Contrary to what their name might imply, black holes aren't just vast areas of nothingness. A black hole is an astronomical object whose gravitational pull is so strong that nothing can escape it, not even light. Known as the "event horizon," the "surface" of a black hole marks the point where the speed required to escape is faster than the speed of light. Once radiation and matter are in, they're in it for the long haul.
But how close is the nearest black hole to Earth (that we know of)? It turns out it’s not that far in the grand scheme of things.
GAIA BH1: the closest black hole to Earth
Gaia BH1 is a mere 1,600 light years away, in the constellation Ophiuchus. This is three times closer to Earth than the previous record holder, an X-ray binary in the constellation Monoceros. Gaia BH1 was discovered in 2022 by the European Space Agency’s Gaia space telescope. The find was made possible by careful observations of the black hole's companion, a Sun-like star orbiting the black hole at about the same distance as the Earth orbits the Sun.
“Take the Solar System, put a black hole where the Sun is, and the Sun where the Earth is, and you get this system,” said Kareem El-Badry, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian and the Max Planck Institute for Astronomy, and the lead author of the paper describing last year’s find. "This is the first unambiguous detection of a Sun-like star in a wide orbit around a stellar-mass black hole in our Galaxy.”
To investigate the system further, El-Badry and his colleagues used the Gemini Multi-Object Spectrograph instrument on Gemini North, which measured the companion star's velocity as it orbited the black hole and provided a precise measurement of its orbital period.
“Our Gemini follow-up observations confirmed beyond reasonable doubt that the binary contains a normal star and at least one dormant black hole,” El-Badry said. “We could find no plausible astrophysical scenario that can explain the observed orbit of the system that doesn’t involve at least one black hole.”
The Gaia BH1 system is unusual because it contradicts scientists' current understanding of how black holes form. BH1 is massive enough to have become a supergiant early in its life as a star. As a matter of fact, it should have already grown large enough to consume its companion star long before it matured into what it is today.
“It is interesting that this system is not easily accommodated by standard binary evolution models,” concluded El-Badry. “It poses many questions about how this binary system was formed, as well as how many of these dormant black holes there are out there.”
Telescopes that can see x-rays, light and other forms of electromagnetic radiation cannot be used to observe black holes directly. However, by studying the effect of black holes on nearby matter, scientists can infer their existence and learn more.
For example, if a black hole travels (yes, they can move) through a cloud of interstellar matter, it will accrete some of that cloud into itself. A similar process can occur if a regular star comes close enough to a black hole. As the star is drawn closer to the black hole, it risks being torn apart.
Due to its increased speed and temperature, the attracted matter releases x-rays into the surrounding space, whose signatures astronomers can detect and use to identify black holes. New evidence suggests that black holes profoundly impact their local environments, whether by emitting powerful gamma-ray bursts, devouring nearby stars, or stimulating the growth of new stars in some regions while stalling it in others.
What are the closest black holes to Earth?
Besides BH1, astronomers have identified a number of other nearby black holes. Ordered from the closest to the farthest, these include:
- A0620-00 (3,300 light-years away). A0620-00 consists of two objects, a K-type main-sequence star and a stellar-mass black hole. The two objects orbit each other every 7.7 hours.
- Gaia BH2 (3,800 light-years away). Gaia BH2 is a binary system consisting of a red giant and what is very likely a stellar-mass black hole. Gaia BH2 was originally discovered as a black hole binary candidate in 2022, alongside Gaia BH1.
- MOA-2011-BLG-191 (5,000 light-years away). OGLE-2011-BLG-0462, also known as MOA-2011-BLG-191, is a stellar-mass black hole isolated in interstellar space, in the direction of the galactic bulge in the constellation Sagittarius.
- GRS 1124-683 (5,400 light-years away). The gamma-ray and X-ray source GRS 1124-683, discovered by the Granat mission and Ginga, is a system containing a black hole candidate.
- XTE J1118+480 (5,700 light-years aways). XTE J1118+480 is a low-mass X-ray binary in the constellation Ursa Major. It is a soft X-ray transient that most likely contains a black hole and is probably a microquasar.
Where do black holes come from?
The remnants of massive stars that explode as supernovae are the primary source of black holes. Neutron stars can't trap light because they aren't massive enough to form from smaller stars. If the star’s total mass is large enough- about three times the mass of the Sun- theoretically, it can be proven that no force can keep the star from collapsing under the influence of gravity. As the star disintegrates, however, a peculiar phenomenon takes place. When a star's surface approaches the event horizon, local time slows down compared to the time recorded by distant observers. After a star's surface reaches that point of no return, the collapse process is frozen in place and the star ceases to shrink.
Black holes created by cosmic collisions are even more massive than those created by supernovae. NASA's Swift telescope first observed transient, intense gamma-ray bursts in the months following its launch in December 2004. The Chandra X-ray Observatory and the Hubble Space Telescope also picked up on the intense explosions, leading researchers to conclude that these powerful explosions result from the merger of a black hole and a neutron star, forming yet another black hole.
Even though we know how black holes are made at their most basic level, we still don't know how they can exist on two such different size scales. On one end, you have the many black holes that are all that's left of once-massive stars. These "stellar mass" black holes range in mass from roughly 10 to 24 times that of the Sun, and can be found across the cosmos. When another star comes close enough to a black hole, part of the matter in its surroundings is sucked into the black hole by its gravity, emitting x-rays that can be detected by astronomers. Based on the number of massive stars in the Milky Way, scientists estimate there could be as many as a billion black holes.
At the other extreme, there are the "supermassive" black holes, millions to billions of times as massive as the Sun. Supermassive black holes are thought to reside at the galactic core of nearly all major galaxies, including our own Milky Way. Astronomers can monitor their effects on nearby stars and gas to help identify them.
Researchers have long held the firm belief that no black holes exist in the intermediate size range. However, recent data from Chandra, XMM-Newton and Hubble bolster the case for the existence of black holes of intermediate size. The accumulation of extremely massive stars, which then collapse to form intermediate-mass black holes, is a potential mechanism for forming supermassive black holes through a chain reaction of colliding stars in compact star clusters. Following this, the star clusters fall to the galaxy's center, where the intermediate-mass black holes merge to form a supermassive black hole.