Yale Bulletin and Calendar

October 19, 2007|Volume 36, Number 7

The larger image shows an artist's representation of M33 X-7, in which a star about 70 times more massive than the Sun revolves around a black hole. The inset is a composite of data from NASA's Chandra X-ray Observatory and the Hubble Space Telescope, showing M33 X-7 (in blue) surrounded by young, massive stars.

Biggest ‘small’ black hole discovered

The discovery of the largest example of a “small” black hole — one formed from the collapse of a single massive star at the end of its lifetime — has led scientists to reevaluate how black holes come into being, according to a report in Nature.

“The theory we operated with for the last decade was that single-star black holes are formed from the remnants of massive stars — the more massive the star, the more massive the remnant,” says Charles Bailyn, the Thomas E. Donnelley Professor of Astronomy and Physics at Yale, and a member of the research team. “But, all of the stellar mass black holes were expected to be in the range of 10 times the mass of the sun or less, since only the core regions of the star would collapse.”

The black hole that the team discovered was 15.65 times the mass of the sun that spawned it, prompting the astronomers to reevaluate the long-standing theory.

The scientists took advantage of a natural phenomenon to make an unusually precise measurement of the black hole. Since black holes can’t be seen (because they trap all matter and light that enters them), they are detected by the gravitational effects they have on nearby stars or other matter that is near them. This team made their calculations by measuring the motion of a star as it orbited the black hole, known as M33 X-7. The black hole completes one orbit every 3.45 days around its massive companion star.

“In this particular case, an eclipse in the system provided the exact orientation and gave mass information far more accurate than any previous reports,” says Bailyn. “Researchers rarely have such accurate points of reference.”

While some other astronomers have identified “small” black holes with masses of 12 to 14 times that of the sun, Bailyn says, “Those data had large margins of error that could still fit within the theory. Finding a black hole with such unusual characteristics points out that our understanding of the evolution of massive stars and the formation of black holes must be incomplete.”

This black hole is also the most distant stellar black hole ever observed and is located in a dwarf galaxy, Messier 33, that orbits Andromeda. Bailyn notes that, “Finding black holes in different and distant locations gives us many more objects to study and opens up the opportunity to find extreme examples that test theoretical limits.”

Lead author on the study is Jerome A. Orosz, associate professor in the Department of Astronomy at San Diego State University, and a former graduate student who studied with Bailyn. Other authors are Jeffrey McClintock, Ramesh Narayan, Joel Hartman, Jiefeng Liu, Lucas Macri, Wolfgang Pietsch, Ronald Remillard, Avi Shporer and Tseve Mazeh.

Scientists combined data from three observatories to make their discovery —NASA’s Chandra X-ray Observatory in orbit around the Earth; the Gemini North 8-meter telescope on the island of Hawaii; and the 2.1-meter and WIYN 3.5-meter telescopes at Kitt Peak National Observatory near Tucson.

For more information on black holes, see Bailyn’s “Black Hole Toolbox”: http://cmi2.yale.edu/bh.

— By Janet Rettig Emanuel

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