About 300 million light-years away, in the heart of galaxy NGC 4889, is a black hole 21 billion times the size of our sun. Researchers from NASA and the European Space Agency (ESA) have revealed that this black hole is possibly the largest black hole that scientists have ever found. Scientists made this discovery after the Hubble Space Telescope took an image of the elliptical NGC 4889 galaxy. The scientists then studied the activity of objects within the galaxy, including its supermassive black hole.

NGC 4889 is located in the Coma Cluster, which is in the constellation Coma Berenices. The Coma Cluster is estimated to contain 10,000 or more galaxies, according to EarthSky.

Just how big is this newly discovered black hole?

The supermassive black hole has an event horizon (or black hole boundary) with a diameter 15 times greater that the diameter of Neptune’s orbit of the sun, scientist say. In comparison, the Milky Way galaxy's own supermassive black hole sports an event horizon just one-fifth of Mercury's orbit of the sun. Also, the Milky Way's black hole is thought to have a mass of just 3 to 4 million times our sun, which is tiny in comparison to the newly found black hole’s mass.

How did scientist find it?

Because black holes engulf everything around them — including light — scientists cannot directly observe them. This makes black holes impossible to see and analyze first-hand. However, scientists can observe the behavior of objects nearby to determine the black hole’s existence and its nature. To understand the phenomenon at the center of NGC 4889, astronomers used tools from the Keck II Observatory and the Gemini North Telescope. These instruments help them calculate the velocities of the stars orbiting the center of NGC 4889. From those calculations, they determined the mass and activity of the black hole.

The black hole is a dormant “sleeping giant,” but when the black hole was active, scientist posit that galaxy NGC 4889 was a quasar, emitting 1000 times more energy than the Milky Way.

This incredible amount of energy comes from the process of “hot accretion,” which occurs when a black hole feeds on whatever matter is nearby. The matter is pulled toward the black hole by its extreme gravity and then forms an accretion disc around the black hole. The accretion disc then heats up and puts out immense amounts of energy in the form of astrophysical jets. Once all the nearby matter has been sucked into the black hole, the black hole runs out of fuel and becomes dormant — the current state of NCG 4889’s back hole.

“The environment within the galaxy is now so peaceful that stars are forming from its remaining gas and orbiting undisturbed around the black hole,” say Hubble researchers. However, NGC 4889’s black hole may not remain quiet forever; as the scientists say, it is “napping quietly as it waits for its next celestial snack.”