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PH-120-02 Spring 2013

Homework Chapter 13

1. Why do the stars of the Galaxy appear to form a bright band that extends around the sky?

Answer:Most of the stars in our Galaxy are found within a disk surrounding the center of the Galaxy.

Our solar system is found within this disk. As we look out at our Galaxy, we see this disk from

within. The result is that we see a bright band of stars that extends around the sky.

4. The galactic halo is dominated by Population II stars, whereas the galactic disk contains

predominantly Population I stars. In which of these parts of the Galaxy has star formation takenplace recently? Explain your answer.

Answer:Population I stars are younger stars with higher metal content, signifying that they were born

more recently than Population II stars. Therefore, if we wish to study the locations of recent star

formation, we want to look in the disk of the Galaxy, not the halo.

9. How do astronomers conclude that vast quantities of dark matter surround our Galaxy? How is

this dark matter distributed in space?

Answer:

By looking at the motions of stars at various distances from the galactic center, astronomers can

determine the distribution of matter throughout our Galaxy. Comparing this measurement withthe amount of luminous matter that can be observed, astronomers have discovered that roughly

90 percent of all matter is in the form of dark matter. Because it appears to have a greater effect

on material farther out in our Galaxy, astronomers believe that the dark matter is distributedthroughout a halo surrounding our Galaxy and is twice as large.

12. In our Galaxy, why are stars of spectral classes O and B only found in or near the spiral

arms? Is the same true for stars of other spectral classes? Explain why or why not.

Answer:Very luminous O and B stars are concentrated near spiral arms because they are very short lived

and die before they move far from where they were created, within spiral arms. Other stars live

longer lives and will disperse throughout the Galaxy before dying.

13. What is the evidence that there is a supermassive black hole at the center of our Galaxy?

How is it possible to determine the mass of this black hole?

Answer:

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Observations of the center of our Galaxy show that it is crowded with stars, suggesting that

something massive prevents them from dispersing. Observations of individual stars show that

some of them have tight orbits around an unseen object and are moving at very high speeds.Only something small and very massive could have this affect on the stars. The only possible

object that fits this description is a supermassive black hole. In addition, variations in X-ray

emission from the center of the Galaxy suggest that this supermassive black hole is surroundedby an accretion disk on the order of a few AU in size. By measuring the periods of orbits of thestars orbiting this object as well as their average distances from it, astronomers have determined

that it has a mass of 3.7 million solar masses.

Homework Chapter 14

4. What is the Hubble classification scheme? Which category includes the largest galaxies?Which includes the smallest? Which category of galaxy is the most common?

Answer:Hubble classification scheme is a method of organizing the various types of galaxies based on

their morphology. The largest galaxies ever observed are giant elliptical galaxies; the smallest

ever observed are dwarf elliptical galaxies. Visually, it appears as if spiral galaxies are the most

common, but looking in our own local group of galaxies, we observe mostly dwarf ellipticals. Itcould be that these are the most common type of galaxy but are just too small and too dim to be

seen at large distances.

5. Which is more likely to have a blue color, a spiral galaxy or an elliptical galaxy? Explain why.

Answer:Spiral galaxies are typically blue or white in appearance while elliptical galaxies are typically red

or yellow. This is because spiral galaxies contain lots of gas and dust and therefore still have

ongoing star formation. When new stars form, they form luminous, blue O and B stars. Thesestars are very short lived and will die quickly. Therefore, they are only observed in galaxies

where new star formation occurs. Since elliptical galaxies have little gas or dust, they dont

make new stars, which means that they dont have any blue O and B stars.

6. Why are Type Ia supernovae useful for finding the distances to very remote galaxies?

Answer:

Type Ia supernovae are useful as standard candles. A standard candle is any object with a well-

defined luminosity that occurs frequently enough to be observed. Type Ia supernovae occur

when a white dwarf in a binary system explodes. They are very luminous and can be seen atlarge distances. While not all Type Ia supernovae have the same luminosity, there is a

relationship between their maximum luminosity and how quickly they fade away, allowing us to

determine their maximum luminosity. As long as a Type Ia supernova can be observed in agalaxy, it can be used as a distance indicator.

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10. Why do collisions between galaxies result in lots of starformation?

Answer:

When galaxies collide, the clouds of gas and dust within them can slam into one another, causing

them to collapse and form new stars.

12. What is gravitational lensing?

Answer:

Gravitational lensing occurs when a massive object lies directly in front of a background object,

distorting the light from the background object. Matter has the property that it can distort space

around it. As beams of light from the background object approach the lensing object, its gravitycauses different beams to curve around it. This has the effect of amplifying the amount of light

we see from the background object. It is very similar to how a magnifying lens works. We dont

normally see this effect on Earth because it takes a lot of mass to cause any amount ofgravitational lensing, and we simply dont encounter objectsthat massive in our everyday lives.