All stars follow essentially the same life cycle, although they vary widely in terms of their size, mass, brightness, temperature, and in some small ways their chemical composition (referred to as metallicity). First, they form out of giant molecular clouds, amassing enough size that they undergo stellar ignition and begin to fuse hydrogen atoms into helium in their cores. Second, for a period ranging from millions to trillions of years depending on the size of the star, they undergo a relatively stable, unchanging lifetime, burning away their hydrogen fuel. Finally, when the hydrogen in their core is depleted, they die, in one of several spectacular ways.
- Stellar Formation -
Stars are theorized to form from giant molecular clouds, enormous masses of dust and hydrogen in the interstellar medium which are in turn the remnants behind from the explosions of previous generations of stars. Over time, this material is drawn inward into an ever-larger central mass, which grows in size, heat, and gravitational strength until the hydrogen in its centre is pushed together, fusing into helium. At that point a star is born: essentially, an enormous fusion reactor.
Stellar ignition cannot occur without enough hydrogen in this single central mass. If the cloud from which it forms is too small, or too dispersed, the resulting mass is called a brown dwarf: something far too large to be a planet, but not large enough to be a star. Brown dwarfs are theoretical objects only: because they are small and emit little light, astronomers have not yet been able to locate one.
- Normal Lifespan -
Left to its own devices (i.e. avoiding interference from companion stars, stellar remnants, etc.), stars spend most of their lifetime undergoing a stable and predictable process of nuclear fusion, referred to as the stellar "main sequence." How hot stars are, and how quickly they burn through their hydrogen reserves, is entirely dependent on how large and massive they are: smaller stars form hard-to-see red dwarfs, mid-sized stars form yellow dwarfs like our Sun, and very large stars form blue giants.
The other components of a stellar lifespan also are dependent on size. Counterintuitively, it is currently believed that the largest stars have the shortest lifespans, measured in just a few million years. This is because, as a result of their enormous size, they force together most of their hydrogen in a very short period of time. Red dwarves, by contrast, fuse only a small amount of hydrogen at a time, and will survive for hundreds of billions, perhaps even trillions, of years. As a matter of fact, no red dwarf has ever run out of fuel in the known universe - yet.
Mid-sized yellow stars, like our own Sun, have lifespans measured in the billions of years. According to our current estimates, our Sun has run through roughly half of its fuel supply at this point in its life. Inevitably, eventually, all of these stars will run out of hydrogen, leading to their deaths.
- Stellar Death -
How a star dies depends on how it lived. The smallest stars, the red dwarfs, simply run out of hydrogen and gradually wink out, becoming dead hulks of free-floating hydrogen. As these cool, they became theoretical objects called black dwarfs - massive, dead spheres of matter. A black dwarf would by definition emit no light; as a result, if they do exist, we are not currently able to see them.
Larger stars, like the Sun, undergo a somewhat more spectacular ending. As the hydrogen in the core becomes depleted, such stars begin to fuse together hydrogen closer to their surface, and then to fuse helium into carbon. These changes cause the star to puff out immensely, becoming a red giant. Eventually, the new fusion processes fail too, and the star compresses again, very rapidly. The outer layers blow off, forming beautiful planetary nebulae (we can see some of these with telescopes, like the majestic Eagle Nebula). The core is left as a glowing ember, or white dwarf. Given enough time, it will cool, becoming a black dwarf.
Even larger stars, the blue giants, undergo even more devastating and exciting deaths. These explode into nova or supernovae, giant cataclysmic explosions in which a star explodes and blows off most of its outer shell. The most powerful supernova ever recorded, in 1006 A.D., lit up the skies nearly as brightly as the moon for three months, even though it was 7000 light-years away.
The remnants of these giant stars are too large to be white dwarfs: they compress in similar form, but in the process become so heavy that their gravity begins to break them down at an atomic level. Stars several times as large as our Sun became neutron stars, incredibly dense spheres just a few miles across. Even larger stars collapse even further, becoming regions of almost impossibly dense energy and gravity called black holes.
This is not the true end, however. The matter blown off, in the nebulae and the supernovae, coalesces into molecular clouds in the interstellar medium. Over time, these give birth to new stars.