By Linda Copman, based on an interview with Andrea Ghez
Animations and images created by Dr. Andrea Ghez and her research
team at UCLA from data sets obtained with the W. M. Keck Telescopes
Animation: This three color animation,
centered on Sgr A*-IR shows, for the first time ever,
the broadband color of Sgr A*-IR throughout an outburst.
The image covers about two hours of observations.
“The larger the telescope the more detail,
or the closer to the center of the Galaxy I can see. Having
access to the Keck Telescopes - the largest telescopes
in the world - was KEY to my success in studying the black
hole at the Galactic Center. So was having high angular
resolution techniques — now in the form of adaptive optics
- which allow this resolution to be achieved. Without AO
the atmosphere blurs the images and you lose the advantage
of having a large telescope.”
— Dr. Andrea
An object becomes a black hole when it is compressed to the
point that gravity overcomes all other forces. While black
holes don't have a physical size, there is a radius associated
with them, called the Schwartzschild radius or “R_s.” This
radius is not a physical size or surface. Rather, it is the
last point at which light can escape the gravitational pull
of the black hole, and therefore the last point from which
scientists can gather any information. Scientists call this
point the “event horizon.”
It is also the radius to which one would have to compress an
object for it to become a black hole. At this point, gravity
would prevail over all other known forces, and the object would
collapse to an infinitesimally small scale and become a black
hole. It turns out that the size of the object when this happens
is the same as the event horizon. Thus, the event horizon of
a black hole is also its Schwartzschild radius, or “R_s.”
Animation: The first detection of infrared
light from plasma falling onto the supermassive black
hole at the center of the Milky Way Galaxy. The location
of the black hole is marked with an arrow. The brightness
variations, which occur on timescales as short as 40
minutes, reveal that the plasma is much more energetic
than previously believed, showing that violent events
occur almost continually.
To prove the existence of a black hole, which doesn’t emit
light like a star does, scientists apply Kepler’s Laws to track
the orbits of neighboring stars. By tracking the objects which
orbit around a proposed black hole, it is possible to determine
how much mass is confined within this black hole and its associated
Sgr A*, the black hole at the center of the Milky Way Galaxy,
was recognized to be an unusual radio source about 30 years
ago, but it wasn’t known to be a black hole back then because
there was no way to say how massive the object was.
Dr. Andrea Ghez’s utilized the powerful telescopes at Keck
Observatory to conduct a groundbreaking experiment and prove
the existence of a supermassive black hole at the Galactic
Center of the Milky Way. Ghez tracked the orbits of stars in
the vicinity of Sgr A* to measure its mass and then confined
that mass to within a specific radius. Ghez’s work was critical
in establishing that the radio source was most likely caused
by light arising from matter falling onto a black hole. Astronomers
can’t see a black hole directly, but they can see matter falling
toward the black hole. The radio source was not the black hole
itself, but matter just outside the event horizon seen falling
towards Sgr A*.
The orbits of stars within the central
1.0 X 1.0 arcseconds of our Galaxy. While every star
in this image has been seen to move over the past 9 years,
estimates of orbital parameters are only possible for
the seven stars that have had significant curvature detected.
The annual average positions for these seven stars are
plotted as colored dots, which have increasing color
saturation with time. Also plotted are the best fitting
simultaneous orbital solutions. These orbits provide
the best evidence yet for a supermassive black hole,
which has a mass of 4 million times the mass of the Sun.
As a result of Ghez’s work at Keck Observatory, we now know
much more about Sgr A*, as well as about black holes in other
galaxies. “Sgr A* is one of the least massive among the supermassive
black holes in the universe. “It’s also not accreting a lot
of matter, so the light associated with the matter falling
toward the black hole is fairly dim,” explains Ghez.
Massive stars, or stars whose mass is more than 20 to 30 times
the mass of the Sun, will end their lives by turning into “little” black
holes. These “little” black holes have a mass of roughly 3
to 10 times the mass of the Sun.
In contrast, the supermassive black holes at the center of
galaxies are typically a million to a billion times the mass
of the Sun. Such supermassive black holes were not predicted
by theory, as were the “little” black holes, but were found
Scientists are still trying to understand how supermassive
black holes form. The consensus today is that these huge black
holes came into existence when their galaxies were first forming. “We
noticed that the mass of a black hole is related to the central
spherical component of its galaxy,” says Ghez. “These
two things are of such different scales that the only way we
know how to induce a correlation between them is for them to
be the outcome of the same process. What this means is that
we think they were born that way,” Ghez speculates.
Black holes, with their seemingly improbable properties, have
become iconic in popular culture. Comparisons between energy-sucking
activities, such as certain kinds of jobs, and black holes
are commonplace. The concept of something which pulls everything
in its vicinity into a deep, dark abyss has captured our imaginations
and, as frequently happens in such instances, misconceptions
about black holes abound. In order to debunk some of these
misconceptions, Dr. Ghez provides the following list of the
real attributes of black holes:
Black holes don’t expand with time. However, they might
gain mass by swallowing up stars and local gas from their
Supermassive black holes will probably “live” forever.
They are quite stable.
Supermassive black holes are likely found in every galaxy.
They can effect the formation and evolution of stars in
their environment. They also provide an important clue
as to how galaxies might form.
Image: Keck Observatory Laser Guide Star
Adaptive Optics image of the Galactic Center. Young stars
are marked with red crosses. The white arrow marks the
position of the supermassive black hole.
Ghez’s research continues to zero in on black holes, particularly
on Sgr A*, the closest black hole to home. Ghez is currently
studying three aspects of Sgr A* that she hopes will elucidate
our understanding of these fascinating objects:
Subtle variations in the orbits of the stars around Sgr A* that the theory of general relativity (GR) predicts:
Such observations could provide a test of GR, which is
the least tested of the four fundamental forces.
Understanding star formation: Sgr A* is a hostile environment
for star formation, and yet astronomers keep discovering
more and more young stars, closer and closer to the black
Understanding the accretion process, evolution, and nature
of our Galaxy’s black hole: Ghez, like many other astronomers,
would like to know how black holes are able to accrete
without emitting too much energy, what their spin is, and
the biggest mystery of all: what the physical description
of a black hole is inside the event horizon.
Photo: Dr. Andrea Ghez at work at Keck
Observatory on the summit of Mauna Kea.
Ghez admits that the first moon landings had a big impact on
her. She attributes her success as an astronomer, at least
in part, to a few great mentors that she encountered during
her years in school. In particular, Ghez recalls her chemistry
teacher Judith Keane, the only female science teacher she had
in high school or college; Hale Bradt, an MIT professor whom
she worked with as an undergraduate on “little” black holes;
and Gerry Neugebauer, her Caltech Ph.D. advisor. “Neugebauer
was the best advisor I could have had - very supportive, very
straight with me, and he gave me lots of independence,” says
“I love the research process. First you have
to figure out what might be an interesting question to
ask given what is currently known and possible to do. I
really enjoy using new techniques. While this can be a
struggle because things are not totally ironed out, you
get to see things in a way that have never been seen before.”
— Dr. Andrea Ghez
Photo: Dr. Andrea Ghez enjoys the train
ride at Griffith Park in Los Angeles with her two sons,
Evan (6) and Miles (2).
Ghez has two children, ages 6 and 2 years, with whom she spends
most of her non-working hours. To relieve stress and maintain
her perspective, she swims with a masters swim club. When asked
what she likes least about her work as an astronomer, she was
hard pressed to think of anything negative. “I can’t think
of anything bad about being an astronomer. . . . I feel so fortunate
to be in this line of business,” says Ghez. Ghez loves being
able to pursue questions that she is curious about, and she
loves being able to work on problems that she defines and feels
Ghez admits that part of the reason she teaches at UCLA is
because this enables her to earn time on the Keck Telescopes. Keck
time is very precious, and Ghez has passed up family vacations,
high school reunions, lots of holidays, and even a wedding
anniversary in order to work at Keck Observatory.
Image: A recent adaptive optics (AO) image
of the region around the Galactic Center from the Keck
Observatory, showing the vast improvement in image quality
made possible by Keck AO technology.
“Keck is an absolute gem! It houses the largest
optical/infrared telescopes in the world. This allows astronomers
who use Keck all sorts of advantages. We can see more distant
objects, and therefore further back in time, so we learn
more about the early universe. We can also see in more
detail. This is what allowed us to demonstrate the existence
of a supermassive black hole at the center of the Galaxy.”
— Dr. Andrea Ghez
Planned improvements in instrumentation, and in particular
a more advanced adaptive optics system, will greatly improve
Keck Observatory’s views of the center of the Galaxy. The Next
Generation Adaptive Optics (NGAO) system proposed for the Observatory
will improve the sensitivity of the orbital measurements Ghez
and other astronomers are able to make. Such measurements will
enable scientists to test the theory of general relativity,
study star formation in the vicinity of a black hole, and understand
how a black hole gains weight.
Image: Image of the space-time continuum
being affected by the gravity of a black hole.
Einstein’s theory of relativity predicts the existence of black
holes, as a place where there is a singularity of the warping
of the space-time continuum. Since density rises to infinity
in a black hole, they warp
the space-time continuum. Yet our current theory of gravity
clearly breaks down within the limits of a black hole. “Presumably,
we’ll understand black holes when we can make the study of
things that are very small (quantum mechanics) work together
with the study of things that have strong gravity (general
relativity),” explains Ghez. That’s why Ghez and other astronomers
want to study gravity in a strong gravity regime, such as a
black hole, where there have been very few tests of the theory.
“We’ll get a better understanding of how our physical world
works. We simply don’t have a perfect description or understanding
today. And an improved understanding is likely to yield insight
into other questions about the origin and evolution of our
universe,” says Ghez.
Visit the Monsters
of the Milky Way home page for clips from a 2006 NOVA documentary
on the supermassive black hole at the center of our Galaxy.
This site includes interviews with experts in the field and
stunning digital simulations of black holes.