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=== Absolute Magnitude ===
The apparent magnitude that a star would possess it if were placed at a distance of 10 parsecs from Earth. In this way, absolute magnitude provides a direct comparison of the brightness of stars. The apparent magnitude of a star is based upon its luminosity and its distance. If all stars were placed at the same distance then their apparent magnitudes would only be dependent on their luminosities. Thus, absolute magnitudes are true indicators of the amount of light each star emits. The value is lower, even into the negatives, for objects that appear brighter, and higher for objects that appear dimmer. Absolute magnitude is usually measured through the visible spectrum. However, it can be measured through the entire spectrum. Absolute visual magnitude, through the visible spectrum, is abbreviated as ''Mv.'' Compare "Apparent Magnitude," below.
=== Accretion ===
=== Apoapsis ===
The point in an orbit when a planet is farthest from any body other than the Sun or the Earth.
=== Apparent Magnitude ===
A measure of how bright an astronomical object, as viewed from Earth, appears to be to the naked Human eye, but calculated to a value that negates any interference from Earth's atmosphere. The value is lower, even into the negatives, for objects that appear brighter, and higher for objects that appear dimmer. Unless otherwise indicated, apparent magnitude is usually measured through the visible spectrum. However, it can be measured through specific wavelengths, such as infra-red. Apparent magnitude is abbreviated as ''m.'' Compare "Absolute Magnitude," above.
=== Arc Minute ===
=== Asteroid ===
(Also "planetoid") These are rocky bodies, the vast majority of which orbit the Sun between Mars and Jupiter. It is thought that there must be around 100,000 in all. The largest asteroid is Ceres which has a diameter of 579 miles. The smallest detected asteroids have diameters of several hundred feet. Together with comets and meteoroids, asteroids make up the minor bodies of the solar system. They are considered to be left over planetesimals from the formation of our solar system. The gravitational pull of Jupiter is thought to have stopped the members of the asteroid belt from forming a planet.
=== Astronomical Object ===
Naturally occurring physical structures that exist in observable space. [[Asteroid|Asteroids,]] [[Moon|moons,]] [[Planet|planets,]] and [[Star|stars]] are examples of single astronomical objects. [[Nebula|Nebulae,]] [[Star system|star systems,]] star clusters, and [[Galaxy|galaxies]] are examples of compound astronomical objects.
=== Astronomical Unit ===
=== Plage ===
Bright regions in the Sun's chromosphere.
=== Planetary System ===
A set of gravitationally bound non-stellar astronomical objects that orbit a [[Star|star]] or [[Star system|star system.]] Compare "Star System," below.
=== Planisphere ===
=== Star Cluster ===
A loose association of stars within the the Milky Way. Examples are the Pleiades (Seven Sisters) Hyades clusters.
=== Star System ===
A set of a few gravitationally bound stellar objects that orbit each other. A set of many such stellar objects is a star cluster. See "Star Cluster," above. Compare "Planetary System," above. See also [[Star system|star system.]]
=== Sunspot ===
