Stars

A star starts out life from what seems like nothing at all. Stars are born in huge clouds of gas that are actually far less dense than the space immediately surrounding Earth.

"These clouds are so spread out, they make the 'vacuum' of the space around the Earth that the space shuttle flies through seem as thick as chicken soup," says Jeff Hester, an astronomer at Arizona State University in Tempe. But because the clouds are so big, they contain a lot of molecules—enough, eventually, to build massive stars.

How big are these clouds that serve as star nurseries? They can be a light-year across—so enormous it would take light one year to cross one. In contrast, it takes light only one-seventh of a second to travel the nearly 25,000-mile distance that equals the circumference of tiny Earth.

The key to star formation is gravity, says Hester. Gravity causes the multitude of spread-out molecules to move toward each other and pulls them toward the center of the cloud. "The cloud starts to collapse under the force of its own gravity," says Hester, who studies the process of star formation. This collapsing process happens relatively quickly (by cosmic standards)—only about 30 million years, or less.

Over time the cloud gets smaller and smaller. As the cloud contracts, it also begins to spin faster. (This is due to a little something called conservation of angular momentum—the same phenomenon that allows a figure skater like Nancy Kerrigan to speed up her spin when she pulls her arms in toward her body. As the mass of gas moves toward the center, the cloud spins faster.) Next, the cloud starts to flatten. "It's just like when you make a simple pizza," says Hester. "The dough, or mixture of flour and water, flattens as it spins." Finally, because gravity becomes so strong in the center of the cloud, the center starts to collapse in on itself as it continues to rotate. At this point, you have a disk that's a few times the size of our solar system. (The disk would be about a couple of light-days across, if you're keeping track of the size of things.) As the disk continues to rotate, matter in the center of the disk starts to move further inward and a big lump forms in the middle of the disk. This lump, says Hester, is a protostar.

What happens to the matter that's left over further out in the disk? In our solar system, it went on to become the planets. (In essence, earth is made up of leftovers.)

Protostars are very hot because so much of the gravitational energy that was once contained in the loose cloud of interstellar gas has been converted into heat. Protostars are spectacular, glowing with dull red light and infrared light. As a protostar emits this light, it continues to shrink and gets hotter and hotter. Finally, it's hot enough for real star business to begin—nuclear fusion.

At high enough temperatures, atoms slam together at incredibly fast speeds. When this happens, lighter atoms like hydrogen can fuse together to make heavier atoms like helium. One reaction releases massive amounts of energy; add all the reactions together, and "that's the energy that makes the stars shine," Hester says. Once nuclear fusion begins, that's truly when a star is born.

After a star "turns on," he says, its power can cause destruction to the surrounding environment. A young star expands, tearing apart the cloud that formed it. New stars often break up neighboring stars before they can form. It's hard to overstate what a powerful process star formation is. Even as they are forming, protostars eject huge amounts of material in jets and streams and create violent solar winds.