The thermonuclear reactions in the core of a protostar cause pressure -- the star wants to fly apart. However, gravity keeps it from flinging its guts into space. This constant interplay between radiation pressure and gravitation dictates the course of a star's life -- and, eventually, the star's end.

The protostar's accretion disk sometimes buds off into small vortices of condensing matter, away from the tumult of the stellar furnace. These protoplanets eventually become the other bodies orbiting in the system. [footnote 2]

The protostar, when it finally "turns on," pushes the gas and dust in its vicinity away. This clears the system out, and leaves the protoplanets orbiting around their young sun. [footnote 3]

After a few million years the system begins to mature. The protostar, which might have been wildly variable -- changing in size and luminosity -- begins to settle down. The protoplanets begin forming their more recognizable cousins, the planets, asteroids, comets, and other interplanetary bodies. The star has now become a main-sequence star, the most populous of all stars in the galaxy. [footnote 4]

The course of a star's evolution is dictated almost entirely by its initial mass. The lower the mass of a star, the longer it will live, and the less spectacularly it will die. The higher the mass of the star, the shorter its life, but the more violent the death. Red dwarfs, the coolest stars, are very small, but can live for hundreds of billions of years -- while blue main-sequence stars, some of the hottest, can burn themselves out in only a hundred million years or so. Our Sun, a yellow main-sequence dwarf, somewhere in between, will live ten billion years, from birth to death.

When a star begins to age, it wanders off the main-sequence. It begins to run out of hydrogen in its core, and the star starts to quiver. It is now a subgiant. It begins to swell, and starts becoming unstable. The reactions in the core are beginning to change, and so the balance between expansion and gravitation becomes upset. At this point in its life, the star can again become wildly unstable and begin to pulsate, change brightness, or even change color.

After a time, the hydrogen in the core runs out almost completely, and it collapses. New reactions, such as helium-to-carbon, begin to take place in the core. These reactions liberate more energy than do the hydrogen-to-helium reactions, and so the star expands. When a star roughly the mass of our Sun or below comes to this stage, it is a red giant. Its radius has increased many times -- so much so that our Sun, when it eventually reaches this stage in five billion years or so, will engulf the inner planets, out to Mars.

As red giants remain in this stage, their cores, as they run out of their new fuel sources, switch to others. Helium-to-carbon, carbon-to-oxygen, oxygen-to-silicon, and so on. [footnote 5] New elements are formed in the cores of stars all the way up to iron. But iron is the limit to nuclear reactions in stars -- iron is too heavy to be fused into higher elements with a gain of energy. The giant has reached its evolutionary dead-end. When a giant reaches the end of its life, and the reactions proceeding in its core are delivering a very large amount of energy indeed, it begins to shed its outer layers, since gravity is not strong enough to hold onto them anymore. These layers are flung into space, leaving a fiery core. Such an object is called a planetary nebula. [footnote 6]

Eventually, when the core runs out of fuel and has converted the majority of its mass into heavy elements, such as iron, it collapses -- there is insufficient outward pressure from thermonuclear reactions with which to keep gravity at bay. It collapses until electron pressure -- the electrical repulsive force between electrons, halts the collapse. Such an object is very dense, and is called a white dwarf (or degenerate star). It is the mass of the Sun compressed into a sphere the size of the earth. [footnote 7] Eventually, the white dwarf cools, venting its energy into space, until it no longer shines. This dead star is called a brown dwarf, or an exhausted star. Such a fate eventually awaits our Sun.

More massive stars, however, do not have such a kindly death. When a star is quite a bit more massive than the Sun, then the powerful reactions in its core turn it into a supergiant -- some supergiants have radii corresponding to Jupiter's, Saturn's, or even out to Neptune's orbit. Supergiant stars are extremely luminous, and have tremendous surface areas. When a supergiant runs out of its fuel (again, iron is the limit), then it begins to quake. Supergiant stars, occasionally, undergo tremendous change. Their cores, with no fuel left to sustain reactions, collapse violently. This collapse triggers a tremendous explosion, called a supernova. Supernovae are so brilliant that they can outshine an entire galaxy. [footnote 8] The outer layers of the star are thrown off violently, and the remaining core has two possible fates.

If the star was only moderately more massive than our Sun, then electron pressure in its core is not strong enough to halt the collapse. The core continues to collapse, plunging in on itself, until the protons and electrons in its core are forced together so violently that they are transformed into neutrons. Thus, when almost all of the core of the star is transformed into neutrons, the collapse is halted by this new pressure. Such an object is called a neutron star. It is the mass of several Suns compressed into the size of the center of a city -- only 10 kilometers in diameter or so. Neutron stars are made up of stuff called neutronium, which is the densest matter known. [footnote 9] The neutron star, still ringing from its formation, slowly cools, and dies. [footnote 10]

However, if a star is quite a bit more massive than our Sun, even the pressure from the neutrons squeezed together in its core is not enough to stop the contraction. Such an object falls in on itself, until the escape velocity of its surface exceeds that of light itself. The object disappears from view in our universe, and is now a black hole, or collapsar. Black holes are so dense that not even light can escape -- hence the reason they are called black.

New study has shown that even black holes can die. A black hole emits radiation like any hot body. As it does, it loses mass, and eventually it dies in a puff of photons and exotic particles. [footnote 11]

It is interesting to note that the early universe, according to cosmologists, was composed of entirely hydrogen and helium. All of the other elements -- the carbon in our bodies, the oxygen in our atmosphere, the silicon in the rocks -- were formed in the process of evolution of the stars. The elements up to iron were formed in the cores of massive stars, and all of the other elements -- those higher than iron -- were formed in supernovae. [footnote 12] The supernova explosion is so powerful that it forces atoms together to fuse into very high elements indeed -- making silver, gold, uranium, and all of the other heavy elements.

We are all undeniably tied to the history of the universe.