. The lunar seas have far fewer craters because they are much younger

than the highlands and formed after the solar system had been cleared of most asteroids and

comets.

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The lunar highlands are so old, even the craters are covered with craters!

Another way to judge the age of lunar craters is to look for signs of erosion. Our Moon has no

atmosphere or liquid water, so its surface features show very little erosion and remain unchanged

for hundreds of millions of years. But the Moon endures a steady but gentle rain of tiny

micrometeorites that wear down the sharper features of impact craters over billions of years. It

also gets a constant blast of charged particles from the Sun. Over hundreds of millions of years,

these two effects begin to wear down the sharper features of craters. So the oldest craters appear

to have smoothed walls and rounded ridges which give them a worn, tired look. The image

below shows two side-by-side simple craters, each about 300 meters across. The left crater is far

older than the right crater.

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Old crater (left) next to young crater (right)

This effect is visible in larger craters also. The left image below shows the large and ancient

crater Clavius which was formed about 3.5 billion years ago. It appears worn and heavily over-

cratered. The right image shows the sharp features of the crater Kepler, formed just 100 million

years ago and still sharply defined against the surrounding surface.

The large crater Clavius (left) spans 200 km and dates back 3.5 billion years. The smaller crater

Kepler (right) is just 32 km across but is just 100 million years old.

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As you tour the Moon during the length of your long and pleasant career as a stargazer, keep an

eye out for the differences in lunar craters and what these differences mean to the age of the

craters themselves and the early history of the solar system.

The Deep Sky This Month

Overview

This month we look at a good range of objects: resplendent open and globular star clusters,

superb double stars, and a fine planetary nebula. Compared to last month, most of these objects

are quite easy to see. And they are favorites of most experienced stargazers. As you master the

art of stargazing, you will return to these objects year after year.

The Ring Nebula (M57)

Now that you’re armed with an understanding about how stars evolve, let’s take another look at a

planetary nebula, the outer layers of a red giant driven off by a last gasp of a helium burning

shell around the core of a mid-sized star. The Ring Nebula is perhaps the most famous of all

planetary nebulae and the best-known object in the constellation Lyra. M57 is located on a line

between γ and β Lyrae, about 3/5 of the way from the former to the latter.

The nebula can be seen even in binoculars, though it appears star-like at such low magnification.

Telescopes give the best view. Even a 3” scope at 50x shows the tell-tale “smoke-ring” shape of

M57, though it is quite small. M57, like most planetaries, can take a good deal of magnification,

so experiment with your equipment and try low to high powers to get the best view.

The outer ring of M57 is lighter than the inner portion, which itself is lighter than the background

sky. The central star, which is intermittently blowing off its outer layers and setting them aglow,

is visible only in larger telescopes (10” to 12” or more).

The Ring Nebula is likely shaped like an hourglass; we are looking at it from the top.

Astronomers have recently determined the detailed 3D structure of M57 (see below). We are

looking at the nebula almost along its polar axis, which is why it appears so symmetrical.

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This illustration depicts a sideways view of the Ring Nebula, as deduced by astronomers using

new Hubble observations. The doughnut-shaped feature in the center of the graphic is the main

ring. The lobes above and below the ring comprise a football-shaped structure that pierces the

ring. Dense knots of gas are embedded along the ring's inner rim. Image courtesy of NASA.

The blue-green light of the nebula is created by rarified oxygen (OIII) ions as they change to a

lower energy state. Red light is created by the same process in hydrogen atoms. Special OIII or

UHC filters can improve the contrast of this nebula (and most planetary and emission nebulae) in

light-polluted sky. We’ll discuss such filters later in the course.

The Ring Nebula, M57, in Lyra as it appears at 200x in a small telescope

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M56

The “other Messier” object in Lyra is the faint globular cluster M56. Located about 4o east-

southeast of gamma Lyrae, this overlooked glob shares the field of view with a 6th-magnitude

red-orange star. The cluster appears small because it is fairly far away, about 31,000 light years.

In a small scope, it remains unresolved... just a grainy smudge that brightens towards the center.

In a 6-8” scope at 200x or more, with good seeing, you may resolve some stars in the outer halo.

But the cluster sits in a lovely star field, which makes it worth a look on a warm summer

evening.

The cluster spans about 9’, so given its distance, it’s about 85 light years across.

Dim globular M56 lies in a lovely star field in southern Lyra

Epsilon Lyrae

Lyra contains many fine arrangements of stars, including some excellent double stars. The

showpiece multiple star of Lyra is the famous “Double Double”, also known as epsilon Lyrae.

The star is easily found just 1.7o east-northeast of Vega.

There are four stars here, all gravitationally interacting with each other. The system is a splendid

test of visual acuity, telescope optics, and steady seeing. Sharp-eyed observers can visually see a

pair of stars here separated by 208” (about 3.5’). Younger observers with normal vision have

little trouble here. Observers over the age of 40 start to notice their age and find the pair more

challenging.

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Epsilon Lyrae in a telescope at 150x

The northernmost star of the epsilon Lyrae system is ε1. The southernmost is ε

2. Binoculars or

any telescope easily separate this pair. Both epsilons are themselves double stars. ε1 consists of

two stars of magnitude 5.0 and 6.1 separated by a tight 2.6”. ε2 is even tighter at 2.4”, with two

evenly-matched components of magnitude 5.3 and 5.4. Reasonably steady atmospheric seeing is

required to split the two pairs of stars. A 3” telescope at 100x will do the job, and higher

magnification will separate the components nicely.

While there is not much color in these mostly-white stars, the system makes a striking image in a

small telescope. ε1 and ε

2 are 0.16 light years apart, much further apart that our Sun and Pluto for

example, and so interact weakly. The stars take at least 500,000 years to revolve around each

other.

More detailed measurements with larger telescope reveal more stars in this system. A fifth

component orbits the pair ε2 , and there may be other nearby stars bound to the system as well,

for a total of ten stars!

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Deep-sky sights in the small constellation Lyra

70 Ophiuchi

Now find the little bull-shape of Taurus Poniatowski, mentioned in the tour of the constellation

Ophiuchus earlier. It jumps out in dark sky, but if you’re having trouble, look for 2nd

-magnitude

Rasalgethi in Hercules and Rasalhauge in Ophiuchus, the two brightest stars in the region. Then

look about 10o southeast of Rasalhague to find 3

rd-magnitude Celabrai, or β Ophiuchi. This is

one of the 5 stars of Taurus P along with the little triangle of 67 Oph, 70 Oph, and 68 Oph (see

below).

The star 70 Ophiuchi in Taurus Poniatowski is one of the best-known and widely studied binary

star systems. It’s relatively nearby, just 16 light years away. You’ll need about 100x to resolve

them cleanly with a telescope. The two components of 70 Oph have magnitude 4.2 and 5.9; the

brighter star is a yellow-gold while the fainter looks orange-red, with some observers reporting a

tinge of violet. Move your telescope out of focus just a touch to see the colors well. Each star has

an intrinsic brightness only a fraction that of our Sun.

The components complete a revolution about each other in just 88 years. And because the system

is only 16 light years away, the stars are close enough to resolve in a backyard telescope. So this

is one of the few double stars you can see move appreciably during a human lifetime.

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The average distance between the stars is about the same as the distance between the Sun and

Uranus. The stars were closest together in 1989. Since then, their separation has quickly

increased from 1.7 arc-seconds to about 5 arc-seconds.

And here’s a bonus for you: Just across the ‘V’ of Taurus Poniatowski lies another pleasing

double star 67 Ophiuchi. Both components are blue-white and are widely separated by 55”. But

there is a large difference in brightness: the primary star is magnitude 4.0 and the fainter star just

to the southeast is magnitude 8.6, a difference of 70x. So look carefully to see it.

The asterism Taurus Poniatowski, the double star 70 Ophiuchi, and the star cluster IC 4665.

IC 4665

Now look for the lovely open star cluster IC 4665 just 1.3o northeast of Cebalrai (beta Ophiuchi).

In dark sky, IC4665 is just visible to the unaided eye. If you’ve got a little light pollution, you’ll

need binoculars to spot it. The cluster is spread out over a full degree, more than twice the

diameter of the full Moon, so it looks fainter than its integrated magnitude of 4.7.

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In binoculars or your finderscope, you’ll see perhaps a dozen stars; a small telescope at 25-35x

shows a few dozen blue-white stars. With an age of 35 million years, this little cluster is young

compared to robust grand-dad open clusters like the Beehive (M44) which looks similar but is

more than 600 million years old. The youth of IC4665 means few if any stars have evolved into

red giants or supergiants. So the color of its stars is fairly uniform. But the apparent structure of

the cluster invites a long gaze. Look for arrangements among the stars, especially short

intertwined arcs. If southwest is “up” in your field of view when you look at IC4665, look

carefully at the inner stars. They form the pattern of the word “HI”, like a big friendly cosmic

greeting. While not obvious at first, it’s a little unnerving when the pattern finally jumps out at

you!

Open star cluster IC 4665 in Ophiuchus

M5

Last month you met the Great Hercules Cluster, which for many observers ranks as the finest

globular cluster north of the celestial equator. But the dazzling globular cluster M5 in the

constellation Serpens, the Serpent, give M13 a run for its money. A little brighter than its

counterpart in Hercules, this tight collection of 500,000 stars sparkles in the eyepiece of a small

telescope like an electric arc. Experienced observers often remark M5 is the more beautiful

object.

This cluster is a splendid object in a large telescope, with a tight sparkling core and innumerable

tiny stars spraying throughout the halo in all manners of lines and arcs. A 4″ to 6″ telescope

gives a lovely view as well, with fewer resolved stars but with an “electric spark” appearing to

emanate from the core of the cluster. Most observers agree the cluster is perceptibly non-circular,

or at least non-symmetric, with the core appearing somewhat brighter to the north.

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You will find the cluster about 7.5º southwest of α Serpentis, Unukalhai, the brightest star in

Serpens Caput. See the map below.

Charles Messier, who included the cluster in his famous list, could not resolve the cluster in his

tiny telescope and believed it was a round nebula. William Herschel had a much better telescope

and became one of the first to enjoy the cluster’s true nature.

Like all globular clusters, M5 is one of the elder statesmen of the Milky Way with an age of

some 13 billion years. M5 is one of the oldest globular clusters and, at a diameter of 130 light

years, one of the largest. The cluster is 24,000 light years away.

M5 as it appears visually in a 8” to 10” telescope.

When you get M5 in your sights, take a few minutes to look for a bright double star just 1/3 of a

degree south and within the same low-power field of view. The star is 5 Serpentis, also known

as Struve 1930 (or Σ1930). The primary star shines at magnitude 5 and its 10th-magnitude

companion lies about 11” to the northeast. The difference in brightness makes this a good

challenge of your observing skills. A small scope can split the stars at 70-80x or more, but the

brighter star tends to overwhelm the fainter.

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Location of globular cluster M5 in Serpens

M10 and M12

This part of the sky from Ophiuchus into Sagittarius is packed with globular clusters. M5 is

arguably the finest. The pair M10 and M12 are fainter but still look good in a small scope. They

are located within the large pentagonal shape of Ophiuchus proper.

Begin with M10. The cluster lies just 1o west of the orange star 30 Ophiuchi. M10 appears

brighter than its neighbor M12, and certainly more concentrated towards the center. The cluster

is visible in binoculars or a finderscope as a dim smudge. In a 4” scope at 70x, you will see some

granularity on the outer edges of the cluster. A 6” or larger scope resolves a couple of dozen stars

towards the center of this relatively nearby star cluster.

M12 is just 3o northeast of M10. The two are visible in the same finder field of view, and even in

a low-focal-ratio telescope at low magnification. The cluster is certainly less dense than M10,

and a good 6” scope may resolve some stars near the core. Try a range of magnification to see

what works best for you.

Some see in globular clusters a very subtle range of hues, with the core appearing straw colored

and the stars in the halo somewhat blue-white. This may be a matter of perception, because most

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globular clusters have very few blue stars. With ages of 10-12 billion years, the blue stars have

long ago ended their lives, with only old, low-mass, yellow-red stars remaining.

M10 lies some 14,000 light years from Earth, while M12 is about 16,000 light years distant.

Since they are physically separated by just 2,000 light years, a planet around a star in either

cluster would see the other cluster shine at brilliant 2nd

magnitude.

Location of globular clusters M10 and M12 in Ophiuchus

Antares

Antares, the brightest star in the constellation Scorpius, ranks as one of the brightest stars

embedded in the sweeping arc of the Milky Way. It’s beautiful from the northern hemisphere,

and even more dazzling from the south, where it lies almost directly overhead from June through

September.

As mentioned above, the ancient Greeks, like many modern-day stargazers, thought this star

resembled Mars, and named it ant-Ares, Greek for “compared to Ares” (Ares was the Greek

name for Mars, the god of war).

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Like all red supergiants, as you now know, Antares will soon run out of fuel in its core. It has

burned hydrogen, then helium, then carbon, and neon, and magnesium, into heavier elements like

iron. As you know from our discussion earlier this month, it will go no further: fusing iron into

heavier elements creates no energy. So when the fuel finally runs out, Antares will collapse and

detonate as a supernova, the biggest explosion known in the universe. From our sky, it will shine

bright enough to cast shadows in the dead of night for many weeks. This could happen anytime,

though forecasting supernovae is less exact than forecasting earthquakes. Astronomers believe

Antares could explode in a week from now, perhaps, or in a century, or in a million years. But

whenever it goes, it will be quite a show. The star Betelgeuse in Orion is in a similar state of

evolution. And no, neither pose a danger to Earth.

Since its days as a hot blue main sequence star, Antares has swelled to immense size. Though

it’s nearly 600 light years away, the star subtends an angle of 0.04 arc-seconds… large enough

for astronomers to measure directly. Simple trigonometry suggests Antares spans some 820x

the radius of our Sun. Were it at the center of our solar system, Antares would engulf the inner

planets, including Earth and Mars, and stretch almost to the orbit of Jupiter.

Antares, which is slightly variable like many end-stage stars, is listed as the 15th or 16th

brightest star in the sky at visual wavelengths. Intrinsically, it’s about 10,000x as bright as our

Sun, and some 60,000x brighter if infrared wavelengths are included.

Antares also presents a good challenge for backyard stargazers with a telescope. The star has a

companion, a blue-giant star called Antares B, that’s nearly 3 arc-seconds away from the reddish

Antares A, but some 370x fainter. Seeing the fainter star is not easy… it’s like trying to look for

a firefly in the glow of a bright streetlight. Still, it’s worth a try. You’ll need a good 8-10″ scope

to separate the two stars, and a magnification of perhaps 200x.

Occasionally, when the Moon passes in front of Antares, bright Antares A is blocked for a brief

time, making it easy to spot Antares B, even with a good 3″ scope. Many observers say the

fainter star looks greenish compared to its brighter red companion. This is likely just an artifact

of our perception, since no stars appear overtly green.

M4

Just 1.5o directly west of Antares lies Messier 4, the brightest of the globular clusters of

Scorpius. M4 is a splendid 6th-magnitude cluster. It appears much more “open” than most

globular clusters, which makes it easier to resolve in a small telescope. The cluster is bright

enough to be observable with the unaided eye. But the nearby glare of Antares makes it all but

impossible to do so. In binoculars, the cluster is easy quarry and appears round and bright and as

large as the full Moon.

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Antares (left) and M4 (right). Fainter and more distant globular cluster NGC 6144 is at top.

Even a 3-4” scope at 80x will resolve some stars, at least in the halo. And an 8” scope will

resolve the cluster to its core and reveal hundreds of yellow-white stars. Most observers notice a

loosely defined “bar” of stars in M4 than runs north-south. To some, the cluster looks a little like

a cat’s eye, with the bar taking the form of the eye’s pupil.

With so many resolvable stars, you will begin to see an amazing array of patterns in M4, defined

not just by the position of stars but the positions of their absences. The dark lanes, with no stars,

seem to radiate and ripple from the center of the cluster. De-focus your telescope slightly and

relax your eye to see this remarkable effect.

At a distance of just 7,000 light years, M4 is one of the closest globs. If you have dark sky and a

4” or larger scope, try to see the much more distant globular cluster NGC 6144 just 1/2 a degree

northwest of Antares. The neglected cluster is more than 30,000 light years away. But it makes a

striking though challenging sight between large M4 and bright Antares.

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The red circle marks the location of ρ Ophiuchi, about 3o north and slightly west of Antares.

Globular cluster M4 is just west of Antares

Rho Ophiuchi

The region around Antares contains a splendid nebula complex centered on the star ρ Ophiuchi

just over the border of Ophiuchus. You can’t see the nebula visually, but it’s a favorite for

astrophotographers. The star ρ Ophiuchi itself, however, is a fine and widely-spaced triple star

system that resolves easily in a small telescope or larger binoculars. All three are blue-white

stars of magnitude 5.0. 7.0, and 8.0 and are separated by a generous 150”.

The brightest component of ρ Ophiuchi is itself a tighter pair of B2V stars separated by 3”. The

stars are magnitude 5.0 and 5.9 and revolve around each other every 2,000 years.

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The Antares region. The triple-star ρ Ophiuchi is at top-middle. Orange-red Antares is at

bottom and M4 is to the right of Antares.

M6 and M7 star cluster (image courtesy of Sky and Telescope)

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M6 and M7

The span between the “stinger” of Scorpius and the tip of the teapot-shaped constellation

Sagittarius to the east holds the two superb open star clusters M6 and M7. Lovely in a small

scope or binoculars and visible to the unaided eye, these two clusters are a dazzling sight for all

stargazers, though they are best seen from as far south as possible.

M6 is among the brightest of dozens of open star clusters that fleck the Milky Way in Scorpius

and Sagittarius. Look for it about 4o north of Shaula in the Scorpion’s tail (see map below). M6

is a respectable 4th magnitude, though its light spreads over an area as large as the full Moon.

You’ll see the cluster without optics in dark sky; a full Moon or city lights make it harder to see,

especially if the cluster is near the horizon.

M6 is often called the Butterfly Cluster, and a glance through a small telescope reveals why. At

40-50x, the cluster has 3 bright stars running through the center (the body of the butterfly), with

two irregular loops of stars on either side (the wings). A little imagination reveals the butterfly’s

“antennae” to the northeast. Experiment with different eyepieces to get the best view.

The cluster lies some 1,600 light years away from us towards the galactic center. It holds a little

more than 300 stars, though you’ll see just a few dozen in binoculars, and perhaps 100 stars in a

6-inch scope. It’s a young cluster… about 100 million year old… so it contains mostly blue

stars, with one orange-giant star in the northeast corner.

Now look 3.5o southeast of M6 to find the cluster M7 set in one of the richest sections of the

Milky Way (see image at top). Though they’re closeby in the sky, the two clusters are not

physically associated. M7 is closer, just 780 light years away. To the unaided eye, in the words

of Stephen J. O’Meara, the spray of light from M7 looks “like the eruption of distant fireworks.”

M7 appears larger and brighter than M6, though both were known in antiquity. But M7 is the

southernmost Messier object, so it’s rarely seen well at northern latitudes, and it presents a real

challenge for observers in northern Europe. The view of this cluster from the southern

hemisphere, however, where it’s high overhead this time of year, is jaw-dropping.

The cluster spans more than a full degree of sky, twice the size of the full Moon. So stick with

binoculars or a low-power eyepiece. At 30x, the center of M7 looks square, or to some, cross-

shaped. Since M7 is twice as old as M6, some of its 80 stars have begun to evolve off the main

sequence and turn orange-red. The hottest and most massive blue stars have, presumably,

burned out long ago.

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Location of star clusters M6 and M7 northeast of the “stinger of Scorpius

What You Have Learned This Month Once you work through the notes and tours of The Art of Stargazing this month, you will have

learned:

Sky Tours

The legends, layout, and position of the constellations Lyra, Ophiuchus, Serpens, and

Scorpius

The stars Vega, epsilon Lyrae, Rasalhague, the asterism Taurus Poniatowski, Antares, the

stars of the Scorpius OB1 association

Science of Astronomy

How mid-sized stars evolve off the main sequence to form red giants, planetary nebulae,

and white dwarfs

How large stars evolve off the main sequence to form red supergiants, supernovae,

neutron stars, and black holes

Examples of evolved mid-sized and massive stars you can see in the night sky

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Observing Techniques

An overview of alt-azimuth and equatorial telescope mounts

How to evaluate a mount for visual observation

A look at go-to and push-to mounts

A look at good choices of telescope mounts currently on the market today

Deep-Sky Objects

The Ring Nebula, M57, in Lyra; open clusters IC 4665, M6, and M7; globular clusters

M56, M4, M5, M10, and M12; double stars 70 Ophiuchi, rho Ophiuchi; multiple star

epsilon Lyrae; a look at the red supergiant star Antares

Solar System Tour

The cause of apparent retrograde motion of the planets

The structure and main types of lunar craters

The age of craters

notes: the art of stargazing month 5: june - july 2013 · welcome to month 5 of the art of...

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