Lehigh University
Lehigh University


Ghez: There is a supermassive black hole in the center of our galaxy


Andrea Ghez, the first speaker at the 2009 Academic Symposium, shook up the world of astronomy with her discovery that some stars are born near the center of the galaxy.

Astronomers had not believed that was possible, and doubters suggested that these stars were not actually young, but rather old ones that acted and looked young.

“I’m from L.A. I’m very familiar with old stars masquerading as youths,” Ghetz quipped.

However, she said, these stars did not apply stellar Botox, and actualy did appear their true age.

“Andrea’s work has helped to illuminate the galactic center for us. It’s also illuminated in some sense our minds,” said George McCluskey, professor of physics, in his introduction. “One thing I always try to tell my students is that you have to keep an open mind about these things. Don’t get hidebound by theory and what nature can and can’t do. Nature can do a lot more than we can imagine.”

‘Black holes on a diet’

Those listening solely to Ghez’s inflictions and intonations might assume that she was enthusiastically reading a favorite bedtime story aloud to her children, not presenting complex research in astrophysics, which she teaches and studies at UCLA.

Ghez’s passion for the stars and her quick wit entertained her audience as she presented in a simplified manner what might have been an abstruse subject. Most of Ghez’s research has attempted to answer the question: If you or I am not at the center of the galaxy, what is?

When Ghez started studying the galaxy’s core approximately 15 years ago, scientists had noticed that some galaxies swirl around very large—or as it is technically termed, supermassive—black holes that are a million to a billion times the size of our sun.

While astronomers cannot see the black holes, they noticed jets of energy spewing from a central location, and observed matter drawing toward that location, heating up and then shining brightly. That glow, she said, is the black hole “feasting on the center of the galaxy.”

Astronomers speculated that other galaxies may pivot around black holes that are less visible because they are not as active—or, as Ghez put it, they are black holes “on a diet.”

Black holes, Ghez explained, are defined as a large mass compressed into an infinitesimally small region that generates a powerful gravitational field from which not even light can escape. If any mass were squeezed beyond a certain point, it would be forced to collapse into a black hole. For example if the earth were pressed to the size of sugar cube, it would become a black hole, she said.

‘We have this horrible beast in the center of our galaxy’

To prove that our own galaxy, the Milky Way, has a black hole at its core, Ghez needed to prove that its center had a mass confined to such a small space that it could only be a black hole.

Her team devised a method to measure the mass and volume of the galaxy’s center by examining orbits of nearby stars. If these orbits were affected by a gravitational pull, that would suggest a large mass, confined to a very small radius. And that would mean the stars were likely to be orbiting around a black hole.

This relatively simple proof had not yet been done, because interstellar dust obstructed astronomers’ view, and they could not distinguish individual stars from the haze.

“Now I’m from Los Angeles, I understand this phenomenon extremely well,” Ghez said. “It’s smog. The size of the dust particles in our galaxy are, in fact, very comparable to the size of the smog in Los Angeles.”

Because of these dust particles, she said, “the center of our galaxy is basically invisible at wavelengths that our eyes can detect.”

However, near infrared light can dodge more dust particles than visible light, and by using the same wave lengths that television remotes do, Ghez can distinguish individual stars in our galaxy.

After more than a decade of stargazing, Ghez has documented the orbits of several stars and determined both the density and size of the Milky Way’s core.

“The conclusion is pretty inescapable,” she said. “There is a supermassive black hole in the center of our galaxy.”

“While we have this horrible beast in the center of our galaxy, we are perfectly safe,” Ghez said, in response to a query posed by Virginia McSwain, assistant professor of physics at Lehigh, during their on-stage conversation held after Ghez’s presentation.

McSwain asked Ghez about her work in astrophysics, and about how she manages her time as a mother (she said you learn to juggle as a professor and perfect the skill as a mother) and whether she had any difficultly being a woman in a male-dominated field (she had not).

Understanding research

Among those attending the lecture were students in professor Steve Peters' earth and environmental studies (EES) senior seminar. Leading up to Ghez’s lecture, the class reviewed physics and astronomy and then read one of Ghez’s research papers.

While astrophysics is not typically considered part of EES, Peters said that his class is built on the principle that upperclassmen at Lehigh should be equipped to understand any research.

“The idea is to give students the awareness that they can read anything and make sense of it. The major that you pick doesn’t matter as much as whether you can read effectively, comprehend important concepts, and ask insightful questions,” he said.

Ghez’s lecture provided Peters’ students the opportunity to explore a field they were not familiar with, and prepared them to study other fields that seemed as out of reach as the stars.

Back to Lehigh hosts celebration of research and scholarship

--Becky Straw

Photo by John Kish IV

Posted on Friday, April 17, 2009

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