Black Holes: Tall, Grande, Venti
NASA: We have booster ignition and liftoff
of Columbia, reaching new heights for women
and X-ray Astronomy.
Martin Elvis: The main thing Chandra does
is take these superb, sharp images.
Cady Coleman: Nothing as beautiful as Chandra
trailing off on its way to work
Narrator: Long before astronomers found evidence
that black holes existed, these exotic objects
have captured imaginations. In the 21st century,
scientists not only have proof that black
holes are real, they continue to make startling
discoveries both about individual black holes
examples and about their populations across
the Universe. Dr. Paul Green of the Chandra
X-ray Center explains what black holes are.
Paul Green: A black hole is created whenever
matter collapses onto itself so densely that
anything that comes too close, even light,
can not escape it. What defines a black hole
is that its gravitational pull is so large
that at a close-enough distance, even the
fastest thing in the Universe, which is light,
cannot escape. There's a term for that distance,
it's called the "event horizon". No event
that occurs closer to the black hole than
the event horizon can ever be seen by an outside
observer like us.
Narrator: If all the material that passes
over the event horizon is lost forever, how
do astronomers ever see a black hole?
Paul Green: We can never see a black hole
itself, because light cannot escape it. But
we can detect light emitted from just outside
the event horizon. When material like gas
and dust falls in towards a black hole, because
there is always some amount of spin to the
material, it swirls into an orbiting whirlpool-like
structure called an accretion disk. The orbital
speeds are different at different places in
this disk, causing friction to heat up the
gas. Near to the black hole, temperatures
are beyond red hot, or even white hot. The
gas becomes "X-ray hot", and gives off profuse
amounts of X-ray light. Such high temperatures
are very rare in the Universe, so X-ray telescopes
are an excellent way to find and study black
holes.
Narrator: Black holes are rather similar to
one another, except when it comes to size
and mass. How does the Universe end up with
black holes in different sizes and masses?
It turns out, it all depends on how and where
the black hole is born.
Paul Green: There are at least 2 types of
black holes for which we have seen many examples
 - stellar black holes, and supermassive black
holes. Stellar black holes are created when
a single very massive star dies. A star more
than about 3 times as massive as the Sun lives
only a short time before it explodes as a
supernova, and its core collapses into itself
to become a black hole. These stellar black
holes range in size from about 2 to 15 times
the mass of the Sun, and can be found scattered
throughout a galaxy like the Milky Way. Supermassive
black holes, on the other hand, may be from
a million to a billion times more massive.
These are found only in the cores of galaxies.
When theyre active, supermassive black holes
also give off copious X-rays, and are known
as quasars. But the nature and origin of supermassive
black holes is much less clear than for their
smaller stellar cousins.
Narrator: While astronomers have known for
many years that black holes come in these
two very distinct small and large categories,
there has always been a question of whether
there is an in-between variety. This quest
for a so-called intermediate-mass black hole
took an interesting twist when astronomers
using Chandra found an intriguing object in
the galaxy known as M82. While not conclusive,
this object suggests that maybe black holes
do, in fact, come in this medium size.
Paul Green: One hypothesis that astronomers
have put forth for the formation of supermassive
black holes is that they begin simply as an
accumulation of smaller stellar black holes.
This requires a very dense cluster of stars,
but it also requires that as pairs of black
holes go into close orbits around each other
in these clusters, they must lose orbital
energy somehow. If a bright star could be
found orbiting such an object, astronomers
could directly measure its mass. Proof of
the existence of these intermediate-mass black
holes would help to complete the evolutionary
chain between stellar and supermassive black
holes, between stars and quasars.
Narrator: We've just begun to explore the
intriguing topic of black holes that are so
fantastic that they seem to belong to the
realm of science fiction. In fact, they are
firmly rooted in the reality of physics. Scientists
can use tools such as Chandra to test their
theories and ideas about black holes, and
as they do, they continue to be amazed by
these mind-boggling objects.
