PHYS 202 Chapter 26

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

-spiral galaxies: spiral shaped; has a halo, bulge and a disk -elliptical galaxies: shaped as an ellipse or sphere; more older stars than newer -irregular galaxies: no regular shape; irregular gravitational force

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?

-that galaxies are moving away from each other -the solar system's size is dictated by the center of our galaxy

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.

-the ratio between the total mass of a galaxy and its total luminosity -elliptical galaxies have more older, massive stars that have already completed their evolution

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

1. Cepheid variables: uses the period-luminosity variable to find the distance 2. Tully-fisher relation: uses the 21-cm radiation to find the rotational rate which can be used to calculate the distance

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

1. Type Ia supernovae: compare the luminosity with the apparent brightness to determine the distance 2. Tully-fisher relation: uses the 21-cm radiation to find the rotational rate which can be used to calculate the distance

Hubble constant

a constant of proportionality in the law relating the velocities of remote galaxies to their distances

spiral galaxy

a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars, winding out from its central bulge

elliptical galaxy

a galaxy whose shape is an ellipse and that contains no conspicuous interstellar material

irregular galaxy

a galaxy without any clear symmetry or pattern; neither a spiral nor an elliptical galaxy

Hubble's law

a rule that the radial velocities of remote galaxies are proportional to their distances from us

type Ia supernova

a supernova formed by the explosion of a white dwarf in a binary system and reach a luminosity of about 4.5 × 109 LSun; can be used to determine distances to galaxies on a large scale

Which of the following statements about the different types (shapes) of galaxies is correct?

collisions and mergers between galaxies can sometimes change a galaxy's type (shape)

One of the main projects being carried out by the Hubble Space Telescope is to measure the distances of galaxies located in groups dozens of millions of lightyears away. What method do astronomers use with the Hubble to find such distances?

finding Cepheid variables and measuring their periods

What method would astronomers use to find the distance to a galaxy so far away that individual stars are impossible to make out (resolve)?

finding the redshift and using Hubble's Law

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

it was the first law to put forward the idea of an expanding universe, and it helps measure distances to remote galaxies

Is the Hubble constant actually constant?

no, the constant changes with the speed of the universe (acceleration or deceleration)

A graduate student in astronomy needs to measure the mass of a spiral galaxy she is studying for her PhD thesis. Which of the following observations would be important for her to make?

obtain the speed at which stars or gas near the outer regions of the galaxy are moving around

The new instrument that made it possible for Edwin Hubble to demonstrate the existence of other galaxies in the early 1920's was:

the 100-inch reflector on Mount Wilson

Edwin Hubble was able to show that (with the exception of our nearest neighbors) the farther a galaxy is from us, the

the faster it is moving away from us

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?

the faster the galaxy is rotating the more massive and luminous the galaxy

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

the galaxies were to far away to see with the technology available, up until now

mass-to-light ratio

the ratio of the total mass of a galaxy to its total luminosity, usually expressed in units of solar mass and solar luminosity; the mass-to-light ratio gives a rough indication of the types of stars contained within a galaxy and whether or not substantial quantities of dark matter are present

Among irregular galaxies, what makes the Large and Small Magellanic Cloud especially useful for astronomers?

they are (for galaxies) very close to us, so they are easy to study

The reason type Ia supernovae are useful to astronomers for determining distances to other galaxies is that

they are very bright, and generally reach the same peak luminosity

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

they are very faint and difficult to see

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

type ia supernovae

redshift

when lines in the spectra are displaced toward longer wavelengths (toward the red end of the visible spectrum)

Before you can use Hubble's Law to get the distance to a galaxy, what observation must you make of that galaxy?

you must take a spectrum of the galaxy and measure the red shift

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

No, because there are two kinds of cepheids and Hubble mixed them up

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

No, because we are also moving away from other galaxies

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

No, spiral galaxies rotate in a systematic way whereas elliptical galaxies do not

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

because they are very dusty

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

dark matter

Which type of galaxy is observed to contain mostly older stars?

elliptical