Galaxies

Galaxies

• Galaxies result from the accumulation of gas on to the proto-galaxies, which were formed by density fluctuations and gravity instabilities in the expanding primordial fireball.

• The proto-galaxies appear to have formed on the ‘surfaces’ of ‘bubbles’, each about 100 million light years in diameter, and in the process the centres of the bubbles became virtually devoid of matter.

There are estimated to be between 100 billion and a trillion (1011 to 1012) galaxies, each containing about 100 billion stars, so the number of stars is between 1022 and 1023. Hubble classified galaxies into three types: elliptical, spiral and irregular. Lenticular (lensshaped) galaxies are intermediate between the elliptical and spiral galaxies. The centres of regular-shaped galaxies are believed to contain massive black holes with masses millions to billions times that of the Sun.