Galaxies

The idea that stars were not just scattered randomnly throughout space but were organised into large groups or galaxies was first suggested from observations of the Milky Way. The Milky Way is a faint, broad, continuous band of light which can only be seen on a clear, moonless night and when there is not too much light pollution from street lamps.

Galileo first noticed, via observations through a telescope, that the band of light was, in fact, a vast collection of very faint stars. Thomas Wright suggested, in 1750, that the continuous loop of the Milky Way could be explained if the solar system were part of a collection of stars shaped like convex lens, We see more stars when we look across the width of this galaxy than we do when looking across its thickness.

As this idea became accepted, astronomers naturally wanted to know how big the galaxy was and whereabouts in the galaxy the solar system was located. Detailed star counts in different sections of the Milky Way, made my Wiliam Herschel and his son John during the late 18th and early 19th century, gave the same results in all directions and suggested that the Sun must be quite close to the centre of the galaxy. Early in the 20th century, however, Harlow Shapley used Cepheid variables to measure the distances of the globular clusters of stars which surround the galaxy.

His measurements indicated not only that the galaxy was approximately 100,000 light years in diameter, but that the Sun was rather closer to the edge of the galaxy than to its centre. The conflicting evidence from star counts was later explained in terms of dust within the galaxy which obscured the more distant stars. Studies of the clouds of hydrogen gas during the 1950's, using radiation in the 21-centimetre radio wave-band, indicated that the Milky Way has a spiral rather than a lens-like structure.

In 1755, Immanuel Kent had developed Wright's idea of the Milky Way as a galaxy of stars and suggested that the luminous patches of light (nebulae) which can be seen in many parts of the night sky might also be galaxies of stars.

Astronomers disagreed about the idea and it was not until 1924 that the existence of separate galaxies was confirmed when Edwin Hubble, using an 100-inch telescope, discovered Cepheid variables in the Andromeda nebula and in other spiral clusters of stars. These turned out to be much further away than the stars in our own galaxy. Hubble had shown that these spiral clusters were indeed separate galaxies of stars.

Hubble was able to use Cepheid variables to measure the distances of many local galaxies, i.e. galaxies up to about 2.5 million light years away. When he calculated the absolute magnitudes of these galaxies, however, Hubble found that they were very similar: the differences in the magnitudes of the brightest and the dimmest galaxies was only about 2.5. Most galaxies are too far away to be able to pick out Cepheid variables but, by assuming that all this galaxies were of average brightness, Hubble was able to map out the Universe to a distance of 500 million light years, a region which covers about 100 million galaxies.

Hubble's measurements showed that although there are local variations in the Universe, so that the solar system, our own galaxy and our local group of galaxies all have their distinctive features, the Universe on a larger scale is very uniform. Just as the Earth's surface when seen from space no longer looks 'bumpy' but perfectly spherical, so the 'lumpiness' of the Universe is smoothed out when seen on a larger scale. The idea that the Universe is everywhere and in all directions essentially the same- i.e. that it is isotropic- is known as the cosmological principle.

Hubble's mapping of galaxies in effect completed the Copernican revolution. Far from being at the centre of the Universe as it was in the Ptolemaic model, the Earth is simply one of the planets of a star which is situated well away the centre of one of the hundreds of millions of galaxies in the Universe. Hubble's detailed study of galaxies also led to what is often regarded as the most important astronomical discovery of the 20th century, i.e that the Universe is not static but expanding.