Friday March 5th 1999
Announcements:

Lecture notes:

Class 24

As usual it is very important to be able to measure distances on very large scales - recurring problem for astronomers.

We have talked about several methods astronomers use to measure distances to cosmic objects.
- parallax
- spectroscopic parallax
- RR Lyrae variable stars
- Cepheid variable stars

However not even Cepheid variable stars can be used to probe quite distant galaxies.  Even with HST Cepheid variables can
only be used to about 15 Mpc - not bright enough.  For comparison the most distant quasar known has a distance of about
40 000 Mpc.

Makes it clear we need other methods.  Again we rely on standard methods.  Need luminosity and apparent brightness to get
distance.
- P-L relation gave us Cepheid L and let us make progress.
- We rely on other tricks for even larger distances.
- can figure out brightest globular clusters in nearby galaxies and compare to more distant ones.  Globular clusters are quite luminous so
can work further away but are assuming brightest globular clusters in all galaxies about the same luminosity.
 

Supernova explosions.
- type 1a from WD collapse assume have constant peak luminosities.
- entire galaxies
    - spiral galaxies have luminosites proportional to rotationrate.
    - measure rotation rate from line broadening can't spatially resolve galaxy.
    - Tully Fischer out to about 200 Mpc which is 650 million year light travel time.

Now astronomers have one final trick for estimating distances.

Based on an extremely important relation discovered by Hubble in 1929.  Hubble noticed that the spectral lines of distant galaxies were always shifted to the red.  Using the Doppler shift this implied that galaxies were receding from us

Hubble made a plot which is shown in updated form here.
 

The Hubble Law
V = H₀D
v = redshift velocity in Km/s
d = distance in MPC
H₀=the Hubble constant = 70 km/(Mpc * s)

Interpreted as expanding Universe!
space is expanding and this is why distant galaxies are flying away from us.
Balloon analogy to explain why more distant galaxies fly away faster.  For big distances we can measure V to get D