QUESTIONS:
What is an invisible star?
Is it made of the same materials as regular stars?
If the stars are indeed invisible, how do you know they exist?
In Dr. Bregman's writeup, he used another star which was close to the
  invisible star; how do you use the other star to guide you to something
  you can't see?
Why was the star near the Rho Oph dark cloud invisable?

ANSWER from Scott Sandford on May 13:
The invisible stars that Jesse Bregman mentioned are not actually invisible,
they're just invisible to us.  These stars are like the ordinary stars you
see in the sky.  They give off light and they are formed in the same way and
from the same materials as ordinary stars.  The invisible stars Jesse
mentioned are only invisible to us because they are behind or embedded in
thick clouds of gas, dust, and ice.  In other words, these stars are
invisible to us in the same way our Sun is invisible to us when a cloud
passes in front of it in the sky.  The Sun is still there and it is still
giving off light, but we can't see it until the cloud gets out of the way.
Even though we can't see an 'invisible' star at visible wavelengths,
i.e. we can't see it with our eyes, we can often still tell it is there by
detecting it at other wavelengths.  Infrared light is very good at
penetrating through interstellar clouds.  As a result, it is often possible
to use special detectors to 'see' the infrared light coming from a star,
even though all the star's visible light is blocked by the cloud.  This is
one of the properties of infrared light that makes infrared astronomy so
useful and it is why it is worthwhile to go to all the trouble of flying
the Kuiper Airborne Observatory (KAO).

It is difficult, however, to study objects that can only be detected in
the infrared.  One of our biggest problems is that, even when we know an
'invisible' star is present in a cloud, it is often hard to study it because
we have no visible counterpart at which to point the telescope.  If we can't
see the star with our eyes, it's very hard to know whether we have the
'invisible' star in the telescope's field of view or not.  We solve this
problem by using what we call 'offset stars.'  An offset star is usually a
star that is in the same direction as the 'invisible' star we want to study,
but instead of being in or behind the interstellar cloud, it is in front of
the cloud where it is easy to see.  When someone discovers an 'invisible'
star using infrared detectors they will quickly measure where it is in the
sky by measuring its separation from other visible stars in the field of
view.  So, for example, if you wanted to study an 'invisible' star that
had been discovered earlier, you would first point your telescope at a
visible star that is in front  of the cloud of dust.  Then you would 'offset'
the telescope a known amount, i.e. move it north or south and east or west
by the correct amount so that you know the 'invisible' star is right where
you want it.  This technique works very well as long as there are visible
stars in the field of view to use for the offset.

It won't work, however, if the cloud is so big or so
close that there are no visible foreground stars in the
same field of view as the 'invisible star.'.  This is the
problem Jesse Bregman mentioned for the star we wanted to study in the
Rho Ophiuchus cloud.  This object was not near any good visible 'offset'
stars, and as a result it was very difficult to get the telescope pointed
in exactly the right direction.

Scott Sandford
Astrophysics Branch
NASA/Ames Research Center