GALAXIES- DISTANCES

The Distances "Ladder" to Galaxies

Measuring the distances to galaxies is a tricky business. We use different methods for different ranges of distances. This can be tricky because each step going out in distance depends upon how well we know the previous steps. The basic methods are (in order of increasing distance)...

Radar ranging is used for getting distances in the solar system. Stellar parallax is good for distances arround the solar neighborhood of the galaxy (a very tiny spherical region around the sun). Spectroscopic parallax is good for getting distance on our side of the Milky Way galaxy, almost all the way to the bulge. Variable stars are good for getting distance throughout our local "suburbs" of the Virgo Cluster. Tully-Fisher and Standard Candle methods are good for getting distance out to about 6% of the size of the universe. Hubble's Law is used for all distances beyond these.

Hubble's Law

In the 1920s, Edwin Hubble realized that the further a galaxy is away from us, the faster it is moving away from us. Thus there is a relationship between distance to a galaxy and the speed at which it is receding. So, we measure the velocity of a galaxy using the Doppler Effect (in a spectrum of the galaxy), and then we can get the galaxy's distance. As we will discuss later in the class, Hubble's Law implies that space itself, and therefore the universe, is expanding.