A black hole is a region in space where the pulling force of gravity is so intense it swallows anything near it, including light. Mainly feared in pop sci-fi, they in fact are an essential part of the structure of the universe.
The strong gravity that comes into play with black holes occurs because matter has been pressed into a tiny space. This can happen at the end of a star's life, making some black holes the result of dying stars. Scientists believe the smallest black holes came about when the universe began. There are black holes at the center of most galaxies, including our own.
Because no light escapes, black holes are invisible to us. But space telescopes with special instruments can help astronomers find black holes.
In this artist's illustration, a supermassive black hole, millions or even billions times the mass of our sun, is seen at the center, surrounded by matter flowing onto the black hole in what's called an accretion disk. This disk forms as the dust and gas in the galaxy fall onto the hole, attracted by its gravity. Also shown is an outflowing jet of energetic particles, believed to be powered by the black hole's spin, according to NASA.
Supermassive black hole
This computer-simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the black hole's event horizon, where no light can escape the massive object's gravitational grip. The black hole's powerful gravity distorts space around it like a funhouse mirror. Light from background stars is stretched and smeared as the stars skim by the black hole.
Astronomers have uncovered a near-record breaking supermassive black hole, weighing 17 billion suns, in an unlikely place: in the center of a galaxy in a sparsely populated area of the universe. The observations, made by NASA's Hubble Space Telescope and the Gemini Telescope in Hawaii, may indicate that these monster objects may be more common than once thought.
Until now, the biggest supermassive black holes - those roughly 10 billion times the mass of our sun - have been found at the cores of very large galaxies in regions of the universe packed with other large galaxies. In fact, the current record holder tips the scale at 21 billion suns and resides in the crowded Coma galaxy cluster that consists of over 1,000 galaxies.
A black hole illustration. According to NASA, there are three main categories of black holes, determined by their mass and size:
1) Primordial black holes, which scientists think can be "as small as a single atom but with the mass of a large mountain." They are believed to have occurred after the Big Bang.
2) Stellar black holes, which are considered mid-sized, though to keep that in perspective they can be up to 20 times larger than our sun. They are created "when the center of a very massive star collapses in upon itself." There may be many of them within our own Milky Way galaxy.
3) Supermassive black holes, which can be a million times larger than the sun.
Collision of two black holes
The collision of two black holes is a tremendously powerful event, shown in this still image from a computer simulation, released February 11, 2016. The collision set off gravitational waves detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO.
Gravitational waves, ripples in space-time first hypothesized by Albert Einstein a century ago, were spotted in a landmark discovery that opens a new window for studying the cosmos.
Spiral galaxy NGC 4639
The barred spiral galaxy NGC 4639 is seen in a photo taken by the NASA/ESA Hubble Space Telescope, released October 16, 2015.
NGC 4639 lies over 70 million light-years away in the constellation of Virgo and is one of about 1,500 galaxies that make up the Virgo Cluster. NASA has reported that NGC 4639 also conceals a massive black hole that is consuming the surrounding gas.
Markarian 231 binary black holes
Markarian 231, the nearest supermassive black hole host galaxy to Earth (about 600 million light-years away), contains binary black holes, meaning two in close orbit around each other, as seen in this illustration.
Like a pair of whirling skaters, the black hole duo generates tremendous amounts of energy that makes the core of the host galaxy outshine the glow of the galaxy's population of billions of stars. The binary black holes are predicted to spiral together and collide within a few hundred thousand years.
"The structure of our universe, such as those giant galaxies and clusters of galaxies, grows by merging smaller systems into larger ones, and binary black holes are natural consequences of these mergers of galaxies," said co-investigator Xinyu Dai of the University of Oklahoma.
Galaxy 1068 quasar
Galaxy 1068, located about 47 million light-years away in the constellation Cetus, is shown in visible light and X-rays in this NASA composite image, released on December 17, 2015.
The X-ray light is coming from an active supermassive black hole in the center of the galaxy. This one has been extensively studied because of its relative proximity to our galaxy.
Stellar-mass black hole 4U1630-47
An artist's illustration of the Cygnus X-1 system with a large blue star on the right and jets emanating from a black hole center of the accretion disc on the left. NASA describes it as a stellar black hole, a class of black holes that comes from the collapse of a massive star. Cygnus X-1 is located near large, active regions of star formation in the Milky Way, as seen in this image that spans some 700 light-years across.
Jets are shown radiating from the poles of the spinning black hole. Like many black holes, 4U1630-47 emits powerful jets of highly charged particles from each of its poles, possibly composed of electrons and protons, but also the nuclei of heavy elements such as iron and nickel.
A computer-generated image shows a black hole has such strong gravity that light is noticeably bent towards it, causing some very unusual visual distortions.
Every star in the normal frame has at least two bright images - one on each side of the black hole. Near the black hole, you can see the whole sky - light from every direction is bent around and comes back towards you.
The original background map was taken from the 2MASS infrared sky survey, with stars from the Henry Draper Catalog superimposed. (The Henry Draper Catalog is a scientific listing of stars first compiled in the early 1900s.) Black holes are thought to be the densest matter in the universe.
"Heartbeats" of flaring black holes
This illustration shows the X-ray "heartbeats" of GRS 1915 and IGR J17091, two black holes that ingest gas from companion stars. GRS 1915 has nearly five times the mass of IGR J17091.
According to NASA, an international team of astronomers believes that it may be the smallest black hole known, weighing less than three times the sun's mass. A fly-through relates the heartbeats to hypothesized changes in the black hole's jet and disk.
It was nicknamed a "heartbeat" because of its resemblance to an electrocardiogram. The pattern until now has only been recorded in one other black hole system.
A singing black hole
Astronomers actually heard a black hole "singing" in the distant galaxy called NGC 1275 in 2003. And, even more unfathomable, they believe it probably has been producing the tone for more than two billion years, identified as a B flat, 57 octaves lower than middle C. Dr. Andrew Fabian of the Institute for Astronomy at Cambridge University in England and leader of the team that made the discovery with the use of the Chandra X-ray Observatory, said the black hole is playing "the lowest note in the universe."
This color composite image, recreated from archival Hubble Space Telescope data, highlights galactic debris and filaments of glowing gas, some up to 20,000 light-years long.
Black hole gobbles star
Observations with X-ray observatories confirmed that a powerful X-ray outburst occurred in the center of RX J1242-11. This X-ray outburst, one of the most powerful ever detected in a galaxy, is evidence for the catastrophic destruction of a star that wandered too close to a supermassive black hole.
This illustration shows how, after a close encounter with another star, the doomed star (orange circle) takes a path toward the giant black hole, where the black hole's enormous gravity stretches the star until it is torn apart. Only a small percentage of the disrupted star's mass (indicated by the white stream) is swallowed by the black hole, while the rest gets flung into the surrounding galaxy.
Milky Way galaxy
This artist's illustration shows a current view of the Milky Way galaxy, where Earth is located. Astronomers have found evidence that there is a supermassive black hole in the center of our Milky Way.
Piranha black holes
An artist's conception of a rapidly growing black hole, known as an active galactic nucleus (AGN), shown in the center of a galaxy.
A disk of hot gas is flowing into a central black hole, and is surrounded by a large doughnut, or torus, of cooler gas and dust. Earlier in the history of the universe, galaxy clusters are thought to have contained a lot more gas than they do today.
"The black holes in these early clusters are like piranha in a very well-fed aquarium," said Jason Eastman of Ohio State University, first author of a study on the phenomena. "It's not that they beat out each other for food, rather there was so much that all of the piranha were able to really thrive and grow quickly."
Black hole surrounded by a galaxy
Artist's conceptualization of the stellar environment, including stars, around a black hole of about 10 billion solar masses. The velocity of stars in orbit and close to the black hole help determine its mass.
Serial black hole eruptions
NASA reported in June 2015 that scientists discovered a series of "eruptions" in a supermassive black hole found in the NGC 5813 group of galaxies, using the Chandra X-ray Observatory.
Multiple eruptions over 50 million years rearranged the cosmic landscape at the center of the galaxies, NASA said.
GRO J1655-40 (in blue) is the second so-called "microquasar" discovered in our galaxy. Microquasars are black holes of about the same mass as a star.
This artist's rendering shows W2246-0526, the most luminous galaxy known in the universe. It is about 12.4 billion light-years from Earth. New research suggests turbulent gas is essentially tearing the galaxy apart.
"The momentum and energy of the particles of light deposited in the gas are so great that they are pushing the gas out in all directions," said Roberto Assef, an astronomer with the Universidad Diego Portales in Santiago, Chile, who worked on a recent study on the galaxy. "A likely finale would be that the galaxy will blow out all of the gas and dust that is surrounding it."
M60-UCD1 monster black hole
Astronomers using data from NASA's Hubble Space Telescope and ground observation found an unlikely object in an improbable place -- a monster black hole lurking inside one of the tiniest, densest galaxies ever known.
The black hole is five times the mass of the one at the center of our Milky Way galaxy. The M60-UCD1 dwarf galaxy crams 140 million stars within a diameter of about 300 light-years, which is only 1/500th of our galaxy's diameter.
If you lived inside this dwarf galaxy, the night sky would dazzle with at least 1 million stars visible to the naked eye. Our nighttime sky as seen from Earth's surface shows 4,000 stars. The discovery suggests dwarf galaxies may actually be the stripped remnants of larger galaxies that were torn apart during collisions with other galaxies rather than small islands of stars born in isolation.
"We don't know of any other way you could make a black hole so big in an object this small," said University of Utah astronomer Anil Seth, lead author of an international study of the dwarf galaxy in a report published in the journal Nature on Sept 17, 2014.
Supermassive black hole
Artist's conception of the environment around the supermassive black hole at the center of Mrk 231.
The broad outflow seen in Gemini data is shown as the fan-shaped wedge at the top of the accretion disk around the black hole. A similar outflow probably exists under the disk as well. The total amount of material entrained in the broad flow is at least 400 times the mass of the sun per year.
Galaxy NGC 4261
Giant disk of cold gas and dust fueled a possible black hole at core of galaxy NGC 4261, one of the brightest galaxies in Virgo Cluster, about 100 million light-years away.
X-rays emanate from heated material falling into black hole. This image, produced with data from NASA's Spitzer Space Telescope in 2004, includes distant objects -- including several supermassive black holes -- that NASA says are nearly invisible to telescopes operating at other wavelengths.
Flare of X-rays around black hole
This diagram shows how a shifting feature, called a corona, can create a flare of X-rays around a black hole. The corona (represented in purplish colors) gathers inward (left), becoming brighter, before shooting away from the black hole (middle and right). Astronomers don't know why the coronas shift, but they have learned that this process leads to a brightening of X-ray light that can be observed by telescopes.
In 2014, NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, and Swift space telescopes witnessed an X-flare from the supermassive black hole in a distant galaxy called Markarian 335. The observations allowed astronomers to link a shifting corona to an X-ray flare for the first time.
In this artist's illustration, turbulent winds of gas swirl around a black hole. Some of the gas is spiraling inward toward the black hole, but another part is blown away.