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SDSU Astronomer Leads Team in Discovery of Largest Known Stellar Black Hole

Phenomenon challenges standard theories of black hole formation

Astronomers announced today the discovery of an exceptionally large black hole in orbit around a huge companion star in the nearby galaxy known as Messier 33. In a study published in the October 18 issue of the journal Nature, a multi-national team led by San Diego State University’s Associate Professor of Astronomy Jerome Orosz confirms the black hole M33 X-7, which – measured at 15.7 times the size of our sun – is the most massiveand the most distant stellar black hole known.

The black hole is part of a binary system in which it orbits a companion star every 3.45 days.  The orientation of the orbit is such that the X-ray source is eclipsed by its companion star every orbital period, which allows for a much more precise determination of the orbital properties of the binary relative to other binary systems that contain a black hole.  The companion star to M33 X-7 also has an unusually large mass, about 70 times that of the Sun. 

“This discovery raises all sorts of questions about how such a big black hole could have been formed in such a close binary system,” said SDSU’s Orosz.  “Standard theories would tell us that either the black hole would end up widely separated from its companion (or not bound at all) or that the black hole would have merged with its companion.   M33 X-7 will be a key system in understanding both the formation of massive stellar black holes and evolution of massive binary stars.”

A stellar black hole is created by the gravitational collapse of a star initially more massive than about 20 times the mass of the Sun.  The gravitational force from the resulting object is so intense that not even light can escape its immediate grasp, making the object invisible.  However, when a black hole interacts with outside matter – as when it draws in gas from an orbiting star – the inward-spiraling gases become heated and emit large amounts of X-ray radiation that can be detected by X-ray telescopes in Earth orbit.  

M33 X-7 was discovered as an X-ray source almost 25 years ago; but only recently was the X-ray source identified with an optical star.  Jerome Orosz and his fellow investigators combined data from NASA’s space-based Chandra X-ray Observatory and the 8.2 meter Gemini telescope on Mauna Kea, Hawaii to measure the masses of the components of M33 X-7 and establish the presence of a black hole. 

Of the 23 campuses in the California State University system, SDSU is the only one with a department of astronomy, and as such the only campus to offer both bachelor's and master's degrees in astronomy. The department is actively engaged in research projects in which students are encouraged to participate under the direction of its eight faculty. The department provides excellent research and instructional facilities with its Mount Laguna Observatory, CCD Laboratory, extensive computer network, and dedicated engineering/support staff.

San Diego State University is the oldest and largest higher education institution in the San Diego region. Since it was founded in 1897, the university has grown to offer bachelor's degrees in 81 areas, master's degrees in 73 areas and doctorates in 16 areas. SDSU's approximately 35,000 students participate in an academic curriculum distinguished by direct contact with faculty and an increasing international emphasis that prepares them for a global future. For more information, visit www.sdsu.edu.

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