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Sagittarius A*, the black hole at the center of the Milky Way
Credit: A. Eckart (U. Koeln), R. Genzel (MPE-Garching), SHARP I, NTT, La Silla Observatory, ESO (via Astronomy Picture of the Day)
Massive black hole in the Milky Way. This animation shows the motion of stars over a period of eight years, as they orbit the supermassive black hole at the center of our galaxy. The black hole, which contains a few million times the mass of the Sun, is invisible in this infrared image, though its gravitational effect is apparent! Its location, marked above by the yellow cross, coincides with a mysterious and highly variable source of radio waves and x-rays known to astronomers as Sagittarius A*.
Centaurus A, home to the closest active black hole
Credit: NASA, SAO, R.Kraft et al. (via the Chandra X-Ray Observatory Center)
Centaurus A is the nearest example of an active galactic nucleus (AGN), located only 10 million light years from us. The black hole at the center of this galaxy is thought to be around 60 million times the mass of our Sun; material around it gets shot off in the form of huge jets which travel at nearly the speed of light and are easily visible in this Chandra x-ray image. Many of the small dots surrounding the central AGN are thought to be microquasars, smaller black holes which shoot off jets of their own as they rip material from companion stars.

Black Holes and Quasars

(Links | Questions)

Black holes sound like they're straight out of a science fiction story: objects so dense that nothing in the universe can escape from their gravitational pull. But over the past few decades astronomers have been steadily building up evidence that black holes are not only real, but, in fact, quite prevalent in the universe.

It is now thought that almost all galaxies contain gigantic black holes in their centers, millions or even billions of times more massive than the Sun. Some of these beasts are among the most violent and energetic objects in the universe - active galactic nuclei and quasars, which shoot off jets even as they suck in surrounding gas - while others, often older ones like the black hole at the center of the Milky Way, are considerably more quiet feeders.

Galaxies are also thought to contain many examples of small black holes, with masses only a few times greater than that of the Sun. Astronomers have detected a handful of these in our galaxy, by observing the light emitted when they shred apart their companion star in a binary system. Several of these small black holes have been dubbed "microquasars" because they produce miniature jets akin to those of their larger cousins.

Theory of Black Holes

Though the concept of a black hole was first proposed in 1783, it was Albert Einstein's 1915 theory of general relativity which put the idea on a firm theoretical footing. Einstein showed that gravity can bend the path of light just as it bends the path of any other moving object - the only reason we don't observe this effect in our daily lives is that light moves fast and gravity pulls weak. When this was confirmed by observations, the idea of a black hole became obvious. If you pack enough material together, its gravitational pull should be strong enough to not only bend light's path but also keep it from escaping, just as the Earth is strong enough to pull back much slower objects (like baseballs) to its surface.

Formation of Black Holes

Regular black holes are thought to form from heavy stars (perhaps those which start off with masses more than 20 or 25 times that of the Sun, but this is still an area of active research). When these stars end their lives in a supernova explosion, their cores collapse and gravity wins out over any other force that might be able to hold the star up.

Eventually, the star collapses so much that it is contained within its Schwarzschild radius, or event horizon, the boundary within which light cannot escape. At this point, the black hole is extremely tiny; a black hole with the mass of the Sun would fit in a small town, while one with the mass of the Earth would fit in the palm of your hand! The material inside the Schwarzschild radius will continue to collapse indefinitely, reaching the point where our understanding of the laws of physics breaks down. But no information from inside the Schwarzschild radius can escape to the outside world.

Supermassive black holes, meanwhile, form differently - perhaps from the merger of many smaller black holes early in the universe's history - and grow over the years as they suck in gas from their surroundings. The formation of these objects and their relationship to the galaxy that harbors them is still an area of active research.

Observing Black Holes

A lot of light

We can't observe black holes directly, but we do see their effect on surrounding material - gas and dust which lets out its last gasp before being sucked into the black hole or flung away in a jet.

Black holes, in fact, are extremely efficient at converting the energy of incoming material into emitted light. The gas which falls into a black hole doesn't plunge in directly, for the same reason the Earth doesn't plunge into the Sun. Instead, it tries to move around the black hole in an orbit, forming what is known as an accretion disk.

Material in the accretion disk slowly spirals inward as it loses energy due to friction - the huge gravitational tides near the black hole are excellent at ripping apart this material and heating it to high temperatures. The inner disks of supermassive black holes reach thousands of degrees Kelvin (similar to the temperatures at the surface of a hot star), while smaller black holes can heat their disks to millions of degrees, where they emit in the x-ray part of the spectrum.

Black holes, therefore, are some of the brightest objects around. Quasars can be detected out near the edge of the visible universe, where they shine with the light of trillions of Sun, while microquasars in our own galaxy can easily be hundreds of thousands of times brighter than the Sun, even though they are typically only ten times as massive.

Fast variations

Since black holes are small, their brightness can vary quickly. The complicated processes going on in the inner parts of the accretion disk are often highly variable, which leads to rapid changes in the amount of light being emitted. The smallest, most active black holes - the microquasars - can double their brightness in only a few seconds and show evidence for variability on much faster timescales, oscillating at hundreds of times per second in some cases.

Energetic jets

Black holes suck material toward them, but some of it gets spit out rather than swallowed. Many black holes eject jets that move away from the accretion disk at nearly the speed of light. These jets have been observed most spectacularly from the centers of nearby galaxies (for example, M87) but also appear in microquasars - in quick, enormously energetic spurts and sputters, as if someone had taken a video of a quasar jet and pressed the fast-forward button.

The processes by which these jets are formed are not well understood, but seem to require magnetic fields - whose presence causes instabilities in the accretion disk that allow material to fling upwards - as well as rapidly rotating black holes, which can feed some of their energy to the magnetic field and to the jet material itself.

The Ask an Astronomer team's favorite links about Black Holes and Quasars:

Previously asked questions about Black Holes and Quasars:

(General questions | Formation and Evolution of Black Holes | Observations of Black Holes | Theoretical questions | Falling into a Black Hole | Quasars | Merging Black Holes)

General questions:

Formation and Evolution of Black Holes:

Observations of Black Holes:

Theoretical questions:

Falling into a Black Hole:

Quasars:

Merging Black Holes:

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