What does a black hole look like?

At the center of our galaxy lies a swirling, energy-spewing supermassive black hole called Sagittarius A* or Sgr A*, for short. For billions of years, surrounding gas and dust have been falling into it. Every 10,000 years or so, it swallows a nearby star.

Sgr A* (pronounced Saj-A-star) is the largest black hole in our night sky, but we don't know what it looks like up close because we've never been able to take a picture of it.

This is actually true of all black holes.

They're ubiquitous in our universe, but they're so small in the sky, we don't have a detailed image of any of them.

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Credit: EHT

Computer simulations of the images EHT researchers hope to generate. The bright regions are hot gas surrounding the black hole. The circular dark region is a shadow cast by the strong gravity of the black hole.

Source / Image Courtesy

This black hole bounty consists of stellar-mass black holes, which typically weigh between five to 30 times the mass of the Sun. These newly identified black holes were found within three light years — a relatively short distance on cosmic scales — of the supermassive black hole at our Galaxy's center known as Sagittarius A* (Sgr A*).

A black hole by itself is invisible. However, a black hole — or neutron star — locked in close orbit with a star will pull gas from its companion. Astronomers call these systems "X-ray binaries".

This material falls into a disk and heats up to millions of degrees and produces X-rays before disappearing into the black hole. Some of these X-ray binaries appear as point-like sources in the Chandra image.

Researchers have found evidence that the normally dim region very close to the supermassive black hole at the center of the Milky Way galaxy flared up with at least two bright outbursts in the past few hundred years.

These images are from a study of Chandra observations taken over twelve years that show rapid variations in the X-ray emission from gas clouds surrounding the supermassive black hole.

The phenomenon, known as a "light echo," provides astronomers an opportunity to piece together what objects like Sgr A* were doing long before there were X-ray telescopes to observe them

NASA's Swift satellite detected a rising tide of high-energy X-rays from a source toward the center of our Milky Way galaxy. The outburst, produced by a rare X-ray nova, announced the presence of a previously unknown stellar-mass black hole.

An X-ray nova is a short-lived X-ray source that appears suddenly, reaches its emission peak in a few days and then fades out over a period of months. The outburst arises when a torrent of stored gas suddenly rushes toward one of the most compact objects known, either a neutron star or a black hole.

Named Swift J1745-26 after the coordinates of its sky position, the nova is located a few degrees from the center of our galaxy toward the constellation Sagittarius.

While astronomers do not know its precise distance, they think the object resides about 20,000 to 30,000 light-years away in the galaxy's inner region. The pattern of X-rays from the nova signals that the central object is a black hole.