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A sparsely populated grouping (mass range 102-103 Msun) of very young, massive stars lying along a spiral arm of the Milky Way, whose spectral types or motions in the sky indicate a common origin. The star density is insufficient for gravitation to hold the group together against shear by differential galactic rotation, but the stars have not yet had time to disperse completely. OB associations are composed of stars of spectral types O-B2; T associations have many young T Tauri stars. The internationally approved designation for associations is the name of the constellation followed by an arabic numeral - e.g., Perseus OB2.
Industry:Astronomy
The time and azimuth at which a celestial body crosses the astronomical horizon of an oberver.
Industry:Astronomy
The change in direction of travel (bending) of a light ray as it passes obliquely through the atmosphere. As a result of refraction, the observed altitude of a celestial object is greater than its geometric altitude. The amount of refraction depends on the altitude of the object and on atmospheric conditions.
Industry:Astronomy
a metalloid element with two main forms, grey α arsenic and β arsenic
Industry:Astronomy
a satellite made by humans which is gravitationaly bound and in orbit of a larger physical object
Industry:Astronomy
In the orbit of a solar-system body, the point where the body crosses the ecliptic from south to north: for a star, out of the plane of the sky toward the observer.
Industry:Astronomy
Globular cluster stars, which are found in that part of the HR diagram that connects the top pf the giant tip with the horizontal branch stars.
Industry:Astronomy
An optical device usually comprising two thin prisms which can rotate to compensate for the elongation of a star image caused by the wavelength dependence of the refractive index of air.
Industry:Astronomy
Decrease in the intensity of light from a celestial body due to absorption and scattering by the Earth's atmosphere. The extinction increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they are at the zenith.
Industry:Astronomy
