13.5: Galaxies, Distance, and Age

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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 _o.

This goes back to rearranging Hubble’s Law V = H _oto d = v/H _oand allows us to infer not only the age of galaxies, but the Universe itself, time. We have a good idea at the value of H _o. So let’s say we can determine how far galaxies are away at varying distances, d . We already know the velocity, V , 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.

V = d/t t = d/VWhere: V is velocity | d is distance | t is time.

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