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The Stages of a Nebula

The word "nebula" was historically used to describe any distinct object visible through a telescope that is not a star or planet. Now, it is used only to describe clouds of interstellar gas and dust. Nebulae are the starting points for the formation of stars and planetary systems. Several stages occur when a nebula transforms from a cloud of gas to a planetary system.

Nebulae
- A nebula is a large cloud that consists of gas and cosmic dust. Hydrogen gas is the main component, followed by helium. Nebulae exist between stars in outer space, and can be as large as several light years across. Different regions within a nebula can have different densities of material. Astronomers believe that star formation takes place within nebulae.
Gravitational Collapse
- In some regions of a nebula, the gas and dust it contains can come together into a "clump." This process is known as gravitational collapse. Astronomers do not yet know what triggers gravitational collapse, but something must cause a small imbalance in the density of the nebula. Gravity is an attractive force with no repulsive component, so any small change is amplified until the originally small imbalance is very large.
Star Formation
- Star formation begins with gravitational collapse. This process gathers together material which form a protostar, the first stage in a star's lifetime. Around the protostar, a spinning disk of gas and dust called an accretion disk, forms. Most of the material in the accretion disk helps to build up the protostar; the rest goes into making any planets that orbit the star.
  
  The energy generated during gravitational collapse must heat the protostar, hot enough for hydrogen fusion to take place. Hydrogen fusion is what powers a star, so if the protostar never gets hot enough for hydrogen fusion to start, a full fledged star is not formed.
Planetary System Formation
- Once a protostar exists, a protoplanetary disk can form around it. The protoplanetary disk is made of gas and dust, as well as other larger pieces of matter that formed earlier in the "clump" during gravitational collapse. Planetesimals, the starting points of planets, are the next step in the formation of a planetary system. When the planetesimals collide, they stick together and, once they reach a certain size, begin to gather more material by gravitational attraction. The largest planetesimals go on to form fully-fledged planets.

Astronomy