Exercises: Galaxies

Collaborative Group Activities

Throughout much of the last century, the 100-inch telescope on Mt. Wilson (completed in 1917) and the 200-inch telescope on Palomar Mountain (completed in 1948) were the only ones large enough to obtain spectra of faint galaxies. Only a handful of astronomers (all male—since, until the 1960s, women were not given time on these two telescopes) were allowed to use these facilities, and in general the observers did not compete with each other but worked on different problems. Now there are many other telescopes, and several different groups do often work on the same problem. For example, two different groups have independently developed the techniques for using supernovae to determine the distances to galaxies at high redshifts. Which approach do you think is better for the field of astronomy? Which is more cost effective? Why?
A distant relative, whom you invite to dinner so you can share all the exciting things you have learned in your astronomy class, says he does not believe that other galaxies are made up of stars. You come back to your group and ask them to help you respond. What kinds of measurements would you make to show that other galaxies are composed of stars?
Look at [link] with your group. What does the difference in color between the spiral arms and the bulge of Andromeda tell you about the difference in the types of stars that populate these two regions of the galaxy? Which side of the galaxy is closer to us? Why?
What is your reaction to reading about the discovery of the expanding universe? Discuss how the members of the group feel about a universe “in motion.” Einstein was not comfortable with the notion of a universe that had some overall movement to it, instead of being at rest. He put a kind of “fudge factor” into his equations of general relativity for the universe as a whole to keep it from moving (although later, hearing about Hubble and Humason’s work, he called it “the greatest blunder” he ever made). Do you share Einstein’s original sense that this is not the kind of universe you feel comfortable with? What do you think could have caused space to be expanding?
In science fiction, characters sometimes talk about visiting other galaxies. Discuss with your group how realistic this idea is. Even if we had fast spaceships (traveling close to the speed of light, the speed limit of the universe) how likely are we to be able to reach another galaxy? Why?
Despite his son’s fascination with astronomy in college, Edwin Hubble’s father did not want him to go into astronomy as a profession. He really wanted his son to be a lawyer and pushed him hard to learn the law when he won a fellowship to study abroad. Hubble eventually defied his father and went into astronomy, becoming, as you learned in this chapter, one of the most important astronomers of all time. His dad didn’t live to see his son’s remarkable achievements. Do you think he would have reconciled himself to his son’s career choice if he had? Do you or does anyone in your group or among your friends have to face a choice between the passion in your heart and what others want you to do? Discuss how people in college today are dealing with such choices.

Review Questions

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

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

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.

If we now realize dwarf ellipticals are the most common type of galaxy, why did they escape our notice for so long?

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

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

Why is Hubble’s law considered one of the most important discoveries in the history of astronomy?

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?

Was Hubble’s original estimate of the distance to the Andromeda galaxy correct? Explain.

Does an elliptical galaxy rotate like a spiral galaxy? Explain.

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

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

What is the most useful standard bulb method for determining distances to galaxies?

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?

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

Is the Hubble constant actually constant?

Thought Questions

Where might the gas and dust (if any) in an elliptical galaxy come from?

Why can we not determine distances to galaxies by the same method used to measure the parallaxes of stars?

Which is redder—a spiral galaxy or an elliptical galaxy?

Suppose the stars in an elliptical galaxy all formed within a few million years shortly after the universe began. Suppose these stars have a range of masses, just as the stars in our own galaxy do. How would the color of the elliptical change over the next several billion years? How would its luminosity change? Why?

Starting with the determination of the size of Earth, outline a sequence of steps necessary to obtain the distance to a remote cluster of galaxies. (Hint: Review the chapter on Celestial Distances.)

Suppose the Milky Way Galaxy were truly isolated and that no other galaxies existed within 100 million light-years. Suppose that galaxies were observed in larger numbers at distances greater than 100 million light-years. Why would it be more difficult to determine accurate distances to those galaxies than if there were also galaxies relatively close by?

Suppose you were Hubble and Humason, working on the distances and Doppler shifts of the galaxies. What sorts of things would you have to do to convince yourself (and others) that the relationship you were seeing between the two quantities was a real feature of the behavior of the universe? (For example, would data from two galaxies be enough to demonstrate Hubble’s law? Would data from just the nearest galaxies—in what astronomers call “the Local Group”—suffice?)

What does it mean if one elliptical galaxy has broader spectrum lines than another elliptical galaxy?

Based on your analysis of galaxies in Table 1 of Properties of Galaxies, is there a correlation between the population of stars and the quantity of gas or dust? Explain why this might be.

Can a higher mass-to-light ratio mean that there is gas and dust present in the system that is being analyzed?

Figuring for Yourself

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.)

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.)

Suppose we could measure the distance to a galaxy using one of the distance techniques listed in Table 1 of The Extragalactic Distance Scale and it turns out to be 200 million light-years. The galaxy’s redshift tells us its recessional velocity is 5000 km/s. What is the Hubble constant?

Calculate the mass-to-light ratio for a globular cluster with a luminosity of 10⁶L_Sun and 10⁵ stars. (Assume that the average mass of a star in such a cluster is 1 M_Sun.)

Calculate the mass-to-light ratio for a luminous star of 100 M_Sun having the luminosity of 10⁶L_Sun.