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X-Ray Astronomy III
The most brilliant steadily shining objects in the universe aren’t giant stars or even galaxies of stars. Instead, they’re big pancakes of gas and dust.
The pancakes are known as accretion disks. They form as gas and dust fall toward a massive central body. The material flattens out, forming a wide, thin disk — like a pancake.
The brightest of these disks are found around black holes. And much of their energy is in the form of X-rays.
The gas and dust don’t fall directly toward a black hole. Instead, they spiral in. The particles ram into each other, creating friction that makes them hotter and brighter. And as they fall closer to the black hole, some of their energy of motion is also converted to radiation. That makes a disk shine brightly.
The inner part of the disk can reach millions of degrees. At that temperature, it radiates most of its energy as X-rays. In fact, many black holes are discovered from the X-ray glow of their accretion disks. The X-rays can reveal the content of an accretion disk, and the speed of material in the disk as it nears the black hole.
The most luminous disks encircle supermassive black holes at the hearts of galaxies. The brightest of these are known as quasars. They can produce so much energy that they’re visible across billions of light-years. Except for a few objects that flicker briefly and then fade away, that makes quasars the most brilliant beacons in the universe.
More about X-ray astronomy tomorrow.