QUESTION:
Why is Jupiter not a star, since it is almost large enough to be one?

ANSWER from Duane Bindschadler, DDS/MAG Team, on December 21, 1995: 
The critical factor that defines whether a body is a star or a planet
is whether or not nuclear fusion occurs within that body. Nuclear
fusion is a process that can only occur under conditions of extremely
intense heat and pressure, so that individual nuclei (the heavy, central
portion of atoms) are slammed together hard enough to overcome the
electromagnetic forces that repel them from one another. Atomic nuclei
are always positively charged, and positively charged particles
repel one another, just at the north poles of two magnets will repel
one another.

In a star or a planet, atoms deep within are pushed toward one another
by the weight of all the overlying material. If pressures are sufficient
and temperatures are high enough (approximately eight million degrees
Kelvin), then fusion is ignited, and that body becomes a star. Jupiter
would probably need to be about ten times as massive as it is to reach
the central temperatures and pressures needed for fusion.

That doesn't mean that Jupiter would be ten times larger, though. In fact,
Jupiter is pretty close to as large as a planet can get. The reason is that
adding more material to Jupiter increases its gravitational field so as to
further compress the material deep within it. So we'd find it very difficult
to make Jupiter any larger than it already is.