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Earth & Space Science: Session 8

A Closer Look: Star Formation

star field
Star field.

New stars are continually being formed in our galaxy. The formation of a star begins in molecular clouds, which are gigantic collections of gas and dust. A new star forms when the densest region of a molecular cloud, called the core, becomes gravitationally unstable, which means that the inward pull from gravity is stronger than the forces pushing outward.

The core then begins to collapse under its own weight, making it hotter and denser, and, as it gets smaller, it rotates faster in the same way that figure skaters spin faster when they pull their arms closer to their bodies. Once it begins to spin fast enough, the rotation of the gas and dust can keep material from moving toward the axis of rotation (the imaginary line about which all of the material is orbiting), but gravity keeps material falling along the axis of rotation. At this point, there is a flattened disk of material orbiting around a proto-star at the center of the disk. Some of the disk's gas and dust will eventually fall onto the star, some will become the planets, moons, asteroids, and other objects that orbit the star, and some will escape the newly forming solar system. Eventually the solar system will settle into a state in which planets, asteroids, comets, and other bodies have stable orbits around the star, and the gas and dust have disappeared. This is the state of our solar system now.

It is possible for the core to fragment as it collapses, and the result is that more than one star is formed. In such cases, a disk of material can form around each star, around a multiple system of stars, or both. The formation of planets in such cases is not as well understood as planet formation is around an isolated star. Furthermore, it is important to remember that stars do not last forever. Eventually all stars run out of the fuel that allows them to shine, and they cool off and become less luminous, which has a significant impact on the planets orbiting the star. Fortunately for Earth, the Sun is expected to last for several billion more years before this is expected happen.

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