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stellar lifetimes range from perhaps 40,000 years to longer than 10 billion
years, astronomers never get to watch a particular star go through all of
its life cycle phases. However, there are millions of stars to look
at around the galaxy, each one at a different point in its life cycle.
These images can be sequenced in a logical manner, based on a knowledge
People are in much the same situation. The human life cycle is about eighty years (infant, toddler, child, adolescent, teenager, adult, mature adult, senior citizen [images]). Most people can both identify the various stages of the human life cycle that observable all around us, and put those stages in sequence. Astronomers have the same task when classifying stars.
By looking at thousands of pictures and using some basic physics, astronomers have developed the sequence of a star's birth, life, and death (see Sequenced Star Life Cycle Page - massive and sun-like). The picture begins with a large cloud of interstellar gas and dust, mostly hydrogen. For reasons that astronomers don't completely understand, these clouds begin to collapse under their own gravity and heat up. These proto-stars eventually become dense enough to sustain nuclear reactions (hydrogen becomes helium) and a star is born.
A star spends the majority of its life cycle converting lighter elements into heavier elements (nuclear fusion) in a state of "equilibrium." The outward force of radiation emitted from the star's core is exactly balanced by its own gravity pulling inward. This balanced state is called hydrostatic equilibrium. Usually, astronomers describe stars in this equilibrium state as being main sequence stars.
Eventually, the interior of the star can't produce nuclear fusion reactions fast enough, and the star becomes unstable. It is then called a variable star. The star contracts and expands and eventually grows enormously large and cools off. This stage of the star is called a red giant (great NASA HST image of Betelgeuse).
Most stars in the red giant phase are in a dramatic disequilibrium. Finally, the outer atmosphere is cast off leaving a white hot stellar core behind. The outer atmosphere is called a planetary nebula and eventually disappears. It's not a planet, but it sometimes looks like one through a small telescope. The remaining tiny core is called a white dwarf. The white dwarf cools off very slowly (maybe over 5 billion years or more) to become a black dwarf, a cold lump of stellar coal made not of carbon, but mostly helium.
A few stars, however, have a much more dramatic ending. If they are sufficiently large, say more than five times the mass of an ordinary star like our Sun, the red giant phase collapses in an enormous implosion called a supernova. This results in a scattering of interstellar matter called a supernova remnant (image) and an exceedingly small stellar core so densely packed that protons and electrons are squeezed together to form neutrons. This is a neutron star. Even more interesting, if the star that collapses is enormous, say more than eight times the mass of our Sun, the supernova is so intense that a black hole, a tiny object with a huge gravity field, is created.
How long a star lives (and how it dies) depends entirely on how massive it is when it begins. A small star can sustain basic nuclear fusion for billions of years. Our sun, for example, probably can sustain reactions for some 10 billion years. Really big stars have to conduct nuclear fusion at an enormous rate to keep in hydrostatic equilibrium and quickly falter, sometimes as fast as 40,000 years.