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Options to save Hubble
NASA Administrator Sean O'Keefe announces plans to examine a robotic servicing mission to the Hubble Space Telescope. (33min 51sec file)
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Station supply ship
Ride along with the Progress 14P resupply ship as it makes the final approach and docking to the International Space Station on May 27 as seen by a camera mounted on the craft's nose. (9min 02sec file)
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Results from Spitzer
Scientists present new discoveries from the Spitzer Space Telescope, including their findings of raw ingredients for life detected around young stars. (53min 03sec file)
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Spacewalk previewed
The Expedition 9 crew describes their upcoming spacewalk in Russian spacesuits, life aboard the space station and the view of Earth in this interview with Bill Harwood of CBS News. (20min 19sec file)
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Progress undocking
The Progress 13P cargo ship departs the International Space Station on May 24 carrying trash and unneeded items to burn up in the atmosphere. (2min 56sec file)
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AP interviews the crew
The Associated Press interviews the two-man Expedition 9 crew living aboard the International Space Station on May 24. (9min 36sec file)
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Atlas launches AMC-11
The Lockheed Martin Atlas 2AS rocket launches from Cape Canaveral carryin the AMC-11 communications satellite. (4min 30sec file)
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Monster lies camouflaged inside nebula's heart
Posted: June 2, 2004

Most galaxies, including the Milky Way, are filled with giant clouds of gas and dust called nebulae that appear as dark silhouettes against the starry background. Nebulae shine only when illuminated or excited by nearby energy sources.

Usually, the energy source is one or more stars. But today at the 204th meeting of the American Astronomical Society in Denver, Colorado, Smithsonian astrophysicist Philip Kaaret (Harvard-Smithsonian Center for Astrophysics) announced that one nebula is illuminated by X-rays from a black hole. Moreover, the brightness of the nebula suggests that the X-ray source may be an intermediate-mass black hole many times larger than most stellar black holes.

A rare intermediate-mass black hole powers a 100 light-year-wide nebula in the dwarf irregular galaxy Holmberg II. Weighing more than 25 times the mass of our Sun, this black hole may be a remnant from the first generation of stars in the Universe. Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics.
This surprising find offers only the second known example of a black hole-illuminated nebula, after LMC X-1 in the Large Magellanic Cloud, and the first example of a nebula powered by an intermediate-mass black hole.

"Astronomers always get excited about new things, and this nebula is certainly something new. Finding it is like getting a royal flush the first time you play poker - it's that rare," said Kaaret.

Initially discovered by Manfred Pakull and Laurent Mirioni (University of Strasbourg), the nebula is located 10 million light-years away in the dwarf irregular galaxy Holmberg II. Two years ago, Pakull and Mirioni noted that it seemed to be associated with an ultraluminous X-ray source.

By combining observations from NASA's Hubble Space Telescope and Chandra X-ray Observatory with those from ESA's XMM-Newton spacecraft, Kaaret and his colleagues, Martin Ward (University of Leicester) and Andreas Zezas (CfA), pinpointed the X-ray source at the center of the nebula. Moreover, the mystery source is pouring out X-rays at a tremendous rate, shining one million times brighter in X-rays than the Sun shines at all wavelengths of light combined.

The observations by Kaaret and his associates indicate that those X-rays are generated by a black hole gobbling matter from a young, massive companion star at a rate of about one Earth mass every four years. That modest accretion rate is sufficient to ionize and light up a huge 100-light-year-wide swath of the surrounding nebula.

The X-ray emissions provide an important clue to the nature of the black hole. Some astronomers have suggested that X-rays from the source in Holmberg II and similar bright sources are beamed in the Earth's direction like a searchlight. Such beaming would make the X-ray source appear brighter than it really is, thereby making the black hole appear more massive than it really is.

Kaaret's data contradict that view, showing instead that the black hole in Holmberg II sends out X-rays evenly in all directions. Therefore, its brightness suggests that it must be more massive than any stellar black hole in our own Galaxy, weighing in at more than 25 times the mass of the Sun and likely more than 40 solar masses. That would rank it as an "intermediate-mass" black hole.

"It's not easy to explain how intermediate-mass black holes form. Since we only have a few examples to study, every new find is important," said Kaaret.

This research will be published in a paper co-authored by Kaaret, Ward and Zezas in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.