Professors discover black hole

While black holes have historically lived up to their name — representing “a black hole” in existing scientific knowledge — a recent discovery by Yale researchers may be lighting the way to greater understanding about the phenomenon.

On Monday, a team of researchers led by Jerome Orosz GRD ’96, astronomy professor at San Diego State University and including astronomy and physics professor Charles Bailyn announced its discovery of the largest-known stellar black hole — a region of space formed when a massive star burns out and collapses — which weighs nearly 16 times as much as the sun. The discovery is challenging dominant views about the origin and development of black holes, researchers said.

Bailyn said the findings will require scientists to rework current ideas on how black holes are created, since the black hole system the researchers found has a much greater mass than existing theories would predict.

“Small black holes are supposed to evolve from the cores of individual massive stars,” he said. “The core regions are expected to have masses 10 times that of the sun or less. In this case the black hole is 15 solar masses, so something different must have happened.”

Apart from containing the largest black hole discovered to date, the system the researchers found was also unique in two other respects, Orosz said.

First, it is in the M33 dwarf galaxy, 2.7 million light years away — about 17 times more distant from Earth than any other known black hole system. The researchers also found a “companion” star that is nearly 70 times the mass of the Sun — the largest-known star of its kind — circling closely around the black hole in a short orbit of 3.5 days, Orosz said.

He said black holes cannot be seen since they trap light, meaning that researchers must rely on indirect methods to identify potential black hole systems.

“The most practical way to find a black hole is to look for sources of intense X-rays,” Orosz said. “Normal stars will not be a source of intense X-rays, so when you see a source of intense X-rays, you can be sure there is a massive and dense object there.”

Bailyn said the gravitational effects of the companion star on the black hole also provide important clues about the relationship among objects in a black hole system. The companion star the researchers found is the only known star of its kind, so massive that it completely eclipses the black hole — called M33 X-7 — every time it completes an orbit, he said.

Researchers said this phenomenon allowed them to make highly accurate calculations about the object’s two masses and the distance between the two objects.

“This system is 100 times farther away from us than the black hole systems in our own galaxy,” said Jeffrey McClintock, a researcher on the team. “It’s astonishing that we can get more precise of a result for a mass of stellar black hole at such a phenomenal distance.”

The researchers used data from NASA’s Chandra X-ray Observatory in orbit around the Earth, the Gemini North telescope on the island of Hawaii and telescopes at Kitt Peak National Observatory to make their calculations.

Researchers said black holes do not currently have practical applications. Instead, they said, interest in black hole behavior is purely academic and fueled by curiosity about natural phenomena.

“Electricity was once thought of as completely irrelevant,” McClintock said. “We have no way of predicting whether black holes will ever be useful in the practical sense of the word. But I see it as getting to know more about the universe we live in.”

Bailyn said black holes, which exist only outside our solar system and cannot be replicated in a lab, are the most extreme manifestation of Einstein’s theory of relativity. The theory provides a description of gravity that predicts that when a large enough mass is concentrated within a small enough region, all matter and energy will curve inwards toward the center of the object’s mass. The black hole, which contains a point-like center of mass from which light cannot escape, manifests this quality, Bailyn said.

The research team is currently working on a paper that will present findings about M33 X-7’s rate of rotation to provide a complete description of the black hole, McClintock said.

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