Elizabeth Cychosz, news editor, email@example.com
Black holes, massive amounts of incredibly dense matter that suck in everything within their reach, used to be the stuff of science fiction, difficult for the mind to fully grasp. Their effects can hold together galaxies, but they cannot be seen with visible light. If so, how do we know they exist?
Joe Shields, a professor in the Department of Physics and Astronomy, illuminated just how astronomers detect these massive objects at Sigma Xi’s Science Café on Nov. 28. His lecture — entitled “Hunting Black Holes with the Hubble” — included how black holes work and how the Hubble Telescope aids in their detection.
Looking up at the night sky, he said, we can see pinpoints of light that look like stars, but emit unusually large amounts of energy. Initially noted for their radio waves, these specks were named quasi-stellar radio sources, or quasars.
“There’s something very exotic, very strange about these objects,” Shields said. Quasars are part of galaxies far, far away from our own, further than the stars we typically see. Because light loses energy as it travels long, universe-spanning distances, the fact that we can detect still large amounts of energy implies that these objects radiate significantly more energy than a star.
Astronomers have determined that these quasars are the centers of distant galaxies and are actually supermassive black holes, Shields continued. This means they have masses one million to one billion times the mass of our sun in a compact space, as opposed to stellar-mass black holes, which have masses on the same order of magnitude as the sun.
When matter is drawn into the black hole, particles collide with incredible force and regularity. “The source of energy is the gravitational energy from objects falling into a black hole,” Shields explained. The heat energy produced by these collisions turns out to be a source of light much more efficient than even the nuclear processes inside the sun. This light then travels for light years across the universe to our eyes.
Quasars are more common in the distant reaches of the galaxy, however. Because they are so far away, their light has taken a long time to get to us. “We’re seeing light from when the universe was a much younger place,” Shields said. Something must have changed since those quasars emitted the light we will see in the sky tonight, he suggested.
“Very likely, as time as passes, the fuel has gone away,” Shields said. The quasars we see are probably now dim, like black holes closer to us. The gases that were drawn into the black hole that made it emit light have been absorbed, and the black hole is no longer a quasar. Actually, there is one of these supermassive black holes at the center of our own galaxy, one that has four million times the mass of our sun, he said.
Astronomers are able to detect black holes by looking at how gasses and lights react when they pass by the vicinity of one, which is called gravitational lensing. “The methods we use depend on things passing by black holes that bend around them,” Shields explained.
The Hubble Telescope has been instrumental in the study of this phenomenon and in the identification of black holes. Its home orbiting the Earth enables it to capture images of the universe without losing focus due to atmospheric refraction of light — what makes stars “twinkle” — so gravitational lensing can be more accurate.
“Hubble was a game-changer in this field,” Shields said.