The force that tends to make objects spherical
is gravity. In the absence of gravity, there
is no preference for shapes, so objects will be in
any shape. Larger bodies exert more gravity, and
so tend more to be round.
Neutron stars, short of black holes, are the
most strongly gravitationally bound objects in the
universe. In black holes, all of the mass is
contained in a single point with no volume.
Thus, neutron stars, which still have finite
volume, are also the most perfectly spherical
objects in the universe.
Most astronomers think that asteroids
are failed planets, which is to say that they are
leftover rock from the birth of the solar system
that never stuck together to form into planets.
While the question of irregular shapes can be
answered in various ways (with various levels of
scientific jargon), the basic reason is that
the asteroids are too small(most are under
200km) for gravity to be able to deform them into
spherical shapes. The strength of the rock
is stronger than the strength of gravity.
Actually, saying that asteroids
are too "small" may not be the right way to phrase
it, since this also depends on density.
Neutron stars are often only about 20km in
diameter, but have a mass of almost 1.5 times the
mass of the Sun. (They are the densest known
objects, in fact.) Because neutron stars are such
extremely massive objects,compressed to such a
high density and bound by a gravitational
force,they would be round in the absence of
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