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Q&A: General Astronomy and Space Science
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Q:
Why are stars and planets round but asteroids aren't?
A:
The shape of an object is determined by the relative strength of
the forces acting on it. In general, there is gravity, pressure,
electromagnetism, and centrifugal force (or rotation). For most
stars, it is a battle between gravity (inward or radially) and
pressure (outward, also radially) so they are round. If the
stars are rotating rapidly enough, they will bulge at the
equator and flatten at the poles. This is especially pronounced
for galaxies, many of which look like flattened disks because of
the importance of rotation. For rocks, electromagnetic forces
are important. Since positive and negative charges cancel each
other out, the force can be in almost any direction, so rocks
and asteroids can have an irregular shape. The Earth is
approximately round because it is larger enough for gravity to
be important, although is has a very slightly irregular surface,
and rotation does cause it to bulge a little.
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