Outside+our+solar+system

Measuring the distance to the stars
A number of different methods have been used to measure the distance to the planets and the stars. New methods have been devised as we have learnt more about the Universe and the physical laws that covern it. Also as technology has improved, it has been possible to do more with older methods. For instance, the ancient Greeks thought that one arguement against the model of the Earth moving round the Sun was that there was no observable parallax to the position of the stars. However, as technology improved using the parallax to measuer the distance to nearby stars became possible.

In rough chronological order the following were the main systems were used:

Parallax Kepler's Laws Cepheid Variable stars Redshift Supernovae

Cepheid Variable Stars
Henrietta Leavitt (1868 - 1921) was an American astronomer. She worked at the Harvard COllege Observatory as a "computer". Studying photographic plates of stars led her to propose a groundbreaking theory:

She saw that some stars oscillated with particular regularity (like a sharks fin), but some had longer periods than others. She proposed that the period of the oscillation depended on its absolute magnitude (how bright it would be at a given distance) and not on its distance from the Earth. This ernabled her to callibrate the variation of the period with absolute magnitude. Then using such a "Cepheid variable star" that was close enough for its distance to be measured by parallax she was able to work out the distance to other Cepheid variable stars.

From this it was discovered that some of the visible Cepheids were over 10 million ly away, and this set the foundations for the measurement of redshift by Edwin Hubble.