Stars

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.