Galaxies:An Important Part Of Universe
. 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
STARS Stars are accretions of gas that radiate energy through nuclear fusion reactions. Stars form from the gravitational contraction of a cloud of gas and dust with a central core forming rapidly and the remainder of the surrounding cloud falling onto the core to form the star. The gravitational collapse continues until the core is both hot and dense enough to initiate the nuclear fusion of hydrogen to helium. The vast amount of energy produced counteracts the gravitational collapse and the star achieves a state of equilibrium which can last up to 10,000 million years in the case of a star the size of the Sun, but only about one million years for a very massive star. Eventually a critical amount of hydrogen will have been used up, nuclear fusion will cease, and the core will again start to contract. This releases gravitational energy, resulting in fusion reactions in the hydrogen envelope which surrounds the core. This ‘shell’ swells up and, because of the reduced temperature, glows red rather than white and the star becomes a ‘red giant’.
WHITE DWARFS
For stars about the size of our Sun, core collapse continues until the electron fields surrounding the nuclei of the atom become compressed and the core density reaches about 100,000 times the density of the Earth. Such a core may then be the size of the Earth, but a mass equal to that of the Sun.
BLACK HOLES
For stars in excess of 50 solar masses, the core collapse is so extreme that the whole mass may collapse into a single point. The intense concentration of mass causes a distortion of local space-time to such an extent that even radiation (i.e. light) cannot escape from the ‘sphere of influence’, which theoretically extends to a radius of 29.5 km for a ten solar mass core, and would therefore appear as a ‘black hole’ with a diameter of 59 km.
