Astro 250: Astronomy Bizarre
Spring 2014
Assignment #3
Due: Monday, March 31, 2014
Name: ________________________________
- List two distinct properties of the Cosmic Microwave Background radiation
that have been measured, and describe what we believe those measurements tell
us about the origin and early evolution of our Universe.
-
Using Kepler's third law, adapted by Newton, we can use a measured
orbital velocity (v) at a distance (r)
from the center of a galaxy to compute the mass
of the galaxy (M) contained within that radius
r. The formula is:
v = 0.119 km/s
√(M/r)
where v
is the orbital velocity in km/s, r
is the radius of the orbit in light years, and M
is the mass of the galaxy (in solar masses) contained within the orbit.
-
Observations show that objects in the Milky Way galaxy, and in many other
spiral galaxies, revolve about the galactic center at the same velocities,
regardless of distance from the center, beyond about 20,000 light years from
the center. However, the light from these galaxies is concentrated in the
inner 20,000 light years.
The Sun lies 30,000 light years from the center of the Milky Way, and
moves at 220 km/s in its orbit.
- Compute the mass of the Galaxy contained
within the Sun's orbit using Kepler's third law in the above form, and the
numbers given for the velocity and radius of the Sun's orbit.
-
Several different objects in our galaxy have orbits far beyond the Sun's
orbit. For example, a globular cluster lies 6 times farther away than the Sun
from the center of the Milky Way but has the same orbital velocity as the
Sun. Use Kepler's law to compute M, the mass of the Galaxy
(in solar masses)
that lies within the orbit of the globular cluster. How many times larger is
this mass than that found within the Sun's orbit?
- In the galaxy M87, astronomers have measured the rotation
velocity near its
center with the Hubble Space Telescope. They find a rotation
velocity of 550 km/s
(see
the HST press release on M87 for details) at a distance of 60 light years
from the center. Again, using the above version of Kepler's 3rd Law, compute the mass of the galaxy that lies within 60 light years of the
center. How does your answer compare to that found by the Hubble
astronomers?
- In our discussions of dark matter in cosmology, we will deduce that much of this dark matter, while gravitating, cannot
be made of ordinary "baryonic" matter. What lines of evidence lead to that startling conclusion?