This heating is due to the fact that matter accelerates near black holes. As a result, matter emits lots of light and other radiation as it falls into a black hole. Among the best evidence for the existence of a black hole is a source of x-rays coming from a binary star system in which no star companion is observed. In other words, there are visible stars whose orbital motion tells us that they have an invisible partner. An invisible companion emitting x-rays is sometimes a neutron star, but neutron stars cannot exist above a mass of about two to three times that of the sun.
So if analysis shows that the invisible x-ray-emitting companion has an even greater mass, we know it must be a black hole. Newsletter Get smart. Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. They do this by measuring the visible light, X-rays and radio waves emitted by material in the immediate environment of a black hole. For example, when a normal star orbits around a black hole we can measure the speed of the star by studying the visible light that it emits.
Knowledge of this speed can be combined with the laws of gravity to prove that the star is in fact orbiting a black hole, instead of something else. It also yields the mass of the black hole. Alternatively, when gas orbits around a black hole it tends to get very hot because of friction. It then starts emitting X-rays and radio waves. So black holes can also often be found and studied by looking for bright sources of X-rays and radio waves in the sky.
There are many other types of electromagnetic radiation as well. Radiation that has even smaller wavelengths and even higher energies than X-rays is called gamma-rays. Radiation with wavelengths between those of X-rays and visible light is called ultraviolet light.
We encounter ultraviolet light in our daily lives for example in fluorescent lamps. One of the most important black holes to study is the one found at the center of our Milky Way galaxy. Galaxies can merge and when they do, the supermassive black holes at their centers may also collide. This is the case of NGC where Chandra finds two giant black holes—the bright point-like sources in this middle of the image—are only 3, light years apart. The galaxy Centaurus A is well known for a spectacular jet of outflowing material - seen pointing from the middle to the upper left in this Chandra image - that is generated by a giant black hole at the galaxy's center.
Chandra has also revealed information about smaller black holes throughout Centaurus A. How are black holes created? In general, black holes are created whenever enough matter is squeezed into a small enough space. To turn the Earth into a black hole, we would have to compress all its mass into a region the size of a marble! Stellar mass black holes are formed when a massive star more than about 25 times the mass of our Sun runs out of fuel and its core collapses.
The formation of supermassive black holes is more mysterious. They may be created when stellar mass black holes merge and gobble up matter in their vicinity, or by the collapse of giant clouds of dust and gas. Can X-ray telescopes see a black hole? No light of any kind, including X-rays, can escape from inside the event horizon of a black hole. The X-rays Chandra observes from the vicinity of black holes are from matter that is close to the event horizon of black holes.
Matter is heated to millions of degrees as it is pulled toward the black hole, so it glows in X-rays. How do you find black holes with Chandra if you can't see them? Searching for black holes is a tricky business. One way to locate black holes is to search for the X-radiation from a disk of hot gas swirling toward a black hole.
Friction between particles in the disk heats them to many millions of degrees, and they produce X-rays. Such disks have been found in binary star systems composed of a normal star in a close orbit around a stellar-mass black hole and, on a much larger scale, around the supermassive black holes in the centers of galaxies. What happens to objects when they get too close to a black hole? Objects can orbit a black hole without any serious consequences as long as the size of their orbit is much greater than the diameter of the event horizon of a black hole, which is about 30 kilometers for a stellar black hole, and many millions of kilometers for a supermassive black hole.
But, if any object gets too close, its orbit will become unstable and the object will fall into the black hole. Is all matter in the disk around a black hole doomed to fall into the black hole? No, sometimes gas will escape as a hot wind that is blown away from the disk at high speeds.
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