Astronomers Uncover Black Hole Closer to Earth Than Ever Before

Gemini North Telescope on Hawaii Reveals First Dormant, Stellar–Mass Black Hole

Astronomers have discovered the closest black hole to Earth, the first unambiguous detection of a dormant stellar-mass black hole in the Milky Way. Its close proximity to Earth, a mere 1,600 light-years away, offers an intriguing target of study to advance understanding of the evolution of binary systems.

The U.S. National Science Foundation provided funding support for the work. The astronomers used the Gemini North Telescope on Hawaii, operated by NSF’s NOIRLab, one of the twin telescopes of the International Gemini Observatory.

The discovery was made possible by making observations of the motion of the black hole’s companion, a sunlike star that orbits the black hole at about the same distance as the Earth orbits the sun. Though there are likely millions of stellar-mass black holes in the Milky Way galaxy, those few that have been detected were uncovered by their energetic interactions with a companion star. As material from a nearby star spirals in toward the black hole, it becomes superheated and generates powerful X-rays and jets of material. If a black hole is not actively feeding, it is dormant and not directly detectable.

This dormant black hole is about 10 times more massive than the sun and is located about 1,600 light-years away in the constellation Ophiuchus, making it three times closer to Earth than the previous record holder.

“Take the solar system, put a black hole where the sun is, and the sun where the Earth is, and you get this system,” explained Kareem El-Badry, an astrophysicist and lead author of a paper published in Monthly Notices of the Royal Astronomical Society.

The team relied not only on Gemini North’s superb observational capabilities but also on Gemini’s ability to provide data on a tight deadline, as the team had only a short window in which to perform their follow-up observations.

“As part of a network of space- and ground-based observatories, Gemini North has not only provided strong evidence for the nearest black hole to date but also the first pristine black hole system, uncluttered by the usual hot gas interacting with the black hole,” said NSF Gemini Program Officer Martin Still.

A black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping. Albert Einstein's theory of general relativity, which describes gravitation as the curvature of spacetime, predicts that any sufficiently compact mass will form a black hole.[4] The boundary of no escape is called the event horizon. In general relativity, crossing a black hole's event horizon traps an object inside but produces no locally detectable change. General relativity also predicts that every black hole should have a central singularity, where the curvature of spacetime is infinite.

Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century. In 1916, the first solution of general relativity that would characterise a black hole was found. By the late 1950s, this solution began to be interpreted physically as a region of space from which nothing can escape. Black holes were long considered a mathematical curiosity; it was not until the 1960s that theoretical work showed they were a generic prediction of general relativity. The first widely accepted black hole was Cygnus X-1, an x-ray source proposed as a black hole binary through several studies between 1971 and 1974.

Black holes typically form as part of a supernova event when massive stars collapse at the end of their life cycle. After a black hole has formed, it can grow by absorbing mass from its surroundings. Supermassive black holes of millions of solar masses may form by absorbing stars and merging with other black holes, or via direct collapse of gas clouds. There is consensus that supermassive black holes exist in the centres of most galaxies.

Quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the rate of emission being inversely proportional to the mass. This causes the black hole to lose mass very slowly, provided it is not accreting matter. However, even the smallest class of black holes observed, stellar black holes, are gaining mass from the cosmic microwave background faster than they are losing mass via Hawking radiation.

The presence of a black hole can be inferred through its interaction with matter and electromagnetic radiation such as visible light. Matter falling toward a black hole can form an accretion disk of infalling plasma, heated by friction and emitting light. In extreme cases, this creates a quasar, some of the brightest objects in the universe. Merging black holes can be detected by the gravitational waves they emit. If stars are orbiting a black hole, their motions can be used to determine the black hole's mass and location. In this way, astronomers have identified numerous stellar black hole candidates in binary systems and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.