Galaxies are swarms of stars isolated from other stars and held together by
their own gravity. They come in a variety of sizes. Small galaxies may contain only a few million
stars but large ones may contain 1012 (a trillion) stars.
Within a galaxy, each star follows its own orbit, held there by the
gravitational attraction of the other stars in the system.
In addition to their stars, most galaxies also contain clouds of gas and dust. The dust in
such clouds absorbs light and you can see such clouds silhouetted against the background
of stars in many galaxy pictures.
1.Do we live in a galaxy?
2. What is the name of our galaxy?
Up until the early 1900's, many astronomers thought the Milky Way was the only galaxy.
They knew of dim, fuzzy patches of light in deep space, but most astronomers thought
them to be satellites of the Milky Way.
3. What made astronomers believe these fuzzy patches were other galaxies?
4. How did astronomers first measure the distance to galaxies?
Measurements of nearby galaxies showed that they are typically about a
million or so light years apart. Today, astronomers have seen galaxies as far
away as 12 billion light years.
Studies of galaxies soon showed that most were moving away from the Milky Way.
5. How did astronomers discover that galaxies are mostly moving away from the
6. What else was soon discovered about their motion?
A galaxy's distance, D, is related to its speed, V, by V = HD, where H is called the
Hubble constant. If V is measured in km/sec and D is measured in millions of parsecs
(megaparsecs, abbreviated mpc), then H is about 65 (km/sec)/mpc.
Note: 1 megaparsec is about 3 million light years.
Astronomers are still unsure about the value of H. It is very difficult to measure
The above relation, called Hubble's Law, allows us to find the distance to a galaxy if we
know its recession speed, or vice-versa.
For example, suppose a galaxy is 5 mpc away from us. How fast is it moving according
to the Hubble law?
Given that V = HD and that D = 5 mpc, then V = 65 (km/sec)/mpc x 5mpc =
8. If a galaxy is 15 mpc away from us, what is its speed according to the Hubble
The Hubble Law is extremely important to astronomers because it allows them to find a
galaxy's distance if they know its recession speed.
9. How can the recession speed be found?
Galaxies come in a wide variety of shapes, but most have one of three main
shapes: spirals, ellipticals, and irregulars.
11. What type of galaxy is the Milky Way ?
Spiral galaxies themselves divide into two main groups: ordinary and barred
spirals. In ordinary spirals, the arms emerge from a small round-ish core.
In barred spirals, the arms emerge from the end of an elongated "bar." Many astronomers believe the Milky Way is actually a barred spiral.
12. How do astronomers measure the mass of a galaxy?
13. Which of the following laws is used to find a galaxy's mass?
A galaxy's mass affects the orbital motion of its stars and gas because its mass determines
its gravity and the gravity in turn controls the motion of the stars and gas. Therefore,
astronomers can work backwards from the motions to get the galaxy's mass.
The law they use states that for a star orbiting with a period P in years at a distance a in
AU from the center of a galaxy, the galaxy's mass, M(in solar masses) is given by
Note that in using this expression, we normally have to express the star's distance from
the galaxy's center in AU, NOT parsecs or light years. If we have the distance in parsecs,
it is easy to convert to AU by simply multiplying by 2x105, the number of
AU in a parsec.
For example, suppose a star is orbiting a galaxy 10,000 pc from the center
with a period of 150 million years. What is the galaxy's mass?
To use the modified form of Kepler's third law, we first express a in AU. If
a=10,000 pc, then a is 10,000x2x105 = 2x109 AU.
Now plugging into our expression, we find
16=3.6x1011 Solar Masses.
Measurements of a galaxy's mass such as we have described above often disagree
strongly with the mass deduced from the light of the visible stars and gas.
15. his discrepancy has led astronomers to deduce that__________
16. What have astronomers proposed dark matter might be?
Astronomers understand only approximately how galaxies form and what
determines their shape (spiral, elliptical, or irregular). The basic picture of galaxy formation is
similar to a scaled-up version of star formation. That is, galaxies form when gravity
causes a huge gas cloud to collapse not long after the birth of the Universe. As the cloud
shrank in size and its density increased, stars began to form in it.
Because the cloud was so huge and its gas so tenuous, it took tens of millions of years to
collapse. In such a long time, some of the stars that formed in the cloud evolved to the
end of their life and exploded as supernovas. Although the cloud originally contained
only hydrogen and helium, the supernova blasts mixed the heavy elements made in these
stars into the gas, enriching it with the elements from which Earth and we ourselves