Typical stars are balls of hot glowing gas which vary enormously in mass, size, temperature (or colour) and brightness. Stars are born within the nebulae of galaxies in much the same way as galaxies themselves, from gravitationally collapsing pockets of hydrogen and helium and any other interstellar material that happens to be around. As this process continues, the core temperatures of newly forming stars steadily increase until they glow and eventually shine as hydrogen undergoes nuclear fusion forming yet more helium and releasing vast amounts of energy. At this point, the point of hydrogen burning, stars are said to join the Main Sequence. The life cycle of Main Sequence stars is determined by their mass:
|High mass stars||
High mass stars (stars with masses greater than three times the mass of the Sun) are the largest, hottest and brightest Main Sequence stars and blue, blue-white or white in colour. High mass stars use up their hydrogen fuel very rapidly and consequently have short lives. High mass stars pass through a Red Supergiant stage before dying catastrophically in supernovae explosions. Remnant stellar cores which survive supernovae can be sufficiently dense to form neutron stars (a star composed almost entirely of neutrons about 10 to 20 kilometres in diameter), pulsars (rapidly rotating neutron stars) or black holes (objects smaller than neutron stars with a gravitational attraction so strong that not even light can escape). Supernovae events are important, if rare, within galaxies. All elements heavier than hydrogen and helium are produced within stars at different stages in their life cycles. Supernovae release these elements into space making them available to be incorporated within later generations of stars.
|Low mass stars||
Low mass stars (stars with masses less than half the mass of the Sun) are the smallest, coolest and dimmest Main Sequence stars and orange, red or brown in colour. Low mass stars use up their hydrogen fuel very slowly and consequently have long lives. Low mass stars simply die by burning up their fuel to leave behind white dwarfs (contracted low mass stars about the size of the Earth) which themselves cool and contract further to black dwarfs.
|Stars of intermediate mass follow a life cycle somewhere between the two other extremes described. Our nearest star, the Sun, is a pretty good example of an intermediate mass star and its life cycle is likely to be typical of others. Astronomers estimate the Sun to be about 4.6 billion years old and middle aged in stellar terms. In another 4.6 billion years or so the Sun will use up the rest of its hydrogen fuel, its core will collapse, its core temperature rising by a corresponding 100 million degrees. Under these conditions helium will begin to burn producing heavy elements such as carbon and oxygen. At the same time the Suns outer layers will expand and cool to only a few thousand degrees. The Sun will enter old age as a Red Giant (not to be confused with a Red Supergiant), its expansion engulfing the innermost planets including the Earth. After a few million years or so it will shed its outer layers of gas leaving behind a small, dense, hot white dwarf. Eventually what is left of the Sun will cool, fade and die|