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A Closer Look: Star Formation
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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