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absolute zero
The coldest possible temperature. It is the lowest temperature on the Kelvin temperature scale, equivalent to -459.67 F (–273.15 C). At this temperature, molecules have reached their minimum energy and cannot transfer heat, but they are never completely motionless because they retain energy to vibrate. As of 2021, the lowest temperature achieved in experiments was 38 picokelvin (3.8 x 10-11 K), or 38 trillionths of a degree above absolute zero.
absorption
The process in which atoms or molecules retain the energy delivered by electromagnetic radiation that strikes them. When an atom absorbs light, its electrons are excited into higher energy states.
accretion
Growth by the gradual accumulation of additional matter. In astronomy, the term describes growth by the gravitational attraction of matter in the form of gas, dust, rocky or icy debris, and other objects. Young stars, planets, and black holes grow in this way.
accretion disk
A relatively flat, rapidly rotating cloud of gas or dust surrounding any massive object growing in size by attracting matter through gravity (accretion), such as a newborn star or a black hole.
active galactic nucleus (AGN), active galaxy
A class of galaxies with central regions that appear much brighter than ordinary galaxies and emit unusually large amounts of X-ray, ultraviolet, and other radiation. The central region is called the galactic nucleus (plural, nuclei). Astronomers think supermassive black holes at the centers of AGN produce the additional energy as matter stored in vast accretion disks falls into them. Seyfert galaxies, radio galaxies, quasars, and blazars are all AGN. The term is sometimes shortened to active galaxy.
afterglow
Lower-energy electromagnetic radiation, including X-rays, visible light, and radio waves, that often follow the initial burst of gamma rays in a gamma-ray burst. Afterglows can persist for days to weeks, gradually dissipating with time. The BeppoSAX satellite discovered the afterglow phenomenon in 1997.
ammonia
A molecule composed of one atom of nitrogen and three of hydrogen (NH3). It can be found as either a gas (in planetary atmospheres and accretion disks) or a solid (as ice on the surfaces of dust grains, comets, and planets). Ammonia was the first multiatomic molecule found in interstellar space (1968).
amplitude
The size of a wave from the top of a wave crest to its midpoint.
angstrom (Å)
A metric unit of length equal to 0.1 nanometer (10-10 meters), used to specify wavelen …