Q&A: General Astronomy and Space Science

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.