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Smallest Known Black Hole Found
smitty777 writes "Adding to the recent black hole discoveries of gas clouds and a quasar accretion disc, Forbes is reporting on a recent discovery by NASA's Rossi X-Ray Timing Explorer (RXTE) on the smallest known black hole. From the article: 'If the astronomers' calculations are correct, this black hole is located about 16,000 to 56,000 light years away from Earth (a more precise distance hasn't yet been determined). The black hole itself is only about three times the mass of the Sun, which means that the original star was just barely big enough to form a black hole.'"
Replying to myself: it appears that the minimum is related to the Tolman–Oppenheimer–Volkoff limit for the maximum mass of a neutron star, which isn't known to great accuracy. Wikipedia cites a 1996 journal article with an estimate of "approximately 1.5 to 3.0 solar masses".
10 PRINT CHR$(205.5+RND(1)); : GOTO 10
I may be wrong as I'm not a physicist, but as I understand it that chandrasekhar limit applies only if a black hole is formed through a star collapsing. If a black hole is formed by some other means then its mass could be something entirely different, including the minimum limit for forming one. As wikipedia so helpfully lists there are two known ways for a black hole to form: gravitational collapse and high-energy collisions. There could be some as-of-yet-unknown means, too.
The more precise answer to the OP's question thus would seem to be: the Planck mass
NIH Restricts Use of Chimpanzees in Labs - and how is this a science article?
Is that supposed to be a trick question? If this goes through, it's a significant constraint on animal research (the science angle that you're looking for) that could expand in the future.
Here is the scientific paper. It makes no claim whatsoever about the mass of IGR J17091-3624. On p. 6, they say:
Figure 5 implies that if IGR J17091-3624 emits at Eddington, then either it harbors the lowest mass black hole known today (< 3Msolar for distances lower than 17 kpc), or, it is very distant. Such a large distance, together with its b ~2.2deg Galactic latitude, would imply a significant, but not necessarily implausible, altitude above the disk
Here is the NASA press release summarizing the paper for people who aren't scientists. It quotes the lead author as saying:
Just as the heart rate of a mouse is faster than an elephant's, the heartbeat signals from these black holes scale according to their masses
The Forbes article morphs this into "NASA Satellite May Have Found The Smallest Known Black Hole," and says, "An international team of astronomers utilizing NASA's Rossi X-Ray Timing Explorer (RXTE), believe that they've identified a candidate for the smallest known black hole[...]"
The slashdot summary says:
The black hole itself is only about three times the mass of the Sun[...]
This is completely incorrect. It's a candidate for a very low mass black hole. What that means is that they're suggesting that astronomers do follow-up observations on this object and actually determine its mass, which may be unusually low.
It is of very great interest to relativists and astronomers to find the smallest black holes. There is a limit called the Tolman-Oppenheimer-Volkoff limit on the largest mass that a neutron star can have. There are big theoretical uncertainties in this number, but it is probably around three solar masses. However, we don't know for sure whether anything too massive to be a stable neutron star necessarily becomes a black hole. There have been all kinds of goofy objects hypothesized by theorists that might be intermediate between neutron stars and black holes, including black stars, gravastars, fuzzballs, quark stars, boson stars, and electroweak stars. Observing a low-mass black hole narrows the gap in mass between the heaviest stable neutron star and the lightest black hole, leaving less wiggle room to believe in these exotic objects.
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I am a physicist, although not an astronomer. Indeed, microscopic black holes (less than the earth mass) are speculated to exist. They're called primordial black holes and must be created in the early universe.
They're candidates for the sources of gamma ray bursts.