Accretion Disks
Accretion disks are structures formed by dust, gas, and debris that massive astronomical objects, such as black holes, attract due to their intense gravitational pull. When matter spirals towards a black hole, it follows a flat, rotating path, creating the disk. As the material in the accretion disk rotates, friction generates significant heat, producing X-rays that astronomers detect to locate black holes, which cannot be observed directly. These disks can also form around young stars, potentially leading to the creation of planets, or around dying stars like white dwarfs in binary systems, where gravitational forces draw material from a companion star.
Supermassive black holes, found at the centers of most large galaxies, can have accretion disks that emit enormous amounts of heat and light, resulting in the formation of quasars—some of the brightest objects in the universe. In 2011, scientists utilized the Hubble Space Telescope to observe a quasar's accretion disk, marking a significant advancement in understanding these distant celestial phenomena through innovative observational techniques. Overall, accretion disks play a crucial role in the life cycle of stars and the dynamics of galaxies, providing insights into the universe's structure and evolution.
Accretion Disks
FIELDS OF STUDY: Astrophysics; Observational Astronomy; Sub-planet Astronomy
ABSTRACT: Accretion disks are relatively flat, rotating collections of debris and gas that form around black holes, young stars, or other large celestial objects with great gravitational force. Accretion disks can produce large amounts of energy and light, and they are one way scientists can identify the location of a black hole.
Black Hole Markers
Accretion disks are one way that scientists detect the presence of black holes. Some black holes form when a massive star dies in a supernova explosion and then collapses in on itself. For a star to collapse into a black hole, its mass must be about twenty times that of the sun or greater. Other black holes are formed by the collision of dense objects, such as neutron stars. The tremendous gravitational power of a black hole is the result of a massive amount of matter being compressed into a relatively small area. The very center of a black hole is called a "singularity." A singularity is a point in space where mass density, and thus gravitational force, is essentially infinite.
The enormous size and power of a black hole pulls in everything within range, including light. Because of this, scientists cannot locate or study black holes directly using any of the usual instruments that detect electromagnetic radiation. Instead, they look for objects and phenomena that accompany black holes. One such object is an accretion disk. Accretion disks are made up of the dust, gas, and other debris that massive objects such as black holes attract. Because of the law of conservation of angular momentum, this material continues to orbit the massive object as it approaches, following a spiral path and forming a flat, rotating mass. The friction produced by this rotation causes the material in the accretion disk to heat up and generate x-rays. The x-rays then propagate through space. Scientists look for these x-rays in order to identify the probable location of a black hole.
Other Accretion Disks
The largest black holes can be more than a billion times as massive as the sun. These supermassive black holes are believed to be at the center of the galactic nuclei of most massive galaxies, including the Milky Way. If the accretion disk of a supermassive black hole produces enough friction, it generates a tremendous amount of heat and light, more than is produced by the rest of the galaxy combined. This phenomenon is called a quasar. The term "quasar" comes from "quasi-stellar radio source," as quasars were first thought to be individual starlike objects that emitted strong radio waves. In fact, a quasar is a type of active galactic nucleus (AGN), which is a galactic nucleus that emits more radiation than can be produced by the stars it contains. Quasars are believed to be the brightest objects in the universe. They are often more than a hundred times brighter than their surrounding galaxies. Quasars last as long as there is matter in the accretion disk to fuel them.
Accretion disks also form around young stars as they exert their gravitational forces on the space around them. Some of the material in such a disk may eventually form planets that orbit around the new star.
On the other end of the stellar life span, accretion disks can also form when a white dwarf, a type of dying star, is one half of a binary star. The white dwarf compresses and grows both smaller in size and heavier in mass, increasing its gravitational power. This can cause it to draw material from its partner in the binary system, forming an accretion disk.
Bringing Accretion Disks into View
In 2011, scientists at the National Aeronautics and Space Administration and the European Space Agency were able to use the Hubble Space Telescope to study a quasar accretion disk. Because quasars are at the center of distant galaxies, too far to study in detail with even the most powerful telescopes, the scientists developed a new technique: they used a galaxy between Earth and the quasar as a gravitational lens. When the stars in this galaxy passed in front of the quasar, their gravitational fields amplified the quasar’s light and made it easier to see. This was the first time scientists were able to gather direct information about a quasar and its accretion disk instead of relying on theoretical data.
Principal Terms
- angular momentum: the momentum, or force of motion, of a rotating object or system, determined by the object’s mass, speed, and distance from the point about which it is rotating.
- binary star: two stars that orbit around a common center of mass.
- black hole: a region of space with a gravitational pull so strong that even light cannot escape it.
- galactic nucleus: the center of a galaxy, relatively small but with a high concentration of stars.
- quasar: short for "quasi-stellar"; an extremely bright celestial object that produces very large amounts of energy.
- white dwarf: a small, very dense star that has exhausted nearly all of its fuel and is nearing the end of its life cycle.
- x-ray: high-energy electromagnetic radiation with a very short wavelength.
Bibliography
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