General Astronomy/The Life of High Mass Stars

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A red giant is a terminal stage for most stars. When the star starts to run out of available nuclear fuel, it can no longer completely oppose the inward pull of gravity and shrinks. However, the shrinkage increases pressures in the core, which now more compact rises in temperature. Meanwhile the increased core temperature pushes the remaining gas of the star outward into a large gaseous atmosphere. Interestingly, although the outer layers have expanded, they cool and become red, forming a red giant. A red giant star whose core has reached a temperature of 100 million Kelvins, but whose outer layers are much cooler. Red giants are truly enormous. One with the mass of our Sun may extend outward past the orbit of the Earth.

Red giants last perhaps a 100 million years or so, by which time it again begins to run out of available nuclear fuel. This is followed by a new collapse. What happens now is dependent on the mass of the core. If it is less than 1.44 times the mass of the Sun, the collapse crushes the core further into a white dwarf, and since the white dwarf cannot contract any further due to quantum mechanical reasons, additional material falling onto it rebounds and are blow off into space to form a large and often beautiful planetary nebula.

However, if the star is significantly more massive than our Sun, and its core is more than 1.44 times the mass of the Sun, the final collapse is great enough to crush even the Earth-sized white dwarf, pushing electrons into protons to form neutrons and liberate vast amounts of space. Thus it crushes down to about the size of a city and becomes an almost unimaginably compact and dense object known as a neutron star.

A pulsar is a rapidly spinning neutron star. Other stars spinning this fast would explode, but the pulsar does not because of its high density. Pulsars have a powerful electric field, throwing off electrons. These stars get their name from the signal pulses thrown off by their rotation.

If the original star was much more massive than the Sun and its core more than 3 times the Sun's mass, then the force of gravity crushing it at this stage is so great that not even the neutrons can withstand it, and they too are crushed to become perhaps Nature's strangest creation, the black hole.