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Astronomy Terms & Glossary

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Essential astronomy vocabulary and concepts.

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A-type star

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  1. 1A-type star
    A class of main-sequence star having spectra dominated by Balmer absorption lines of hydrogen, typically blue-white or white in color, measuring 1.4-2.1 solar masses with surface temperatures of 7,600-10,000 kelvin
  2. 2Absolute magnitude
    A measure of a star's absolute brightness, defined as the apparent magnitude the star would show if located at a distance of 10 parsecs (32.6 light-years)
  3. 3Accretion disk
    A roughly circular mass of diffuse material in orbit around a central object (star or black hole), acquired from an external source, with friction causing it to spiral inward
  4. 4Active galactic nucleus (AGN)
    A compact region in the center of a galaxy displaying much higher than normal luminosity, with characteristics indicating the luminosity is not produced by stars
  5. 5Albedo
    A measure of the proportion of total solar radiation received by an astronomical body that is diffusely reflected away, measured on a scale from 0 (total absorption) to 1 (total reflection)
  6. 6Aphelion
    The point at which a body orbiting the Sun is furthest from the Sun
  7. 7Apoapsis
    The point at which an orbiting body is furthest from its primary
  8. 8Apogee
    The point at which a body orbiting the Earth (such as the Moon or artificial satellite) is furthest from Earth
  9. 9Apparent magnitude
    A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value in the absence of atmosphere; lower magnitude means brighter object
  10. 10Apsis
    One of the two extreme points of distance between an orbiting body and its primary – either periapsis (minimal distance) or apoapsis (maximal distance)
  11. 11Artificial satellite
    An object intentionally placed into orbit by humans, often around Earth but also around other Solar System bodies
  12. 12Ascending node
    The orbital node at which an orbiting object moves north through the plane of reference or away from the observer
  13. 13Asterism
    Any pattern of stars recognizable in Earth's night sky, which may form part of an official constellation or be composed of stars from multiple constellations
  14. 14Asteroid
    A minor planet of the inner Solar System, primarily found orbiting between Jupiter and Mars, composed of mineral and rock rather than dust and ice
  15. 15Asteroid belt
    The circumstellar disc in the Solar System located roughly between Mars and Jupiter, occupied by numerous small Solar System bodies ranging from dust to asteroids
  16. 16Astrobiology
    An interdisciplinary field studying the origins, evolution, distribution, and future of living systems in the universe, including research on extraterrestrial life
  17. 17Astrometric binary
    A binary system where evidence for an unseen orbiting companion is revealed by periodic gravitational perturbation of the visible component
  18. 18Astrometry
    The branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies
  19. 19Astronomical unit (AU)
    A unit of length used primarily for measuring distances within the Solar System, defined as exactly 149,597,870.7 kilometers (92,956,000 miles)
  20. 20Astrophysics
    The branch of astronomy that employs principles of physics and chemistry to determine the nature of astronomical objects, examining properties like luminosity, density, temperature, and chemical composition
  21. 21Atmosphere
    An envelope of gases surrounding an astronomical body held in place by gravity, with no clearly defined exterior boundary but growing increasingly tenuous with altitude
  22. 22Axial precession
    A slow, continuous, gravity-induced change in the orientation of an astronomical body's axis of rotation; for Earth this occurs over a cycle of approximately 25,771.5 years
  23. 23Axial tilt
    The angle between an object's rotational axis and its orbital axis, or between its equatorial plane and orbital plane; Earth's axial tilt causes the seasons
  24. 24Azimuth
    An angular measurement of an object's orientation along the horizon relative to true north; combined with altitude, it defines an object's current position in spherical coordinates
  25. 25Barycenter
    The common center of mass about which two or more bodies of a gravitationally bound system orbit; it is one of the foci of each body's elliptical orbit
  26. 26Big Bang
    The prevailing cosmological model for the origin of the observable universe, depicting a starting condition of extremely high density and temperature followed by ongoing expansion
  27. 27Binary star
    A star system consisting of exactly two stars orbiting around their common barycenter
  28. 28Black hole
    A concentration of mass so compact that it creates a region of space from which not even light can escape; the outer boundary is called the event horizon
  29. 29Blazar
    An active galactic nucleus (AGN) with a relativistic jet directed very nearly towards the observer
  30. 30Brown dwarf
    A substellar object too low in mass to sustain nuclear fusion of hydrogen-1 in its core; it can still generate energy from gravitational contraction and deuterium fusion

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