QUESTION:
How long does it take for a nebula to form into a star?  How do you know?

ANSWER from Tom Roellig on October 25, 1995:
First of all, let me say that this was a very good question. The time it
takes for a nebula to collapse into a star and start the nuclear
burning of its hydrogen into helium depends on the size of the star.
For a star the size of the sun, it takes about 10,000,000 years to
collapse. For bigger stars, it is a shorter time - as low as 100,000
years for the biggest stars. For stars smaller than the sun it takes
longer than 10,000,000 years. All of these times are too long to allow
us to watch any one object out in space and see this happen. Instead,
we have calculated what we would expect to see at all the different
stages when a nebula collapses into a star. We then look out into
space and observe a lot of different objects. Since we can see some
objects in each of these stages, we believe that our calculations are
right. Also, if our calculations show that a certain stage of collapse
takes a long time, then we would expect to see more objects stuck in
this stage than we see in other stages.  Since this is in fact what we
see, this gives us further confidence that our calculations are right.

Dr. Thomas L. Roellig