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Chapter 16

AY 101 Chapter Notes - Chapter 16: Spiral Galaxy, Accretion Disk, Cosmic Distance Ladder

Course Code
AY 101
Raymond White

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The Big Picture
The picture could hardly get any bigger than it has in this chapter. Looking back through both space and
time, we have seen a wide variety of galaxies extending nearly to the limits of the observable universe. As
you look back, keep sight of these “big picture” ideas:
The universe is filled with galaxies that come in a variety of shapes and sizes. In order to learn the
histories of these galaxies, we must also consider how the universe itself has evolved through time.
Much of our current understanding of the structure and evolution of the universe is based on
measurements of distances to faraway galaxies. These measurements rely on a chain of techniques in
which each link in the chains builds upon the links that come before it.
It has been less than a century since Hubble first proved that the Milky Way is only one of billions of
galaxies in the universe. This discovery, and his subsequent discovery of the universe’s expansion,
provided the foundation on which modern cosmology has been built. Measurements of the rate of
expansion tell us that the universe began about 14 billion years ago.
Although we do not yet know the complete story of galaxy evolution, we are rapidly learning more.
Galaxies probably all began as protogalactic clouds, but they do not always evolve peacefully. Some
galaxies collide with their neighbors, often with dramatic results.
The tremendous energy outputs of quasars and other active galactic nuclei, including those of radio
galaxies, are probably powered by gas accreting onto supermassive black holes. The centers of many
present-day galaxies must still contain the supermassive black holes that once eneabled them to shine as
Summary of Key Concepts
What are the three major types of galaxies?
Spiral galaxies have prominent disks and spiral arms.
Elliptical galaxies are rounder and redder than spiral galaxies and contain less cool gas and dust.
Irregular galaxies are neither disk-like nor rounded in appearance.
How are galaxies grouped together?
Spiral galaxies tend to collect in groups that can contain several dozen galaxies.
Elliptical galaxies are more common in clusters of galaxies, which contain hundreds to thousands of
galaxies, all bound together by gravity.
How do we measure the distances to galaxies?
The chain of methods by which we measure galaxy distances begins with radar ranging in our own
solar system and parallax measurements of distances to nearby stars, then relies on standard candles to
measure greater distances. Important standard candles include Cepheid variable stars, which obey a
period-luminosity relation that allows us to determine their luminosities and distances, and white
dwarf supernovae that can be seen even at enormous distances.
What is Hubble’s law?
Hubble’s law tells us that more-distant galaxies are moving away faster: v = H0 x d, where H0 is
Hubble’s constant. It allows us to determine where a galaxy’s distance from the speed at which it is
moving away from us, which we can measure from the redshift of its spectrum.
How do distance measurements tell us the age of the universe?
The inverse of Hubble’s constant tells us how long it would have taken the universe to reach its present
size if the expansion rate had never changed; when combined with data on how it has changed with
time, we estimate the universe to be about 14 billion years old. A lookback time equal to this age
therefore marks our cosmological horizon, beyond which we cannot see.
How do we study galaxy evolution?
find more resources at oneclass.com
find more resources at oneclass.com
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