Wednesday, 8 July 2009

Distance Scale

Distance Scale:
The most important value for an astronomical object is its distance from the Earth. Since
cosmology deals with objects larger and brighter than our Sun or solar system, it is impossible to
have the correct frame of reference with respect to their size and luminosity as there is nothing to
compare extragalactic objects with.

Before the 1920's, it was thought that galaxies were in fact objects within our own Galaxy,
possibly regions forming individual stars. They were given the name ``nebula'', which we now
use to denote regions of gas and dust within galaxies.
At the turn of the century Cepheid variable stars, a special class of pulsating stars that exhibit a
particular period-luminosity relation, were discovered. In other words, it was found that their
intrinsic brightness was proportional to their period of variation and, hence, could be used for
measuring the distances to nearby galaxies.
In the late 1920's, Hubble discovered similar Cepheid stars in neighboring galaxies as was found
in our own Galaxy. Since they followed the same period-luminosity relation, and they were very
faint, then this implied that the neighboring galaxies were very far away. This proved that spiral
`nebula' were, in fact, external to our own Galaxy and sudden the Universe was vast in space and
Although Hubble showed that spiral nebula were external to our Galaxy, his estimate of their
distances was off by a factor of 6. This was due to the fact that the calibration to Cepheids was
poor at the time, combined with the primitive telescopes Hubble used.
Modern efforts to obtain an estimate of Hubble's constant, the expansion rate of the Universe,
find it necessary to determine the distance and the velocities of a large sample of galaxies. The
hardest step in this process is the construct of the distance scale for galaxies, a method of
determining the true distance to a particular galaxy using some property or characteristic that is
visible over a range of galaxies types and distance.
The determination of the distance scale begins with the construction of ladder of primary,
secondary and tertiary calibrators in the search for a standard candle.

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