Homework 8
Due: Thursday, 30th May
Chapter 23 and 24
Show your work when required!
2. a) What does the term standard candle mean? (1 mark)
b) Give two examples of standard candles. (2 marks)
c) What science can be done using the two standard candles listed in b) (2 marks)?
3.) The amount of mass internal to a point in the galaxy is given by M = rv^2/G where G is the gravitational constant, M is the mass in the orbit, r is the radius of the orbit and v is the speed at which the star is rotating round the galaxy. At the position of the Sun, 8kpc, there is 1x10^11 M_sun of mass within the orbit of the sun. The sun is moving at 220 km/s.
a) If we move the sun to 16kpc, how much mass is within it's orbit if we still move at 220 km/s? (2 marks)
b) If we move the sun to 32kpc, how much mass is within it's orbit if we now move at 200 km/s? (2 marks)
c) We see few stars and gas between 16 and 32 kpc, yet you should find the mass in the orbit increases. What does this tell us? (1 mark)
4. a) Our Galaxy's neighbor, the Andromeda galaxy (M31), is about 800 kpc away. If there were only Hubble expansion what would be the speed of M31 as measured from Earth. Take the Hubble constant to be 70 km/s/Mpc. (2 marks)
b) In reality, M31 is moving towards us at a speed of 120 km/s due to the gravitational pull of the Milky Way. Assuming that M31's speed remains constant, how many years would it take for M31 to collide with the Milky Way? (Use 1kpc = 3 x 10^{16} km and 1 year = 3x10^7 seconds). (2 marks)
5. Describe two general differences between spiral and elliptical galaxies (shape in all it's form only counts as one difference). (4 marks)