chapter 27: stars and galaxies

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley

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Chapter 27:

STARS AND GALAXIES


This lecture will help you understand:

• Observing the Night Sky• The Brightness and Colors of Stars• The Hertzsprung-Russell Diagram• The Life Cycles of Stars• Black Holes• Galaxies


Observing the Night Sky

Observing the Night Sky:constellations are groups of stars named over antiquity

familiar constellation is Ursa Major, the Great Bear.



The monthly constellations seen in the night sky change as Earth’s path around the Sun progresses.


Observing the Night SkyCan you see that during a solar eclipse, the darkened daytime sky would show constellation positions as normally seen six months earlier or later?



The Big Dipper is a well-known constellation. The pairs of stars at the end of its bowl point to Polaris, the North Star.



The seven stars of the Big Dipper are at very different distances from Earth.



A time-exposure of the night sky shows streaks of stars from our “carousel Earth.”


Knowing the names of the constellations tells us much about the

A. stars that comprise them.

B. people in the cultures that named them.

C. difference between stars and planets.

D. All of the above.

Observing the Night SkyCHECK YOUR NEIGHBOR


Knowing the names of the constellations tells us much about the

A. stars that comprise them.

B. people in the cultures that named them.

C. difference between stars and planets.


Explanation:

The names of constellations tell us nothing about the makeup of the stars that compose them. They are more interesting historically.

Observing the Night SkyCHECK YOUR ANSWER


The Brightness and Colorsof Stars

A star’s color indicates its temperature:

• a red star is cooler than a blue star

• a blue star is almost twice as hot as a red star (blue light has almost twice the frequency of red light in accord with T)


Which of these stars radiates light of the longest wavelength?

A. Red star.

B. Yellow star.

C. Blue star.

D. Violet star.

The Brightness and Colors of StarsCHECK YOUR NEIGHBOR


Which of these stars radiates light of the longest wavelength?

A. Red star.

B. Yellow star.

C. Blue star.

D. Violet star.

Explanation:

The longest wavelength is emitted by the star with the lowest frequency, the red-hot star. (If the question had asked for the highest frequency, that would be emitted by the violet star.)

The Brightness and Colors of StarsCHECK YOUR ANSWER


The Brightness and Colors of Stars

We measure the brightness of a star in two ways:

• apparent brightness—the brightness as it appears to us

• luminosity—the intrinsic brightness, independent of how bright it appears

The luminosity of stars is compared to that of the Sun, which is noted LSun.


We measure the Sun’s luminosity as LSun. If we were on a spaceship twice as far away from the Sun, its apparent brightness would appear

A. the same.

B. half as much.

C. one quarter as much.

D. four times as much.

The Brightness and Colors of StarsCHECK YOUR NEIGHBOR


We measure the Sun’s luminosity as LSun. If we were on a spaceship twice as far away from the Sun, its apparent brightness would appear

A. the same.

B. half as much.

C. one quarter as much.

D. four times as much.

Explanation:

In accordance with the inverse-square law, twice as far away means one quarter the brightness.

The Brightness and Colors of StarsCHECK YOUR ANSWER


Graph of intrinsic brightness versus surface temperature for stars

Note: positions that form a main sequence for average stars, and exotic stars above or below the main sequence.The H-R diagram is to an astronomer what the Periodic Table is to a chemist.

The Hertzsprung-Russell Diagram


On the H-R diagram, the Sun is

A. an average star.

B. seen to be special.

C. a low-temperature star.

D. especially bright.

The Hertzsprung-Russell DiagramCHECK YOUR NEIGHBOR


On the H-R diagram, the Sun is

A. an average star.

B. seen to be special.

C. a low-temperature star.

D. especially bright.

The Hertzsprung-Russell DiagramCHECK YOUR ANSWER


A dying star that has collapsed to a small size and is cooling off would appear in which part of the H-R diagram?

A. Lower left.

B. Upper left.

C. Lower right.

D. Upper right.

The Hertzsprung-Russell DiagramCHECK YOUR NEIGHBOR


A dying star that has collapsed to a small size and is cooling off would appear in which part of the H-R diagram?

A. Lower left.

B. Upper left.

C. Lower right.

D. Upper right.

Explanation:

Such a star would be a white dwarf.

The Hertzsprung-Russell DiagramCHECK YOUR ANSWER


The Life Cycles of Stars

Life cycle of stars:• begins as a nebula• advances to a protostar• becomes a star when fusion in its core occurs

Depending on its mass, the star may become a red giant and then burn out to become a white dwarf.



White dwarf:

• cools for eons until it is too cold to emit light

• if part of a binary, pulls matter from its partner, which can lead to a nuclear blast (nova)



Final stage of more massive stars is collapse, then an explosion called a supernova.

Remnant of a supernova is the Crab Nebula.


The source of energy in the Sun and stars is

A. chemical reactions.

B. thermonuclear reactions.

C. Both of the above.

D. None of the above.

The Life Cycles of StarsCHECK YOUR ANSWER


The source of energy in the Sun and stars is

A. chemical reactions.

B. thermonuclear reactions.

C. Both of the above.

D. None of the above.

Explanation:

The Sun and other stars are balls of plasma, much too hot for chemical reactions to occur.

The Life Cycles of StarsCHECK YOUR ANSWER


Stars Sizes

Planets

Sun Vega


Stars Sizes

Sun

Arcturus

Alpha Ceti

Vega

(Red Giant)

(Red Giant)


Stars SizesSun Arcturus

Alpha Ceti

Vega

Betelgeuse A

(Red Supergiant)


Stars Sizes

Sun

Arcturus

Alpha Ceti

Vega

Betelgeuse A

(Red Hypergiant)

Cephei A


Stars Sizes

Sun

Arcturus

Alpha Ceti

Vega

Betelgeuse A

(Red Hypergiant)

Cephei A

All these stars are within 3000 light-years of Earth, which means they are our immediate neighbors within the Milky Way Galaxy.


Black Holes

Black hole:

• what remains when a supergiant star collapses into itself

• named because gravitation at its surface is so intense that even light cannot escape


Black HolesBlack hole

Why gravitation at the surface of a star increases when it collapses

star shrinks to half its radius gravitation at its surface increases by 4 (inverse-square law)


When a star collapses to one-tenth size, gravitation at its surface becomes

A. one tenth as much.

B. the same.

C. 10 times as much.

D. 100 times as much.

Black HolesCHECK YOUR NEIGHBOR


When a star collapses to one-tenth size, gravitation at its surface becomes

A. one tenth as much.

B. the same.

C. 10 times as much.

D. 100 times as much.

Explanation:

This follows from the inverse-square law introduced in Chapter 4.

Black HolesCHECK YOUR ANSWER


When a giant star collapses to become a black hole, gravity is greatly increased

A. at it surface.

B. at its center.

C. in all surrounding space.




When a giant star collapses to become a black hole, gravity is greatly increased

A. at its surface.

B. at its center.

C. in all surrounding space.


Explanation:

It is important to know that gravitation increases mainly at the surface of the collapsed star. Gravity at the surface before collapse is the same at that same distance from the center of the black hole after collapse.



If the Sun collapsed to become a black hole, the orbit of Earth would

A. remain unchanged.

B. be pulled inward toward the black hole.

C. spiral outward away from the black hole.

D. be a straight-line path.



If the Sun collapsed to become a black hole, the orbit of Earth would

A. remain unchanged.B. be pulled inward toward the black hole. C. spiral outward away from the black hole.D. be a straight-line path.

Explanation:

F = G(m1 m2)/d2. Letting this equation guide our thinking, we see that none of its terms differ. Although the density of the black hole has greatly increased, its mass is the same before and after collapse. Because the mass of Earth and the solar black hole are the same, and distance between centers is the same, the force holding Earth in orbit wouldn’t change. Equations nicely guide thinking!



Galaxies

Galaxy:

• huge assemblage of stars, interstellar gas, and dust

• most familiar—Milky Way


Galaxies

Three types of galaxies:• elliptical• irregular• spiral

This is a giant elliptical galaxy M87


GalaxiesA pair of irregular galaxies—

the Large Magellanic Cloud and neighboring Small Magellanic Cloud


This is Spiral Galaxy M83, thought to be much like our Milky Way.

Galaxies are not the largest things in the universe. There are clusters of galaxies, and then galaxy superclusters—larger than can be imagined!

Galaxies


GalaxiesSome galaxies are known as active galaxies and are emitting huge amount of energy.By comparison, these active galaxies emit many orders of magnitude more energy than our own Milky Way! Two examples of active galaxies include:

• Starburst galaxies • Galaxies with an active galactic nucleus


GalaxiesStarburst galaxies form stars at a very high rate. They result from violent disturbances, such as the collision between two galaxies.

This image shows the aftermath of the collision of two spiral galaxies. Areas in blue are regions of rapid star formation.


GalaxiesSome active galaxies have supermassive black holes in their centers into which large amounts of matter is falling, sometimes causing jets that extend thousands of light years from the galactic center (called an active galactic nucleus, or AGN).

This jet coming from M87 consists of charged particles being accelerated to velocities near the speed of light.


The motion of individual stars in a galaxy normally follow

A. elliptical orbits around the center of the galaxy.

B. completely random paths.

C. straight lines for the most part.

D. circular orbits around the center of the galaxy.

GalaxiesCHECK YOUR NEIGHBOR


The motion of individual stars in a galaxy normally follow

A. elliptical orbits around the center of the galaxy.

B. completely random paths.

C. straight lines for the most part.

D. circular orbits around the center of the galaxy.

Explanation:

Within galaxies, stars move in elliptical orbits around the center of the galaxy.

GalaxiesCHECK YOUR ANSWER


Clusters and SuperclustersThe Milky Way Galaxy and its neighboring galaxies are

known as the Local Group.


Clusters and SuperclustersOur local group is situated between the Virgo and Eridanus clusters, which all together make our Local Supercluster.


Clusters and SuperclustersOur Local Supercluster is part of a network of superclusters.


Clusters and SuperclustersAs far as we can see, superclusters hold together like a foam

within which there are bubbles of super large voids.

chapter 27: stars and galaxies

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