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Astronomical distance

Astronomical distances are simply so huge as to defy the imagination. For instance, the distance to the nearest star besides the sun (Proxima Centauri), is about:

40,000,000,000,000 kilometers

For this reason astronomical distances make use of the speed of light to scale huge distances to comparatively small time periods. The distance to an astronomical body is often, therefore, referred to by the time it takes light to reach the body. For instance, the sun is 150 000 000 km away or 8 light minutes away. Alpha Centauri is 4.23 light years distant.

However even the vast light year is often too small a unit, the furthest known galaxies are at a distance of about 13,000,000,000 light years.

Another often used unit, on the same order of magnitude as the light year is the parsec. 1 parsec is approximately 3.26 light years.

However, for objects within our own solar system, the smaller unit of the astronomical unit (AU) is used.

Measuring distances

Distance measurements began with astronomers of the renaissance using parallax across the face of the earth to determine the distance to nearby heavenly bodies. This method was eventually applied to stars (see parallax). The distances to nearby stars so determined were recently radically improved by the Hipparcos mission.

However, the parallax method soon fails as distances grow larger and becomes practically impossible at distances greater than about 200 light years. But there is a way out of this dilemma. In the early 1900s an astronomer named Henrietta Leavitt examined the relationship between the absolute magnitude and period of the variable stars known as Cepheid variables. She discovered that the period of the stars is directly proportional to their absolute magnitude. So, by determining the periods of these stars and thus their absolute magnitudes and comparing these with their apparent magnitude, the distance to the star can be determined.

By calibrating this by using some Cepheid variable to which the distance is known by the parallax method, we can find the distances to vastly more distant objects.

This method can be extended another step. If a cepheid is discovered in another galaxy, then that galaxy's distance can be determined. If it is part of a cluster, then the approximate distance to all the galaxies in the cluster can be found. By Hubble's law it is thought that the velocity of recession of an object is proportional to its distance from us. The exact speed is determined by Hubble's constant, H0. This can now be found from the cepheid method and thus distances to the most distant reaches of the universe can be found.