The smallest black hole ever

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Illustration of a black hole and its surrounding disk Credit: NASA

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April 7, 2008 Using measurements taken by the Rossi X-ray Timing Explorer satellite, NASA scientists have identified the smallest known black hole in the universe. At 3.8 times the mass of our Sun and estimated at only 15 miles in diameter, the black hole known as XTE J1650 is also close to the smallest size thought to be theoretically possible for such an object.

A star is basically a huge nuclear reactor at core, and a black hole is formed when it runs out of fuel and collapses upon itself to form an incredibly small but incredibly massive "singularity", a point in space with so much gravitational force that even light cannot escape. Stars need to be several times more massive than the Sun for a this to occur, if not a Neutron Star, rather than a Black Hole, will be the result. Astronomers believe that the minimum mass required for the creation of a Black Hole is around 3 times the weight of our Sun, meaning that the newly discovered Black Hole in the Milky Way Galaxy binary system named XTE J1650-500 is almost as small as they come.

The research was conducted by Nikolai Shaposhnikov and Lev Titarchuk at NASA’s Goddard Space Flight Center in Greenbelt, Md. Scientist have been aware of the presence of a lightweight black hole in the system since not long after its discovery in 2001 but its mass has never been measured with any accuracy. This information has only been discovered using a new technique that links the mass of the Black Hole to the X-rays being radiated from the surrounding discs of hot gases that are being dragged inwards by the object's massive gravitational forces. By measuring the regular pattern created by these X-rays (known as quasi-periodic oscillation, or QPO), the mass of the black hole can be established with X-rays are emitted on a shorter timescale for smaller black holes.

Shaposhnikov and Titarchuk verified their method by applying it to black holes whose masses had been measured by other techniques. "In every case, our measurement agrees with the other methods," says Titarchuk. "We know our technique works because it has passed every test with flying colors."

The measurement of the black hole's mass is due to high-precision timing observations made by NASA’s Rossi X-ray Timing Explorer, a small satellite that launched in late 1995 and made the first observations of the XTE J1650-500 in 2001.

Einstein's equations predict that a black hole with 3.8 times the mass of our Sun would be only 15 miles across -- the size of a city. “This makes the black hole one of the smallest objects ever discovered outside our solar system,” says Shaposhnikov.

Previously, the smallest known black hole would weigh about 6.3 Suns. The black hole that center of the Milky way by comparison, is thought to have a mass somewhere around three million times that of our Sun and others are estimated to be billions of times more massive.


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