Why are stars never born alone?

The reason why stars are never born alone is that it is fairly
difficult to make stars, and you need a whole bunch of gas to make it
work. In order for a star to form, gas must collapse under the
influence of gravity and heat up in the center until fusion begins.
But there are other forces in nature which counteract the force of
gravity. If the gas is hot enough, then the motions of the gas
particles and the pressure that creates can be enough to stop the gas
from collapsing. This heat can come from the radiation from all the
stars in the Universe, which is absorbed by the atoms of gas. One way
to prevent this is to have a large enough mass of gas that the outer
edges absorb most of the radiation, allowing the interior to cool and
start to collapse. Also, magnetic fields can stop gas from
collapsing. There, too, a larger mass of gas helps you by increasing
the gravitational force so that it is larger than the magnetic force.
So you need a bunch of gas, something like 10,000 times the mass of
the Sun (very rough number) to get conditions favorable to star
formation. However, once the gas starts collapsing, it doesn't all go
into one star. The gas cloud starts fragmenting into smaller pieces,
and so a bunch of stars are created. This process is not very well
understood, and there is a lot of active research in this area. I
hope this helps!
Jennifer Johnson

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