Chap 25 and 26

Question 1

Explain why we see the Milky Way as a faint band of light stretching across the sky.

Answer 1

The galaxy has a disk shape and we are within the galaxy.

Question 2

Explain where in a spiral galaxy you would expect to find globular clusters, molecular clouds, and atomic hydrogen.

Answer 2

• Globular cluster: Halo
• Molecular clouds: Disk and spiral arms
• Atomic hydrogen: disk and spiral arms

Question 3

Describe several characteristics that distinguish population I stars from population II stars.

Answer 3

• Population I stars: younger, in disk, have higher metallicity
• Population II stars: older, found in halo globular clusters, have lower metallicity

Question 4

Describe the evidence indicating that a black hole may be at the center of our Galaxy.

Answer 4

The stars move faster and orbit around the center, and the x-rays from accretion disk

Question 5

Suppose the Milky Way was a band of light extending only halfway around the sky (that is, in a semicircle). What, then, would you conclude about the Sun’s location in the Galaxy? Give your reasoning.

Answer 5

The Sun would be closer to the edge of the galaxy and therefore not making one side of the Milky Way as luminous.

Question 6

The globular clusters revolve around the Galaxy in highly elliptical orbits. Where would you expect the clusters to spend most of their time? (Think of Kepler’s laws.) At any given time, would you expect most globular clusters to be moving at high or low speeds with respect to the center of the Galaxy? Why?

Answer 6

They will spend most of their time away from the center, since they are further from the center, they will be moving at slower speeds

Question 7

Consider the following five kinds of objects: open cluster, giant molecular cloud, globular cluster, group of O and B stars, and planetary nebulae.
1. Which occur only in spiral arms?
2. Which occur only in the parts of the Galaxy other than the spiral arms?
3. Which are thought to be very young?
4. Which are thought to be very old?
5. Which have the hottest stars?

Answer 7

1.) Open cluster, O and B, and giant molecular clouds
2.) Globular cluster and planetary nebulae
3.) Open cluster, O and B, and giant molecular clouds
4.) Globular cluster and planetary nebulae
5.) Open cluster, O and B, and giant molecular clouds

Question 8

The dwarf galaxy in Sagittarius is the one closest to the Milky Way, yet it was discovered only in 1994. Can you think of a reason it was not discovered earlier? (Hint: Think about what else is in its constellation.)

Answer 8

The black hole is obscuring our view

Question 9

The Sun orbits the center of the Galaxy in 225 million years at a distance of 26,000 light-years. Given that a3=(M1+M2)×P2,
where a is the semimajor axis and P is the orbital period, what is the mass of the Galaxy within the Sun’s orbit?

Question 10

The best evidence for a black hole at the center of the Galaxy also comes from the application of Kepler’s third law. Suppose a star at a distance of 20 light-hours from the center of the Galaxy has an orbital speed of 6200 km/s. How much mass must be located inside its orbit?

Answer 10

Use Kepler’s law

Question 11

Describe the main distinguishing features of spiral, elliptical, and irregular galaxies.

Answer 11

Spiral: Disk and spiral arms
Elliptical: No spiral arms or disk
Irregular: Randomly shaped and has no familiar structure

Question 12

Why did it take so long for the existence of other galaxies to be established?

Answer 12

Astronomers had no method of determining distance to objects so far away and could only see so far

Question 13

Explain what the mass-to-light ratio is and why it is smaller in spiral galaxies with regions of star formation than in elliptical galaxies.

Answer 13

A lower mass-to-light ratio indicates a greater proportion of bright, actively-burning/ larger stars, while a higher ratio suggests a larger fraction of less luminous or non-luminous matter like dark matter.

Question 14

What are the two best ways to measure the distance to a nearby spiral galaxy, and how would it be measured?

Answer 14

Cepheid variable and type 1a supernova

Question 15

What are the two best ways to measure the distance to a distant, isolated spiral galaxy, and how would it be measured?

Answer 15

Hubble constant, Tully- Fisher, and type 1a supernova

Question 16

What does it mean to say that the universe is expanding? What is expanding? For example, is your astronomy classroom expanding? Is the solar system? Why or why not

Answer 16

It means space is expanding, the classroom and solar system are too small to be considered expanding.

Question 17

Why does the disk of a spiral galaxy appear dark when viewed edge on

Answer 17

The dust blocks the view by absorbing visible light

Question 18

What causes the largest mass-to-light ratio: gas and dust, dark matter, or stars that have burnt out?

Answer 18

Dark matter can create the largest mass-to-light ratios, since it dominates the mass of large galaxies and galaxy clusters without adding to the total luminosity.

Question 19

When comparing two isolated spiral galaxies that have the same apparent brightness, but rotate at different rates, what can you say about their relative luminosity?

Answer 19

Applying the Tully-Fisher method, the faster spinning galaxy should be more massive and thus more luminous.

Question 20

If all distant galaxies are expanding away from us, does this mean we’re at the center of the universe

Answer 20

No, because all galaxies would experience the same, therefore none are the center.

Question 21

Is the Hubble constant actually constant?

Answer 21

No, it changes with time and gets faster

Question 22

According to Hubble’s law, what is the recessional velocity of a galaxy that is 108 light-years away from us? (Assume a Hubble constant of 22 km/s per million light-years.)

Question 23

A cluster of galaxies is observed to have a recessional velocity of 60,000 km/s. Find the distance to the cluster. (Assume a Hubble constant of 22 km/s per million light-years.)