appulse The closest approach of one celestial object to another, as viewed from a third body. The brighter the object appears, the lower its magnitude. A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. apparent magnitudeĪlso called visual brightness (V). apogee The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is furthest from the Earth. apoapsis The point at which an orbiting body is furthest from its primary. aphelion The point at which a body orbiting the Earth's Sun is furthest from the Sun. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. Am star A chemically peculiar star belonging to the more general class of A-type stars. albedo feature A large area on the surface of a reflecting object that shows a significant contrast in brightness or darkness ( albedo) compared to adjacent areas. Syrtis Major (center) is a prominent dark albedo feature on Mars. cloud cover and geological or environmental surface features). upper atmosphere versus surface), and by local variation within these layers (e.g. The albedo reported for an astronomical body may vary widely by the spectral and angular distribution of the incident radiation, by the "layer" of the body being measured (e.g. It is a dimensionless quantity typically measured on a scale from 0 (indicating total absorption of all incident radiation, as by a black body) to 1 (indicating total reflection). albedo A measure of the proportion of the total solar radiation received by an astronomical body, such as a planet, that is diffusely reflected away from the body.
A galaxy hosting an AGN is called an active galaxy. active galactic nucleus (AGN) A compact region in the center of a galaxy displaying a much higher than normal luminosity over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object. accretion disk A roughly circular mass of diffuse material in orbit around a central object, such as a star or black hole. It is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light-years. A B C D E F G H I J K L M N O P Q R S T U V W X Y ZĪ A-type star absolute magnitude A measure of a star's absolute brightness.
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