Astro Physisc HW

Homework #9

Due date: Tuesday, May 2nd

1 ) If the sun were only 4:0 kpc from the galactic center, how long would it take us to orbit once around the galaxy? Ignore any effects due to dark matter.

Hint: Remember, as we saw in class, that the stellar mass density is not uniform. You will need to account for this as well.

2 )

M31 is another name for the nearby Andromeda galaxy.

3 )

4 )

You do not need to discuss sources of error.

5 ) As observed from earth, a certain galaxy has a redshift z = 2:5.

How long did it take for the light from this galaxy to reach us?

Assuming the Hubble constant is actually a constant, how big was the universe when the light left this galaxy? You should express your answer as a fraction of the present size of the universe.