Biggest stellar mass black hole found

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Oct 30, 2007
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More cool astronomy news: astronomers have found the most massive stellar black hole so far, weighing in at about 24 - 33 times the mass of the Sun.

Drawing of IC 10 X-1 by artist (and my friend and ex-coworker!) Aurore Simonnet. Courtesy of NASA.

Stellar mass black holes are called that because they are in the same mass range as stars (supermassive black holes are millions or billions of times the Sun's mass and reside in galactic cores; intermediate mass BHs have hundreds of solar masses and are generally seen in star clusters, where "food" for the black hole is plentiful and they can grow easily). Stellar mass black holes form when a massive star explodes. The outer layers blow off, but the core collapses inward. If the mass of the core is high enough, it forms a black hole.

Picture of IC 10. Courtesy of NASA, Adam Block/NOAO/AURA/NSF

The black hole just found is in another galaxy, a nearby dwarf galaxy named IC 10 (the black hole itself is called IC 10 X-1). It's a little less than 2 million light years from us, putting it substantially closer than even the Andromeda Galaxy. If a black hole has a companion star orbiting it, then it can siphon off and gobble down matter form that star. As the matter falls in to the hole, it heats up... a lot. It can get to millions of degrees, and give off X-rays.

That's how this one was found; the Chandra X-ray Observatory spotted its emission. The observations also indicated that the X-rays dipped in brightness with a regular period, which is almost certainly due to our view of the black hole being blocked by the companion star as it orbits the hole. We're seeing black hole eclipses!

More than just being a curiosity, this gives us critical info on the hole: by observing the orbiting stars, the orbits and therefore the masses can be calculated (by using Kepler's Laws, first formulated in the early 1600s!). When this was done (using Swift), astronomers were surprised to find the black hole was at least 24 solar masses. As far as we know, black holes in our own Galaxy are formed with much lower mass than that, and it doesn't look like this one could have eaten enough of the other star to get this big. It really looks like it was this massive to start with.

This means that something odd is going on. A likely explanation is that the star that formed the black hole originally had fewer heavy elements (heavier than helium, that is, like carbon and manganese) than usual, so it would have had a weak solar wind. During its life, a star can lose a lot of mass through its solar wind. If this star didn't have a strong wind, then it could have retained a lot of its original mass, forming a more massive black hole.

More observations of the black hole are planned to narrow down the mass (the observations have only been able to get a lower limit on the mass; it might be bigger). This also makes it possible that such portly black holes exist in our Galaxy too, though none has ever been found. We think that there are millions of black holes in the Milky Way, but since most of them are loners, we don't see them; they only announce their presence when they're eating material. There could be lots of black holes like IC 10 X-1 closer by, but we just don't see them. Don't worry-- there's none close enough to hurt us. I mean close in an astronomy sense, like a quadrillion kilometers or so. :-)

Finding oddballs, objects at extremes, pushes our ideas hard, letting us know where they are strong and where they are weak. So finding objects like IC 10 X-1 is more than just an announcement of a record breaker, it's actually yet another tool in our workshed helping us understand the Universe.