Astro 201: Exercise: Hubble Law answers

For the following problems, assume that H₀ = 50 km/s/Mpc.

1. What method from chapter 5 does one use to get the velocity of a galaxy?

   A: The Doppler shift!

2. About how far away is a galaxy with velocity 7000 km/s?

   A: Rewrite the Hubble law d = v/H₀ and find d=7000/50 Mpc = 140 Mpc.

3. How fast is a galaxy moving away from us if it is 1000 Mpc distant?

   A: 50000 km/s.

4. The Andromeda galaxy is 0.77 Mpc away. Calculate its expected radial velocity. In fact, the Andromeda galaxy is coming toward us at about 200 km/s. Why? What is going on?

   Individual galaxies can be moving several hundred km/s under the influence of gravity from nearby galaxies or clusters of galaxies. To this extent, they do not participate in the "Hubble flow." The Hubble law is good for an average of lots of galaxies, but can be off by several 100 km/s for any one individual galaxy.