Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
We just found the first one within 1,000 light-years of us. But there’s probably one much, much closer.
For a long time, black holes were known to exist only in the imaginations of theorists.
Both inside and outside the event horizon of a Schwarzschild black hole, space flows like either a moving walkway or a waterfall, depending on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape. (ANDREW HAMILTON / JILA / UNIVERSITY OF COLORADO)
With enough mass concentrated into a small enough volume, not even light moves fast enough to escape.
When a black hole and a companion star orbit one another, the star’s motion will change over time owing to the gravitational influence of the black hole, while matter from the star can accrete onto the black hole, resulting in X-ray and radio emissions. (JINGCHUAN YU/BEIJING PLANETARIUM/2019)
First predicted in 1916 in General Relativity, the first one wasn’t discovered in space until 1964: Cygnus X-1.
Cygnus X-1, the first observationally discovered black hole (from 1964), is thought to be a star orbiting a black hole, with X-ray emission as imaged here by NASA’s Chandra X-ray observatory showing the spatial density of the signal. (NASA/CXC)
When black holes orbit another object, they can siphon and accelerate matter, creating X-ray and radio signatures.
This illustration of a black hole, surrounded by X-ray emitting gas, showcases one of the major ways black holes are identified and found. Based on recent research, there may be as many as 100 million black holes in the Milky Way galaxy alone. (ESA, RETRIEVED VIA HTTP://CHANDRA.HARVARD.EDU/RESOURCES/ILLUSTRATIONS/BLACKHOLES2.HTML)
Black holes also gravitate; we can infer a black hole’s presence as it influences its companions.
Black holes are regions of space where there’s so much mass in such a small volume that there exists an event horizon: a region from within which nothing, not even light, can escape. Yet this doesn’t necessarily mean that black holes suck matter in; they simply gravitate, and can remain in stable binary, trinary, or even larger star systems just fine. (J. WISE/GEORGIA INSTITUTE OF TECHNOLOGY AND J. REGAN/DUBLIN CITY UNIVERSITY)
We later discovered X-ray and radio emissions from galactic centers, indicative of supermassive black holes.
The second-largest black hole as seen from Earth, the one at the center of the galaxy M87, is shown in three views here. At the top is optical from Hubble, at the lower-left is radio from NRAO, and at the lower-right is X-ray from Chandra. These differing views have different resolutions dependent on the optical sensitivity, wavelength of light used, and size of the telescope mirrors used to observe them. These are all examples of radiation emitted from the regions around black holes, demonstrating that black holes aren’t so black, after all. (TOP, OPTICAL, HUBBLE SPACE TELESCOPE / NASA / WIKISKY; LOWER LEFT, RADIO, NRAO / VERY LARGE ARRAY (VLA); LOWER RIGHT, X-RAY, NASA / CHANDRA X-RAY TELESCOPE)
Even without visible companions, we’ve identified dozens of black holes directly: through their gravitational waves.
A still image of a visualization of the merging black holes that LIGO and Virgo have observed as of the end of Run II. As the horizons of the black holes spiral together and merge, the emitted gravitational waves become louder (larger amplitude) and higher pitched (higher in frequency). With the third observing run now complete, the number of black hole-black hole mergers seen has increased from about a dozen to more than 50. (TERESITA RAMIREZ/GEOFFREY LOVELACE/SXS COLLABORATION/LIGO-VIRGO COLLABORATION)
Sagittarius A*, at the center of the Milky Way, is the closest supermassive black hole, some 25,000 light-years distant.
The supermassive black hole at the center of our galaxy, with an X-ray flare as imaged by Chandra. 19 years of Chandra data allows us to better remove any instrumentation errors; so it will be with the EHT data in the radio, which suffers the additional effects of atmospheric turbulence. (X-RAY: NASA/UMASS/D.WANG ET AL., IR: NASA/STSCI)
A smaller one — just 6.6 solar masses — orbits a Sun-like star just 3,500 light-years away: V616 Monocerotis.
The X-ray source V616 Monocerotis is fascinating from a mass perspective: a black hole of ~6.6 solar masses is orbiting a low-mass star just 40% as heavy as our Sun. But twice in recorded history, once a century ago and once in 1975, it has become an X-ray nova. If that was indicative of a pattern, we should see it flare again in the 2030s. (SLOAN DIGITAL SKY SURVEY)
This artist’s impression shows the orbits of the objects in the HR 6819 triple system. This system is made up of an inner binary with one star (orbit in blue) and a newly discovered black hole (orbit in red), as well as a third object, another star, in a wider orbit (also in blue). (ESO/L. CALÇADA)
The system HR 6819, located in the constellation of Telescopium (in the southern hemisphere, about 20 degrees south of the ‘base’ of the teapot in Sagittarius), is a 5th magnitude star with two optical components and one invisible (black hole) component. If you live at tropical latitudes or below, you should be able to easily see it from May through September. (DIGITIZED SKY SURVEY 2, N. RISINGER (SKYSURVEY.ORG))
As our methods and surveys continue to improve, closer black holes will inevitably be discovered.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
