# 177 Galaxies, Distance, and Age

There are a number of characteristics which galactic shapes, brightness, outputs, and specifics can tell us about galaxies.Galactic distances are more difficult to determine for the more-distant galaxies. Methods like radar and parallax are of no use. So astronomers have developed a standard galactic brightness ; there is a relationship between a spiral’s luminosity and how fast it rotates. Called the Tully-Fisher Relationship , the faster a galaxy spins, the brighter the galaxy. So once the spin rate is determined, the brightness and then distance can be determined.

Another method is to examine a galaxy’s white dwarf supernovae luminosities. This provides another “standard” brightness with which we can compare.

### Galactic Distance and Position versus Galactic Age

Most of the galaxies are moving away from each other; each with a velocity of V. This infers that the galaxies must have been closer together at one time. Recall that Hubble’s Law is a relationship between velocities and distances, and is related to expansion through the Hubble Constant, H .

This goes back to rearranging Hubble’s Law V = H to d = v/H and allows us to infer not only the age of galaxies, but the Universe itself, time. We have a good idea at the value of . So let’s say we can determine how far galaxies are away at varying distances, . We already know the velocity, , from determining the distance; this is the velocity each galaxy is moving. Since you know the velocity and distance, use a simple formula and you can now calculate the time.