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How Is a Protostar Different From a Star?

The universe was born in a blaze of glory, an unimaginably powerful explosion that not only filled all space, but created all space --- all space and everything in it. Most of what was in it was hydrogen and helium. The hydrogen and helium atoms, like all matter, exerted gravitational attraction on other matter. That gravitational attraction was responsible for the formation of the first stars, and the same process is responsible for the formation of stars today. A review of the steps of stellar formation will illustrate the difference between stars and protostars.

Gravitational Attraction
- The force of gravity is dependent upon how much mass an object has. The heavier an object, the stronger its gravitational attraction. So a few hydrogen atoms get together, attract a few more atoms, and before you know it, there's a chunk or clump of hydrogen and helium. The chunk exerts an even stronger gravitational attraction, growing larger, exerting more attraction. The cycle continues until there's a dominant mass, pulling everything else towards it.
Rotation
- As atoms, clusters and clumps fall towards the center of the dominant mass, they don't just zip in on a straight line to the center. The clumps swirl about, spiralling into the center. But, just like an ice skater bringing her arms closer to her body, the closer the swirling clumps get to the center, the faster they spin. But now the cloud is more dense, so the clumps get absorbed into what becomes a swirling ball. Because of the swirling, there's now competition with gravity. Along the axis of the spinning mass, gravity is still unopposed, but in the plane of the spin, the rotation fights against gravity, slowing or stopping further collapse.
Snapshot of a Protostar
- Clouds of gas condense into protostars --- which could turn into a star and planets.
  
  At this point if you could look at the massive collapsing cloud, you would see a gigantic rotating ball sitting in the middle of a spinning disk. As described in the previous section, two forces are opposing each other: gravity and rotational forces. The situation is dynamic. Balance --- equilibrium --- has not been reached. This is a protostar. Enough material has accumulated so that there is a well-defined structure, but it is still one fuzzy body, with no distinctions.
Heading Toward Stardom
- As material continues to collapse into the center, the central body becomes denser. As the density increases, and the mass grows, the pressure and temperature at the interior of the central body gets higher. At the same time, out in the spinning disk gravity is collecting other material together. Particles and clumps that rotate close to each other fall together, coalescing into their own bodies, still rotating about the central mass. Back at the central body, more mass collects and the pressure grows. As the pressure grows, the temperature grows. If the mass is high enough, making the pressure high enough, making the temperature high enough, then within the central mass, fusion will occur --- it will become a star. So, if a protostar is big enough, it will eventually become a star, burning nuclear fuel. If the protostar is not big enough to start fusion, then it will remain a protostar forever.

Astronomy