Possibly the most fascinating aspect of astronomy is its intimate connection with origins. As we have seen, the mass distribution due to the stars in a galaxy has remained virtually unchanged since the time the galaxy formed. Hence, the structure of galaxies should give us some clues about their origin.

The formation of our Galaxy must have taken place some ten billion years ago. This estimate follows from three independent age determinations. Firstly, the chemical composition of the Galaxy, and especially the relative abundances of certain radioactive elements, has been used as an atomic clock. Secondly, the theory of stellar evolution enables us to determine the ages of the stars. Thirdly, observers have found that the Universe expands; calculating backwards in time by using the theory of cosmology , we can demonstrate that there was a time when the Universe was so hot and dense that no galaxies, nor even stars, could exist. These three clocks give roughly the same readings. How, then, did galaxies form ?

The answer to this intriguing question is not known, but certain aspects of galaxy formation have become clearer in recent years. Cosmology tolls us that no objects as dense as stars could have existed in the Universe until it had cooled by expansion to a temperature below 10000K. From this we conclude that a galaxy formed from a more or less uniform gas cloud. At the time the critical temperature (10000K) was reached, the cosmic gas consisted almost entirely of hydrogen (75 per cent of the total mass) and of helium (25 per cent). The hydrogen, which until then had been ionized, cooled during further expansion and finally re-combined at 3000 K to form neutral hydrogen. The Universe had already expanded so far, that only gravity remained important to govern the motions of the gas on the large scales involved in galaxy formation. Consequently, a galaxy must have formed from a gas cloud collapsing under its own gravitational attraction.

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