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.'"

interesting, but vaguely in line with estimates

[Trepidity]

There have been a bunch of claims of black holes roughly in the range 3-4x solar masses, some subsequently revised upwards (this one [wikipedia.org] made some news in 2008, and there are some other candidates as well). The "normal" range for stellar black holes is roughly 3-30x solar masses, according to current understanding.

Anyone have a link to a good explanation of the current estimated values for a minimum? My understanding is that there isn't really a theoretical physical minimum (black holes can exist at any size), but that there's a mass level beneath which astrophysicists consider it very unlikely that conditions would have really existed to produce a black hole through stellar collapse of a star. But I can't seem to find a solid estimate of what that number is, just these sorts of indirect references to 3x being "close" to the minimum (looking at Google Books, I find an old textbook that also mentions 3.2x as "just above" the theoretical minimum, but doesn't elaborate).

Some of the last science for RXTE

[Anonymous Coward]

This is the probably the last big science release for Rossi X-ray Timing Explorer (RXTE). RXTE will be turned off at the end of this year and disabled. This is mostly because of NASA budget reductions and reduced prioritization by peer review committees, but also because of its aging on-board systems. The systems were designed for a two year life-time but lasted for more than sixteen! The satellite will probably remain in orbit for quite a few more years - passive - before re-entering the atmosphere. The scientific community will lose the only working X-ray observatory that can measure the fast heartbeats of black holes and neutron star systems, and do complicated monitoring observations.

RXTE has done a lot of great science in the past 16 years, some of it featured [slashdot.org] here [slashdot.org] on Slashdot [slashdot.org]. The legacy will live on though, since the data archive will remain publicly available. Sometimes great science can come from the archive as well!

(speaking for myself only) CM

Hawking evaporation limit

[TuringCheck]

A black hole that would grow by absorbing radiation instead of shrinking at the ~4K average temperature of the universe can be around the mass of the Moon. Such a small black hole cannot form just by gravitational collapse but can go on indefinitely unless the universe cools down.

The question here was about the minimum mass a star can have to become a black hole instead of remaining a neutron star - or maybe something more exotic but still fighting gravity. The Tolman–Oppenheimer–Volkoff equation gives an estimate but since noone managed to observe exactly what happens with matter at the densities found in a neutron star there are still a lot of assumptions.

Re:interesting, but vaguely in line with estimates

[Anonymous Coward]

Evaporation is an issue for very small black holes, and usually important to question of primordial black holes that have been around since when the early universe could have created them without requiring a star collapse. While a kilogram sized black hole would evaporate on the order of femtoseconds, and even a million kilogram one would be gone in a minute, the evaporation time goes with mass cubed. So by the time you get to a single solar mass, it is 10^67 years to evaporate, as it is radiating at about 10^-28 W. Even if you could squeeze the Earth into a sphere about a centimeter across to get a black hole, it would last for 10^50 years.