The center of the Milky Way galaxy, with the supermassive black hole Sagittarius A* (Sgr A*), located in the middle. Photo by NASA Chandra X-ray Observatory.
The notion of a black hole started as a mathematical construct, a consequence of Einstein’s theory of Relativity. They are entities so massive that even light bends inward towards them, never to be seen. We know black holes exist because they effect the things around them—leaving clues that help scientists like Professor Meg Urry piece together a picture of what they are. As an observational astronomer, she says:
“The theorists can make a lot of things happen. But to me I have to see it in the data to believe it.”
Dr. Urry is not just interested in any black hole, but supermassive black holes that sit at the center of galaxies. She has observed that these black holes have unusual properties, including a special relationship to the galaxy in which the black hole lives.
Supermassive is an interesting adjective for a black hole, because black holes already have a reputation for being very massive objects. So what’s supermassive? Imagine trying to hide something like the Earth in your refrigerator. You’d have to do a lot of squishing to get it to fit. If you succeed, you’d have a black hole. Now imagine trying to hide the sun and one million of its best friends in your refrigerator. You’d have to do quite a bit more squishing. But this time you’d have a supermassive black hole.
She explains that a supermassive black hole at the center of a galaxy pulls matter towards it, but because such a giant has so much pull, it causes the matter coming towards it to fly into a very fast and tight orbit around the black hole without actually crashing into it the way you’d imagine. Think back to the skating championships in the last winter Olympics. The elegant skater, with her arms outstretched, starts a beautiful spin. Then she pulls her arms inward and her spinning rate increases. That’s angular momentum. And that’s the same sort of increase in speed that happens around the supermassive black holes.
Due to this speeding up, some material goes towards the hole, but to preserve angular momentum, some material flies outward as energy. Professor Urry and her colleagues spend a lot of time detecting this energy. These types of supermassive black holes are what powers active galactic nuclei (AGN)—where the center of a galaxy produces more radiation than the rest of the galaxy itself.
Professor Urry points out that one of the most exciting discoveries in the last 15 years or so is that the mass of a supermassive black hole seems to know about the mass of the galaxy it’s in. How could that be? She explains there could be a feedback loop whereby the black hole emits radiation as it grows, thereby ionizing gas that forms stars.
Indeed, we are in exciting times! In the podcast Speaking with : Meg Urry on supermassive black holes, Meg Urry talks about a merger of black holes would create gravitational waves. At the time of the podcast (October, 2015), no one had reported detecting a gravitational wave. Unbeknownst to the world, scientists detected a gravitation wave on September 14, 2015. It took several months for them to verify it was a true result. The wave indicates a black-hole merger. Just recently, there was a report of Supermassive Black Holes Observed in Close Dance.
Professor Urry is not only an amazing researcher, but she is also a champion of science and a champion of women in Astronomy. See her recent piece Trump's proposed STEM budget cuts a grave mistake where she says: "Revolutionary scientific discoveries can seem obscure and even unimportant when they are first made -- but can have enormous impact decades downstream, reaching into every part of our lives and rendering unimaginable a time when they were not there."