AST 102 Chapter 23: Galaxies

The event that settled the Shapley-Curtis "debate" about the nature of spiral nebulae was the discovery of

Cepheid variable stars in the Andromeda Nebula.

Edwin Hubble determined that the Andromeda Galaxy was a star system in its own right outside the Milky Way by studying

Cepheid variables.

How does the composition of distant galaxy clusters compare with the composition of nearby galaxy clusters?

Distant clusters contain a greater percentage of spirals.

Very hot gas emitting X rays has been discovered filling the space between galaxies in clusters of galaxies. How has this gas affected the "missing mass" problem for these clusters?

It supplies about 10% of the "missing mass," leaving 90% still missing.

On the basis of galactic motions in the Local Group, what is likely to happen to the Milky Way within the next 4 billion years?

It will collide with the Andromeda Galaxy, triggering an intense burst of the birth of new stars in the gas and dust clouds of the two galaxies.

What conclusion can we draw from the observation that elliptical galaxies appear to contain little or no dust or gas?

Little if any star formation is taking place in these galaxies.

In the collision between two galaxies, why is it highly unlikely that stars from the galaxies will collide with one another?

Stars in a galaxy are very widely separated from each other, with lots of space between them.

The galaxy distribution map made by the Two-Degree Field Galaxy Redshift Survey resulted in two pie-shaped wedges. Why?

These regions were chosen to avoid the obscuring dust lanes in the plane of the Milky Way.

What is the general distribution of galaxies in the universe?

They are grouped into clusters and superclusters, which in turn are distributed around huge, apparently empty volumes of space.

The method used to find the distances to the very farthest galaxies is

Type Ia supernovae peak brightnesses.

Which of the following distance measurement techniques allows us to reach farthest out into the universe?

apparent magnitude measurements of type Ia supernovae

The key discovery about Cepheid variable stars that led in the 1920s to the resolution of the question of whether spiral "nebulae" were separate and distant galaxies or part of our Galaxy was the

direct relationship between their pulsation period and their absolute brightness or luminosity. A measurement of apparent brightness of a variable star could then be used to determine the distance to a "nebula" containing it.

The Tulley-Fisher relation is useful for finding the

distances to distant galaxies.

Hubble classified the galaxies he saw into

elliptical, spiral, and barred spiral galaxies.

Observations of very distant galaxies have shown that these galaxies are bluer than nearby galaxies of similar Hubble type. The bluer color indicates that

galaxies had a much greater star formation rate when they were very young than they do now.

The double images and short arcs of light that are found around some galaxies (examples of which are seen in Figures 23-32 and 23-33 of Universe, 10th ed.), are caused by

gravitational lensing by the galaxies of the light from a single, distant object very far behind the galaxy.

What gives rise to the extensive X-ray emission detected from many rich clusters of galaxies?

hot, intergalactic gas at temperatures of up to 100 million K

What is believed to have been the most important factor that determined whether a protogalaxy in the early universe evolved into a spiral or an elliptical galaxy?

initial rate of star formation

. Which two observed parameters of a Cepheid variable star are required in order to use it to determine the distance to a galaxy?

its average brightness or apparent magnitude and its period of variability

An Sa galaxy has a

large nuclear bulge and tightly wound spiral arms.

The Local Group, the cluster of galaxies to which our Milky Way Galaxy belongs, is a

poor, irregular cluster of three to four dozen galaxies.

"Standard candles," which are essential tools for measuring distances to remote galaxies, are

stars and other objects of known intrinsic brightness whose apparent brightness can then be used to find the distance.

In spiral galaxies, the relationship between nuclear bulge and tightness of spiral arms seems to be that

the larger the nuclear bulge of the galaxy, the tighter the spiral arms are wound together.

One result of the analysis of the properties of clusters of galaxies that is very surprising and difficult to account for is that

the mass of the easily visible matter (stars and so on) appears to be insufficient to hold these systems together gravitationally.

The Hubble flow is a general recession of distant clusters and superclusters of galaxies away from us such that

the more distant they are, the faster they are moving away from our Galaxy.

According to Hubble's law, if a very distant galaxy B is three times farther away from our Galaxy than another galaxy A, then galaxy B is receding from us ____ times as fast as Galaxy A.

three

The protogalaxies we observe when we look out 13 billion light years away display all of the following characteristics except one. Which is the exception?

widely separated in space