A black hole discovered in the spiral galaxy NGC 300 sets a new distance record at six million light years from the Sun, and with a mass of twenty times that of the Sun it is also the second most massive stellar-mass black hole known.

The record-breaking black hole was discovered by astronomers using ESO's Very Large Telescope (VLT) in Chile. “This is the most distant stellar-mass black hole ever weighed, and it’s the first one we’ve seen outside our own galactic neighborhood, the Local Group,” says Paul Crowther, Professor of Astrophysics at the University of Sheffield and lead author of the paper reporting the study that appears in the journal Monthly Notices of the Royal Astronomical Society.

Stellar-mass black holes are the dense end products of collapsed, massive stars and generally weigh no more than about twenty solar masses. In contrast, supermassive black holes – found in the centre of most galaxies – boast masses equivalent to millions or even billions times that of the Sun. The new discovery brings the number of known black holes with masses greater than fifteen times that of the Sun to three, but the known twenty stellar-mass black holes in our own Galaxy weigh up to just ten solar masses.

As well its impressive mass and distant location, the black hole was also found to be entwined with a Wolf-Rayet star, also weighing in with twenty solar masses, that will soon become a black hole itself. Wolf-Rayet stars are near the end of their lives and expel most of their outer layers into their surroundings before exploding as supernovae, with their cores imploding to form black holes.

Clues to the existence of this strange pair came in 2007 with observations from NASA's Swift observatory of a bright X-ray source in NGC 300 – originally discovered by ESA's XMM-Newton X-ray observatory – that showed periodic, extremely intense X-ray emission, a tell-tale sign of the presence of a black hole. Using the FORS2 instrument on the VLT the presence of the black hole was confirmed, and detailed observations revealed that the pair are bound in a waltz around each other in a 32-hour period. Furthermore, the black hole is stripping the Wolf-Rayet star of its matter.

“This is indeed a very ‘intimate’ couple,” notes collaborator Robin Barnard. “How such a tightly bound system has been formed is still a mystery.”

Systems comprising a black hole and a companion star are not uncommon, but only one other system of this type has previously been seen. Nonetheless, astronomers are beginning to see a connection between black hole mass and galactic chemistry. “We have noticed that the most massive black holes tend to be found in smaller galaxies where ‘heavy’ chemical elements are less abundant,” says Crowther. Heavy elements are chemical elements heavier than helium, such as oxygen, silicon and carbon. “Bigger galaxies that are richer in heavy elements, such as the Milky Way, only succeed in producing black holes with smaller masses.” In other words, a higher concentration of heavy chemical elements increases the amount of matter that the star sheds, resulting in a smaller black hole when the remnant finally collapses.

What of the future of NGC 300's curious couple? Within the next one million years, say astronomers, the Wolf-Rayet star will explode in spectacular supernova explosion and collapse into a black hole. “If the system survives this second explosion, the two black holes will merge [over the following few billion years], emitting copious amounts of energy in the form of gravitational waves as they combine,” says Crowther. “Our study does however show that such systems might exist, and those that have already evolved into a binary black hole might be detected by gravitational wave observatories like LIGO or Virgo.”

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