How close is the nearest black hole to Earth?

All across the Universe, massive stars collapse and die.

The anatomy of a very massive star throughout its life, culminating in a Type II (core-collapse) Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive core-collapse supernovae typically result in the creation of black holes, while the less massive ones create only neutron stars.

From core-collapse supernovae, neutron stars and black holes form.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. Direct collapse, while still under investigation, may be triggered by a stellar companion.

Stars and gas directly collapse, forming black holes.

This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions, and could lead to direct collapse black holes of an estimated ~40,000 solar masses. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the formation of stars and the growth of galactic structures.

Finally, neutron star mergers create black holes, too.

Artist’s illustration of two merging neutron stars. The rippling spacetime grid represents gravitational waves emitted from the collision, while the narrow beams are the jets of gamma rays that shoot out just seconds after the gravitational waves (detected as a gamma-ray burst by astronomers). Mass, in an event like this, gets converted into two types of radiation: electromagnetic and gravitational. About 5% of the total mass gets expelled in the form of heavy elements.

These black holes roam the Universe, devouring whatever matter contacts their event horizons.

On September 14, 2013, astronomers caught the largest X-ray flare ever detected from the supermassive black hole at the center of the Milky Way, known as Sagittarius A*. In X-rays, no event horizon is visible at these resolutions; the “light” is purely disk-like. However, we can be certain that only matter remaining outside the event horizon generates light; matter passing within it gets added to the black hole’s mass, inevitably infalling into the black hole’s central singularity. Many types of transients are now known to exist across many different wavelengths of light.

Inspiraling, merging objects emit gravitational waves, allowing black hole detections terrestrially.

A mathematical simulation of the warped space-time near two merging neutron stars that result in the creation of a black hole. The colored bands are gravitational-wave peaks and troughs, with the colors getting brighter as the wave amplitude increases. The strongest waves, carrying the greatest amount of energy, come just before and during the merger event itself. What occurs outside the event horizon is not practically affected by whether there is a ring singularity at the center, or some other, extended object that is non-singular.

We also detect the X-rays emitted by black holes feeding off of binary companions.

When a massive star orbits a stellar corpse, like a neutron star or black hole, the remnant can accrete matter, heating and accelerating it, leading to the emission of X-rays. These X-ray binaries were how all stellar mass black holes, until the advent of gravitational wave astronomy, were discovered, and are still how most of the Milky Way’s known black holes have been found.

These X-ray binaries, traditionally, have revealed the closest black holes: several thousands of light-years distant.

The most up-to-date plot, as of November 2021 (past the end of LIGO’s third data run but before the start of the fourth), of all the black holes and neutron stars observed both electromagnetically and through gravitational waves. While these include objects ranging from a little over 1 solar mass, for the lightest neutron stars, up to objects a little over 100 solar masses, for post-merger black holes, gravitational wave astronomy is presently only sensitive to a very narrow set of objects. The closest black holes had all been found as X-ray binaries, until the November 2022 discovery of Gaia BH1. The mass “border” between neutron stars and black holes is still being determined.

However, two other methods hold promise: microlensing and black hole-star binaries with detached orbits.

If a black hole were on a collision course with Earth, we wouldn’t have any warning from the black hole itself, but it would distort and bend the light from background objects, revealing its presence if we looked closely enough. The fact that mass bends spacetime, regardless of what types of light it gives off, is a key to finding black holes that may be hiding in the nearby Universe, and for giving us advance warning against a potential spaghettification event.

Microlensing occurs whenever a mass intervenes between a luminous object and ourselves.

When a gravitational microlensing event occurs, the background light from a star gets distorted and magnified as an intervening mass travels across or near the line-of-sight to the star. The effect of the intervening gravity bends the space between the light and our eyes, creating a specific signal that reveals the mass and speed of the object in question.

The characteristic brightening pattern reveals the interloper’s mass and other properties.

The relativistic, light-bending effects shown here are caused by the strong gravitational lensing effects of a foreground black hole. Both the background of the Milky Way and a lensed star are shown here. This method would reveal both a lensed star in a detached binary orbit with the black hole as well as an interloping black hole that caused a microlensing event.

Meanwhile, black holes orbiting normal stars will influence the star’s observed motion and position.

By tracking a star’s redshift-and-blueshift over time, a candidate companion’s mass can be uncovered.

The idea of the radial velocity (or stellar wobble) method is that if a star has an unseen, massive companion, whether an exoplanet or a black hole, observing its motion and position over time, if possible, should reveal the companion and its properties. This remains true, even if there’s no detectable light emitted from the companion itself.

Observing its changing position over time should match the companion candidate’s predictions, confirming its partner.

Overview of the radial velocities for Gaia-BH1 as obtained by the LAMOST survey and from follow-up observations with the MagE, GMOS, XSHOOTER, ESI, FEROS and HIRES spectrographs. Points with error bars are measurements, gray lines are drawn from the posterior when jointly fitting thse radial velocity spectra and the Gaia astrometric constraints.

The ESA’s Gaia mission leveraged this method, discovering today’s nearest black hole: Gaia BH1.

Just 1560 light-years away, this record is temporary.

Gaia BH1, at ~10 solar masses, with an orbital period of ~180 days, and located just 1560 light-years away, now holds the record (as of 2022) for closest black hole known to our Solar System.

Upcoming missions, like Nancy Roman, should reveal even closer black holes.

This illustration compares the relative sizes of the areas of sky covered by two surveys: the upcoming Nancy Roman Telescope’s High Latitude Wide Area Survey, outlined in blue, and the largest mosaic led by Hubble, the Cosmological Evolution Survey (COSMOS), shown in red. In current plans, the Roman survey will be more than 1,000 times broader than Hubble’s, revealing how galaxies cluster across time and space as never before, enabling the tightest constraints on evolving dark energy, and revealing more microlensing events, including possibly extremely close black holes, than ever before. Euclid is wider-field than Roman, but with inferior depth, resolution, and wavelength coverage.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.