The key to gasses and matter condensing is the force of gravity, which causes all matter to attract other matter. Initially in a nebula, one small area might be slightly denser (and therefore have slightly more mass) than another area. The mass in the denser area will attract other mass, eventually forming a small ball that continues to collect mass from the surrounding nebula in a process called accretion. Once the object collects enough mass, it begins to collapse, again under the force of gravity, and the star becomes much hotter.

As a gas clump collapses, the gas particles get closer and closer together and bump into each other more, generating heat. The initial clump in the gas cloud forms a protostar, an object that gives off infrared and microwave radiation, but is not yet performing nuclear fusion. The clump eventually forms a disk with the protostar in the center, and the material surrounding the protostar in the disk will eventually either form more stars, or turn into a planetary system like our own.

Mature Stars

Once a protostar collapses enough and becomes sufficiently dense, fusion begins in the core of the star. The pressure from the fusion reactions keeps the star from continuing to collapse. After fusion starts, the star ejects most of the remaining dust and gas surrounding it by producing strong winds. This phase is called T-Tauri after the first star observed doing this. The material is thrown away from the star in huge jets of gas and dust. Once the star settles down and stabilizes, a main sequence star is left. During this stable phase, the star fuses hydrogen atoms together to form helium, and gives off heat and light in the process. Over time, the original cluster loses many of its stars due to gravitational forces from other stars; single stars (known as field stars) are the ones that have escaped along the way.

Since mature stars lose their companions quite early, if you see a cluster of stars together in the sky, they are probably young. One observable star cluster is the Pleiades. They are visible in the constellation Taurus, and are shaped like a very small version of the Little Dipper. The Pleiades, much younger than our Sun, are only about 80 million years old.

Red Giants

A star spends most of its lifetime in a stable, main-sequence phase. This phase must come to an end when the star has used up all its fuel, and converted all its hydrogen into helium. When the nuclear reactions in the star's core stop, the star starts to collapse, again, due to an imbalance between gravity and the support that came from internal energy. This collapse causes the gas to compress and heat up, and fusion begins in the layer outside the core. As the star continues to collapse, heat increases and speeds up fusion—much like the cycles that occurred during the protostar phase. The increased heat and activity in the core makes the star brighter again, and the layer of hydrogen gas surrounding the star, the envelope, starts to expand and cool and the star becomes first a subgiant, then a red giant.

A red giant is basically a star with a bloated outer surface. Since the star's energy is spread out over a larger area as its surface expands, the surface itself becomes cooler and has a redder color, hence the name red giant. In our solar system, when the Sun becomes a red giant star a few billion years from now, it will expand beyond the orbits of Mercury and Venus. Although Earth most likely will not be eaten up by the Sun, Earth's water and atmosphere will be boiled off, making it uninhabitable by today's standards.

Size Matters

Once a star reaches the red giant phase, its mass becomes important. If the star is much more massive than our Sun, it will continue to expand and become a supergiant. Betelgeuse, located in the constellation Orion, is a supergiant. Betelgeuse is so big that it would fill our solar system out to the orbit of Jupiter!

Some stars, if they are massive enough, will go through core fusion, at which point they start fusing helium and eventually heavier elements. When this new fuel runs out, the star collapses again, turns back into a red giant or supergiant, then cycles through core fusion and collapse repeatedly. The number of times a star can cycle through these steps depends on its mass—the greater its mass, the more frequent its cycles. Each time the star goes through a fusion cycle, it creates heavier and heavier elements through nucleosynthesis. Small stars like our Sun can generate only enough heat to create lighter elements such as carbon and oxygen.

Eventually, a star will run out of fuel. If it was a low-mass star to begin with (up to five times the mass of our Sun), then it will eject its outer layers as the core shrinks down. This ejected material forms a planetary nebula, and the leftover remnant of the star's core at the center is called a white dwarf star. One example of a planetary nebula is the Ring Nebula (see Chapter 10). Others, like the Dumbbell Nebula and the Cat Eye Nebula, have more asymmetrical and complex shapes due to different formation processes.