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Extreme Universe: Smallest Black Hole January 10, 2009

Posted by jcconwell in Astronomy, Black Holes, Extreme Universe.
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Not all records in astronomy are about the big stuff.  Good information comes in small packages. A classic case is XTE J1650-500, the smallest or lightest Black Hole measured.

Discovered in 2008 the lowest-mass known black hole belongs to a binary system.. The black hole has about 3.8 times the mass of our sun, and is orbited by a companion star, as depicted in this illustration.

smallbhjpg

Credit: NASA/CXC/A. Hobar

Using a new technique, two NASA scientists have identified the lightest known black hole. With a mass only about 3.8 times greater than our Sun and a radius of  11 kilometers, the black hole lies very close to the minimum size predicted for black holes, or the maximum mass neutron star that originate from dying stars.

The search for the smallest black holes is  important because of the information they tell us about neutron stars, which have a critical upper mass thought to no larger than 3 time the mass of the sun. This upper mass, very similar to the upper mass of white dwarf , which is about 1.4 Solar Masses called Chandrasekhar’s limit, is harder to calculate for the case of a neutron star.

Neutron stars have three extra complications, their rapid spin, the necessity of using general relativity to describe the gravitation, and most important, the nuclear forces at densities the exceed that of normal nuclei. Depending on the nature of the force you can get equations that relate the pressure and densities that very by a factor of 2 or more. resulting in different maximum mass neutron stars that depend on the nuclear force.

Thus the smallest black holes put constraints on the possible type of matter that make up neutron stars.

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