How Do We Know There Are Black Holes?

Motion of "S2" and other stars around the central Black Hole
An international team of astronomers, lead by researchers at the Max-Planck Institute for Extraterrestrial Physics (MPE), has directly observed an otherwise normal star orbiting the supermassive black hole at the center of the Milky Way Galaxy.

Black holes are among the most mysterious and fascinating features of the universe, captivating scientists since the 18th century, including Albert Einstein and Stephen Hawking. They are often described as consuming their surrounding gas, the result of gravity so intense that nothing can escape its pull, not even the fastest known traveler in the universe: light itself. But if black holes don’t emit or reflect light, which means we can’t see them, how do astronomers know they are there?

The answer actually applies to many subjects studied in physics and deep-space astronomy—when you can’t observe something directly, or you can’t explain something you are seeing, you make educated guesses based on what you do see: the effect on other objects. On Earth, you can know it’s a windy day without stepping outside, because a flag is flapping. Astronomers know there is a black hole when the stars or gas around it are distorted or otherwise changed. These effects show up in a few ways.

Astronomers can observe a star accelerating in orbit around an unseen companion, rather than a detectable binary companion star (see video above). By measuring the orbiting star’s rate of acceleration, astronomers can calculate the mass of the object pulling on it; when this mass is so large that nothing else can explain it, astronomers conclude it is a black hole.

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Artist’s illustration shows the black hole Cygnus X-1 pulling matter from a companion blue star. Credit: NASA
In other instances, X-ray telescopes can observe electromagnetic radiation from a star that comes close to a black hole and is pulled apart by its gravity. These black holes accumulate cosmic matter around themselves in a swirling pattern called an accretion disk. Gas particles in the disc accelerate and collide, heating to millions of degrees and giving off the detectable X-rays.

Black holes causing these types of phenomena are at least three times the mass of the Sun and are classified as stellar black holes.

On a larger scale, supermassive black holes have a mass of more than one million Suns, and so must develop and grow very differently than stellar black holes. According to observations of intense gravitational attraction and energy in the center of galaxies made by the Hubble Space Telescope since the early 1990s, there is evidence for supermassive black holes at the heart of nearly all large galaxies, including our Milky Way.

With greater mass and thus greater gravity, supermassive black holes attract more matter that becomes hotter and more volatile. In some cases this super-heated matter shoots off into the universe as jets of gas that can be millions of light-years long. Astronomers have observed these gas ejections, known as outflow, and see it as a way of regulating galaxy expansion and the birth of new stars.

To see the dynamic dance of dust, stars, and gas clouds being shred apart and falling into a black hole, astronomers need powerful telescopes. Hubble made significant progress in confirming and studying galactic supermassive black holes.The James Webb Space Telescope’s powerful infrared instruments will see through the cosmic dust and analyze the details of outflow, providing new information about this process and its role in galaxy evolution.

Webb will also be able to “see” deeper into space, and thus further back in time, to the formation of the first stars, galaxies, and perhaps black holes. Astronomers think early black holes developed at a much faster rate than black holes that are closer/more recent, which means observing these earlier cosmic citizens would shed light on the nature of their descendants, our neighbors we know well and have studied for decades.

Updated: Sep 01, 2017