Chapter 28Stars and

Their Characteristics

Origin of the Universe

• Big Bang Theory –– about 10-20 bya all matter in the universe

existed in a hot dense state about the size of an atom (tiny).

– That matter sort of exploded and began expanding a great speeds.

– The expansion speed slowed down (is still expanding) and temperatures cooled and stars and galaxies were formed.

Evidence of Big Bang

• In 1929 astronomer Edwin Hubble found redshifts – evidence galaxies are moving away from each other.

• In 1964 radio astronomers Arno Penzias and Robert Wilson discovered radiation (cosmic background radiation) left over from the big bang. (Detected this in infrared and radio telescopes – so we can hear it buzzing through radiowaves).

Spectral Analysis

• We can’t always get a sample of a piece of the Universe.

• So we depend on light !

Spectral Analysis

• Light is a form of Electromagnetic Radiation. – Electromagnetic radiation = energy that travels in

waves (radiowaves, x-rays, etc)– Length of the waves determine the characteristics of

the electromagnetic radiation. – The types of electromagnetic radiation can be

arranged in a continuum called the Electromagnetic Spectrum (longest wavelengths at one end and shortest wavelengths at the other end)

Electromagnetic Spectrum

• Visible white light is actuallymade up of light of various colors, each with a different wavelength. (colors seen in rainbow or when light passes through a triangular prism.)– Red light has the longest wavelength, violet has the

shortest wavelength. • Electromagnetic waves emitted by an object

provide information about elements within it or its motion. (use this to learn about distant stars)

The intensity of the electrons bouncing around in their levels makes those wavelengths, and

therefore shows us colors (bigger jump = longer wavelength and shows as red light).

Spectroscope

• Spectroscope – tool astronomers use to separate starlight into its colors (uses a prism to split light, gathered by a telescope, into a spectrum)

• Break light into 3 different types of spectra:– Continuous Spectrum– Emission Spectrum– Absorption Spectrum

Types of Visible Spectra

• Continuous Spectrum – unbroken band of colors, which shows that its source is emitting light of all visible wavelengths.– Emitted by:

• Glowing solids, such as the hot filament of an electric light

• Glowing liquids, such as molten iron• The hot, compressed gases inside stars

Types of Visible Spectra• Emission Spectrum – series of unevenly

spaced lines of different colors and brightnesses. The bright lines show the source is emitting light of only certain wavelengths.– Emitted by:

• Glowing thin gases (every element has its own color signature)

Types of Visible Spectra• Absorption Spectrum – a continuous spectrum

crossed by dark lines. – Dark lines form when light from a glowing object

passes through a cooler gas, which absorbs some of the wavelengths.

– Elements absorb the same wavelengths that they would emit if they were in the form of glowing gases.

– A stars absorption spectrum indicates the composition of the star’s outer layer.

Spectral Analysis• Each element has a unique spectral

signature:• Determined by arrangement of electrons. • Lines of emission or absorption arise from

re-arrangement of electrons into different energy levels.

Hydrogen

Spread a rainbow of color across a piano keyboard(Developed by Shirley Burris, Nova Scotia)

Then, “play” an element

Hydrogen

More Musical Elements

Now play another elementHelium

CarbonAnd Another

Getting a Handle on Water

Oxygen

All together now ...

Hydrogen

Water

Doppler EffectEvidence of a star’s motion relative

to Earth. • If lines on the spectrum are shifted

toward the red side then object is moving away = Red Shift

• If lines on the spectrum are shifted toward the blue side then the object is moving toward you = Blue Shift

Spectral Analysis Explains what is going on in Space!

• Astronomers use spectral analysis to identify what is going on in space.– How fast stars are moving away from us.– What life stage a star is in.– The chemical makeup of a star or a planet.

Now that we can tell if a star is moving toward or away from us lets learn more about stars

… measuring their brightness, their distances, their life cycles…

If we imagined that the distance from the Earth to the Sun was

1 Centimeter…..

1 Centimeter

Sun

Earth

How far away do you think the next nearest star would be???

?

2.5 Kilometers1.5 miles

In real distance, the next closest star would be 300,000 times the distance from the Earth to the Sun, or……

EarthProxima Centauri

39 Trillion miles (that’s 4.24 Light Years!)

Sun

What does 39 trillion miles look like????

Objects in Space are so far apart that units of

measurement used on Earth are not useful.

The distance to the next nearest big galaxy, the Andromeda Galaxy, is 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance.

Andromeda Spiral GalaxyEarth

21,000,000,000,000,000,000 kms

The basic unit of measurement of distance inside of our solar

system is the…

An Astronomic Unit (1 A.U.) is equal to the distance from the Sun to the Earth, which is about 93 million miles.

SunEarth93 million miles

Planets inside Earth’s orbit have distances from the Sun of less than 1 AU.

(Mercury is .4 AU’s from the Sun.)

Sun Mercury.4 AU’s

Planets outside the orbit of the Earth have distances from the Sun of greater than 1 AU.

(Mars is 1.5 AU’s and Pluto is 39 AU’s from the Sun.)

But, Astronomic Units are too small for measuring distances

outside of our own Solar System.

The closest star to the Sun, Proxima Centauri, would be more than 300,000 AU’s from our star, and that’s the closest!

Astronomers use

to measure distances outside our Solar System.

A Light-Year is a unit of Distance.

A Light Year is equal to the distance that light can travel in one Earth year.

A Light Year is equal to 5.3 trillion miles.

Use of Light Years makes the units used in measuring distances in Space smaller, but “small” is pushing it!

The Speed of Light is 186,000 miles per second.

That is almost 8 times around the Earth in 1 second!

Peeoooummmmmmm!!!

The Crab supernova remnant is about 4,000 light-years away.

The Andromeda Galaxy (next closest galaxy) is 2.3 million light-years away.

Our Milky Way Galaxy is about 150,000 light-years across.

The Virgo Galaxy Cluster is 45 million light- years away.

The most distant

Supernova is 10 billion light-years

away.

The most distant Galaxy Cluster is 12 billion Light-years away.

The most distant Galaxy is 13 billion light- years away.

The background radiation from the big-bang is 14 billion light-

years away.

Types of Stars and Their Organization in Space

How are Stars and Planets different?

• Stars emit light, due to nuclear fusion in their center, while planetsonly reflect light.

Binary SystemsSolar systems contain at least one star,

and can contain two or more

1

2

3

Six Star Binary System

Star Cluster contain from tens and hundreds to millions of

stars.Pleiades

“Seven-Sisters”

Open Cluster-Galactic center behind

Large Globular cluster-millions of stars

Center of Globular cluster-Note star density

Spiral Galaxies contain billionsof stars.

Open Clusters of new Stars

Globular Clusters of old Stars

Elliptical Galaxy- Many billions

All old Globular Clusters

A Star is a “self-luminous” (it is giving off light as opposed to reflecting it) sphere of gas that is

undergoing Nuclear Fusion in its center.

Not all stars are the same. In fact, they vary in many ways.

Stars Vary in “Brightness”.Magnitude -How bright an object in Space is, or appears to be.

Luminosity - the true brightness of an Individualunit of a star. The Luminosity of a Star depends on a star’s temperature and size/radius. (how much energy it puts out).

Which star is brighter? (hint: look at temperature)

A. – Temp is 5,000 B. – Temp is 3,000

(The brighter the star the lower the magnitude). Magnitude sequence for stars starting with the brightest is -1, then as stars get dimmer their number/magnitude will increase 0, 1, 2, 3, 4, 5, 6 magnitude, ... etc.

A hotter star is more luminous than a cooler one of the same radius.A bigger star is more luminous than a smaller one of the same temperature.

Absolute Magnitude- is a measurement of the true “brightness” of stars as if all stars were viewed from the same distance. The Absolute Magnitude of a star depends on its Volume and Luminosity.

Apparent Magnitude- Apparent Magnitude is a how bright a star “appears” to be from Earth. The Apparent

Magnitude of a star is affected by Absolute Magnitude(Volume x Luminosity) and Distance from Observer.

Betelgeuse, one of the brightest stars in the Universe, does not appear to be as bright as our Sun, because of its distance from us compared to the Sun’s distance.

The Pink star appears to be the brightest to us from here on Earth, and the Yellow star appears to be the 2nd

brightest.

But… the Pink and Yellowstars are actually less bright than the other stars. (they only appeared brighter because they are closer to us on Earth).

Stars also vary in their:• Mass• density • Volume• interior and surface temperature• rate of fuel-consumption• Color• Main Sequence life-span• what they do when they “die” and what

they become after they “die”.

Stellar Mass

When comparing the masses of different stars, we will use the mass of our star, the Sun, as the standard. A star that is identical to ours would be a star of 1 “Solar Mass”.

Stars vary in mass from a fraction of 1 solar mass, up to 50 times the mass of our Sun, or “50 Solar Masses”.

50 solar mass star The Sun

Red Dwarf star

Stars vary even more in their volume/density

Number represents xSun volume

White Dwarf

Earth

Star Density

Stars vary in their Main Sequence and Giant life-span

So bigger stars tend to “burn out” faster/have a shorter life span than smaller stars.

Stars vary in what they become when they are no longer fusing Hydrogen.

Blue Supergiant

Red SupergiantBetelgeuse

Rigel

Orion’s Belt

Orion Nebula

Stars vary in how they “die”

Supernova explosion

Supernova

Supernova

Native American Petroglyph recording Supernova explosion

Planetary Nebula

Stars vary in what they

become when they “die” (Run out of material

that can be fused to create outward

pressure).

White Dwarf in Binary System

White Dwarfs

White Dwarf in Binary System

White Dwarfs in Globular Cluster

Neutron Stars

Neutron Star

Pulsars

Almost like a very dim lighthouse, pulses light.

Pulsar Cone

Black Holes

A star’s mass determines every other characteristic of the star that

we mentioned earlier.

Life-Span

Density

Volume

Rate of “Fuel” consumption

How it “dies”

Temperature

Luminosity

Life Cycles of Stars

The universe started with the …Big Bang!

– Everything continued to expand, clouds of dust started to gravitate towards each other forming stars.

A star’s life begins in a …Nebula!

– A cloud of gas and dust, consisting mostly of Hydrogen

A star’s life begins…• Gas and dust begin to clump together to form a

Protostar (a baby star).

A star’s life begins…• The smaller a star is the longer it will live.

– Larger stars have more fuel, but they have to burn (fuse) it faster in order to maintain equilibrium.

– Because fusion occurs at a faster rate in massive stars, large stars use all their fuel in a shorter length of time.

– So…A smaller star has less fuel, but its rate of fusion is not as fast. Therefore, smaller stars live longer than larger stars because their rate of fuel consumption is not as rapid.

• The star’s main goal in life is to achieve stability, or equilibrium, where pressure from fusion within the core is equal to the force of gravity pushing down on it (this keeps the star “alive”).

A star’s life begins…

G

E

G

G

G

G

G

Continuous steps occur inside the core of a main sequence star, until there is no more Hydrogen.• Step 1 - Nuclear fusion (hydrogen turning to helium).

Gravity = gas pressure (equilibrium)• Step 2 - Out of fuel• Step 3 - Fusion stops, temperature drops• Step 4 - Core contracts (gravity pulling atoms in)• Step 5 - Increased temperature (more atoms, more

collisions) and density in the core reinitiates nuclear fusion, equilibrium is achieved, and the cycle begins again at Step 1. – This entire process (repeating steps 1 – 5) continues until there

is no more Hydrogen left in the star. – Then the star will start fusing other elements until it has burnt up all the elements.

A star’s life begins…

Life Cycle of a Star like our Sun…

Nebula Protostar Main Sequence Star Red Giant Planetary Nebula White Dwarf

Life Cycle of a Star like our Sun…

• Our sun is at the Main Sequence stage in its life. – When the hydrogen in the core has been used up, the

core shrinks and hydrogen fusion begins in the outer layers,

– which then expands the entire star, turning it into a Red Giant.

– The sun begins to die when helium is fusing into other elements, then the gases at the sun’s surface start to blow away in bursts, called a Planetary Nebula (or halo of gases,

– Resulting in a hot carbon-oxygen core called a White Dwarf.

Life Cycle of a Star With Greater Mass Than Our Sun…

Nebula Protostar Main Sequence Star Red Supergiant Supernova

Black Hole or Neutron Star

Life Cycle of a Star With Greater Mass Than Our Sun…

• Massive stars go through the same life stages as our sun (just on a larger scale) upto the Main Sequence stage,

• Then the massive stars expand into a Red Supergiant,

• Explode into a Supernova, • Then turn into a Black Hole or a Neutron

Star.

Life Cycle of Stars

Hertzsprung-Russell (HR) Diagram

HR Diagram• The Hertzsprung-Russell (HR) Diagram is a tool that

shows relationships and differences between stars (temperatures, brightness, colors, etc.)

• It is something of a "family portrait." It shows stars of different ages and in different stages, all at the same time. – A star in the upper left corner of the diagram would be hot

and bright. – A star in the upper right corner of the diagram would be cool

and bright. – The Sun rests approximately in the middle of the diagram, and

it is the star which we use for comparison. – A star in the lower left corner of the diagram would be hot and

dim. – A star in the lower right corner of the diagram would be cold

and dim.

Hot and Bright

Cool and Bright

Hot and Dim

Cool and Dim

HR Diagram

Main Sequence Line;Core Fusion of H at constant rate; Volume directly related to mass

Main Sequence

. .25-50 Msun

Masses/Luminosity of Main Sequence Stars

Giants

Core fusion of He

Supergiants.. .Supergiants

White Dwarfs

White Dwarfs

“Dead” Star; High temps. Due to compression

Black Holes, Pulsars and Neutron-Stars are not identified on the HR Diagram because they are either

very dim or do not give off energy in the visible wavelengths.

Star Life Cycles

As we have discussed, stars are not all the same.

All of the characteristics of a star are determined by their mass.

Stars with different masses have different life cycles.

Based upon their masses, stars can follow three main “pathways” and fit into three “candidate” groups during the course of their “lives”.

These groups include:

White Dwarf Candidates (less than one “solar mass” to 15 solar masses)

Neutron Star and Pulsar Candidates (16 to 30 solar masses)

Black Hole Candidates (Greater than 30 Solar Masses)

White Dwarf Candidates

Supergiant

Pulsar and Neutron Star Candidates

Pulsar

Supergiant

Black Hole Candidates

Black Holes