Galaxies are often found in groups called clusters of galaxies. Clusters with few
members (tens or so) are called "poor" and clusters with many members (hundreds or
more) are called "rich."
The Milky Way belongs to a cluster with about 40 members.
1.Does the Milky Way belong to a rich or poor cluster?
In the Local Group, the galaxy M31 is heading toward the Milky Way at about 60
km/sec. It is about 2.2 million light years away.
4. If it continues in a straight line at the same speed, how long until it hits the Milky
5. Will this collision pose a problem for you personally? Why?
6. What part of our discussion above might explain this result?
7. If the gas emits strongly at x-ray wavelengths, how hot is? Hint: Use Wien's law.
(The wavelength of the x-rays observed is about 1.0 nm.)
For example, let's calculate the escape velocity of a cluster of galaxies
whose radius is 2 Mpc and that contains 1000 member galaxies, each with a mass
of 1011 solar masses. What is the escape velocity for this cluster?
A few hints to help you solve this problem. First, convert the radius in Mpc
to a radius in meters. To do this, look up the number of meters in a parsec
and then multiply by the number of parsecs in a Mpc.
Now put the values for the mass and radius that you found into the escape velocity
formula, v=(2GM/R) 1/2. To save you needing to look it up, G is
Notice that with the units as given, the velocity you find will be in meters/sec. Thus,
you need to make one last conversion to get this in km/sec. To make that conversion,
recall that 1 km = 103 meters. Thus, divide your answer by 1000 to get
the escape velocity in km/sec.
9. What is the escape velocity for this cluster?
10. The escape velocity of the cluster is
This mass discrepancy led us to believe that the Milky Way (and other
galaxies) are embedded in a halo of dark matter. Astronomers conclude
similarly that galaxies clusters contain large amounts of dark matter.
Astronomers find additional evidence for dark matter in clusters because they
arcs of light within clusters (see figure below). Such arcs are the image of distant
galaxies bent and distorted by the mass within the galaxy cluster. That is, the gravity of
the cluster acts like a lens - a gravitational lens.
11. What are some signs of an active galaxy?
12. Which of the following is not an active galaxy?
13. What theory do astronomers think explains the activity in galaxies ?
14. How does the prevailing model for activity explain the hot, ejected gas seen in
QSO's are among the most distant objects we know.
15.How do we know they are so far away?
16. Given that QSO's are so far away, what can we conclude about their power output?