EIU Astro

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.

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.

In keeping with the end of the year theme of the blog “Extremes in the Universe”. The day after Christmas, an irresistible pull… a point of no return… no we’re not talking about your holiday credit card bill or the after Christmas sales. We are talking black holes.

The heavyweight champion of the year, and at 18 billion solar masses, the largest known black hole in the universe was discovered in January of this year. The biggest black hole beats out its nearest competitor, the black hole in the heart of M87, by a factor of six.

Located in the heart of a quasar called OJ287, it’s at a distance of 3.5 billion light years .

Just like other champions, it had some help. In order to measure the black hole’s mass you need an object orbiting it. A smaller black hole, which weighs about 100 million Solar masses, orbits the larger one on an eccentric path every 12 years. It comes close enough to fly through the disc of matter around the larger black hole twice each orbit, causing a pair of outbursts, coming and going, that make OJ287 suddenly brighten.

General relativity predicts that the smaller hole’s orbit itself should precess over time. This effect  is seen in Mercury’s orbit around the Sun, on a much smaller scale.

In the case of OJ287, the immense gravitational field of the larger black hole causes the smaller black hole’s orbit to precess at an  39° each orbit! The precession changes the timing of when the smaller hole punches through the disc surrounding its larger companion.

About a dozen of the resulting bright outbursts have been observed to date, and astronomers led by Mauri Valtonen of Tuorla Observatory in Finland have analysed them to measure the precession rate of the smaller hole’s orbit. That, along with the period of the orbit, suggests the larger black hole weighs a record 18 billion Suns.

The most recent outburst occurred on 13 September 2007, as predicted by general relativity. “If there was no orbital decay, the outburst would have been 20 days later than when it actually happened,” Valtonen told New Scientist, adding that the black holes are on track to merge within 10,000 years, due to orbital decay caused by gravitational radiation.

Craig Wheeler, of the U of Texas, says the observations of the outbursts fit closely with the expectations from general relativity. “The fact that you can fit Einstein’s theory [so well] … is telling you that that’s working,” he says.