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Astronomy and Astrophysics Glossary

GLOSSARY

absorption spectrum - Dark lines superimposed on a continuous spectrum.

achromatic - Free of chromatic aberration.

almanac - A book or table listing astronomical events.

altazimuth mounting - a mounting with two axes, to allow movement in both horizontal and vertical planes, used with telescopes, antennas and precise surveying instruments.

astrometry - That branch of astronomy that deals with the determination of precise positions and motions of celestial bodies.

astronomical unit (AU) - a unit of length equal to the mean radius of the earth's orbit around the sun (93 million miles).

astronomy - The branch of science that treats of the physics and morphology of that part of the universe that lies beyond the earth's atmosphere.

astrophysics - The part of astronomy that deals principally with the physics of stars, stellar systems, and interstellar material. Astrophysics also deals with the structures and atmospheres of the sun and planets.

atmospheric refraction - The bending or refraction of light rays from celestial objects by the earth's atmosphere.

azimuth - The angle along the celestial horizon, measured eastward from the north point, to the intersection of the horizon with the vertical circle passing through an object.

bands (in spectra) - Emission or absorption lines, usually in the spectra of chemical compounds or radicals, so numerous and closely spaced that they coalesce into broad emission or absorption bands.

"big bang" theory - A theory of cosmology in which the expansion of the universe is presumed to have begun with a primeval explosion.

bolometric magnitude - A measure of the flux of radiation from a star or other object received just outside the earth's atmosphere, as it would be detected by a device sensitive to all forms of electromagnetic energy.

calculus - A branch of mathematics that permits computations involving rates of change (differential calculus) or of the contribution of an infinite number of infinitesimal quantities (integral calculus).

cassegrain focus - An optical arrangement in a reflecting telescope in which light is reflected by a second mirror to a point behind the objective mirror.

celestial equator - A great circle on the celestial sphere 90° from the celestial poles; the circle of intersection of the celestial sphere with the plane of the earth's equator.

celestial poles - Points about which the celestial sphere appears to rotate, intersections of the celestial sphere with the earth's polar axis.

cepheid variable - A star that belongs to one of two classes (type I and type II) of yellow supergiant pulsating stars.

charged-coupled device (CCD) - An array of electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens). A CCD acts like a photographic plate of very high sensitivity.

chromatic aberration - A defect of optical systems whereby light of different colors is focused at different places.

constellation - A configuration of stars named for a particular object, person, or animal; or the area of the sky assigned to a particular configuration.

continuous spectrum - A spectrum of light comprised of radiation of a continuous range of wavelengths or colors rather than only certain discrete wavelengths.

corona - Outer atmosphere of the sun.

coronagraph - An instrument for photographing the chromosphere and corona of the sun outside of eclipse.

cosmic background radiation - The microwave radiation coming from all directions that is believed to be the redshifted glow of the big bang.

cosmic rays - Atomic nuclei (mostly protons) that are observed to strike the earth's atmosphere with exceedingly high energies.

cosmology - The study of the organization and evolution of the universe.

coude focus - An optical arrangement in a reflecting telescope whereby light is reflected by two or more secondary mirrors down the polar axis of the telescope to a focus at a place separate from the moving parts of the telescope.

diffraction - The spreading out of light in passing the edge of an opaque body.

diffraction grating - A system of closely spaced equidistant slits or reflecting strips which, by diffraction and interference, produce a spectrum.

dispersion - Separation, from white light, of different wavelengths being refracted by different amounts.

doppler shift - Apparent change in wavelength of the radiation from source due to its relative motion in the line of sight.

eclipsing binary star - A binary star in which the plane of revolution of the two stars is nearly edge on to our line of sight, so that the light of one star is periodically diminished by the other passing in front of it.

electromagnetic radiation - Radiation consisting of waves propagated through the building up and breaking down of electric and magnetic fields; these include radio, infrared, light, ultraviolet, X rays, and gamma rays.

emission line - A discrete bright spectral line.

ephemeris - A table that gives the position of a celestial body at various times, or other astronomical data.

equatorial mount - A mounting for a telescope, one axis of which is parallel to the earth's axis, so that a motion of the telescope about the axis can compensate for the earth's rotation.

faculus (pl. faculae) - Bright region near the limb of the sun.

flash spectrum - The spectrum of the very limb of the sun obtained in the instant before or after totality in a solar eclipse.

flocculus (pl. flocculi) - A bright region of the solar surface observed in the monochromatic light of some spectral line.

fluorescence - The absorption of light of one wavelength and reemission of it at another wavelength; especially the conversion of ultraviolet into visible light.

focal length - The distance from a lens or mirror to the point where light converged by it comes to a focus.

focal ratio (speed) - Ratio of the focal length of a lens or mirror to its aperture.

focus - Point where the rays of light converged by a mirror or lens meet.

Fraunhofer line - An absorption line in the spectrum of the sun or a star.

Fraunhofer spectrum - The array of absorption lines in the spectrum of the sun or of a star.

galaxy - A large assemblage of stars; a typical galaxy contains millions to hundreds of million of stars.

globular cluster - One of about 120 large star clusters that form a system of clusters centered on the center of the Galaxy.

Greenwich meridian - The meridian of longitude passing through the site of the old Royal Greenwich Observatory, near London; origin point from which longitude is measured on the earth.

Hertzsprung-Russell (H-R) diagram - A plot of absolute magnitude against temperature (or spectral class or color index) for a group of stars.

hypothesis - A tentative theory or supposition, advanced to explain certain or phenomena, which is subject to further tests and verification.

interferometer (stellar) - An optical device, making use of the principle of interference of light waves, with which small angles can be measured.

latitude - A north-south coordinate on the surface of the earth; the angular distance north or south of the equator measured along a meridian passing through a place.

light - Electromagnetic radiation that is visible to the eye.

light year - The distance light travels in a vacuum in one year; one light year equals approximately 6,000,000,000,000 miles.

longitude - An east-west coordinate on the earth's surface; the angular distance, measured east or west along the equator from the Greenwich meridian, to the meridian passing through a place.

luminosity - The rate of radiation of electromagnetic energy into space by a star or other object.

magnitude - A measure of the amount of light flux received from a star or other luminous object.

maser - An radiation; particular acronym for microwave amplification of stimulated emission a device for amplifying a microwave (radio) signal at a wavelength into a coherent beam.

microwave - Short-wave radio wavelengths.

Milky Way - The band of light encircling the night sky, which is due to the many stars and diffuse nebulae lying near the plane of our Galaxy.

monochromatic - Of one wavelength or color.

Newtonian focus - An optical arrangement in a reflecting telescope, in which a flat mirror intercepts the light from the primary mirror before it reaches the focus and reflects it to a focus at the side of the telescope tube.

Newton's laws - The laws of mechanics and gravitation formulated by Isaac Newton.

nova - A star that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by hundreds to thousands of times.

objective - The main lens or mirror of a telescope. The larger it is, the brighter the image is and the sharper it is (i.e., its resolution is better).

opacity - Absorbing power; capacity to impede the passage of light.

optics - The branch of physics that deals with light and its properties.

parallax - An apparent displacement of an object due to a motion of the observer.

parallax (stellar) - An apparent displacement of a nearby star that results from the motion of the earth around the sun; numerically, the angle subtended by 1 AU at the distance of a particular star.

photocell (photoelectric cell) - An electron tube in which electrons are dislodged from a cathode when it is exposed to light and are accelerated to an anode, thus producing a current in the tube, whose strength serves as a measure of the intensity of the light striking the cathode.

photographic magnitude - The magnitude of an object, as measured on the traditional, blue-violet-sensitive photographic emulsions.

photometry - The measurement of light intensities.

photomultiplier - A photoelectric cell in which the electric current generate is amplified at several stages within the tube.

prime focus - The point in a telescope where the objective focuses the light.

prism - A wedge-shaped piece of glass that is used to disperse white into a spectrum.

radio astronomy - The technique of making astronomical observations in radio wavelengths.

radio telescope - A telescope designed to make observations in radio wavelengths.

reflecting telescope - A telescope in which the principal optical component (objective) is a concave mirror.

refracting telescope - A telescope in which the principal optical component (objective) is a lens or system of lenses.

resolution - The degree to which fine details in an image are separated or resolved.

resolving power - A measure of the ability of an optical system to to resolve or separate fine details in the image it produces; in astronomy, the angle in the sky that can be resolved by a telescope.

Schmidt telescope - A type of reflecting telescope invented by Bernhard Schmidt, in which certain aberrations produced by a spherical concave mirror are compensated for by a thin objective correcting lens.

science - The attempt to find order in nature or to find laws that describe natural phenomena.

spectral sequence - The sequence of spectral classes of stars arranged in order of decreasing temperatures of stars of those classes.

spectrograph - An instrument for photographing a spectrum; usually attached to a telescope to photograph the spectrum of a star.

spectroheliogram - A photograph of the sun obtained with a spectroheliograph.

spectrophotometry - The measurement of the intensity of light from a star or other sources at different wavelengths.

spectroscope - An instrument for directly viewing the spectrum of light source.

spectroscopy - The study of spectra.

spectrum - The array of colors or wavelengths obtained when light from a source is dispersed, as in passing it through a prism or grating.

steady state (theory of cosmology) - A theory of cosmology which proposes that the universe has existed and will exist forever in its current form. The observed expansion of the universe being caused by the continuous creation of new matter so that the average density and appearance of the universe remains the same at all times.

theory - A set of hypotheses and laws that have been well demonstrated as applying to a wide range of phenomena associated with a particular subject.

thermal energy - Energy associated with the motions of the molecules in a substance.

thermal radiation - The radiation emitted by any body or gas that is not at absolute zero.

transit - An instrument for timing the exact instant a star or other objects crosses the local meridian. Also, the passage of a celestial body across the meridian; or the passage of a small body across the disk of a large one.

21-cm line - A spectral line of neutral hydrogen at the radio wavelength of of 21 cm.

ultraviolet radiation - Electromagnetic radiation of wavelengths shorter than the shortest (violet) wavelengths to which the eye is sensitive; radiation of wavelengths in the approximate range 100 to 4000 angstroms.

universe - The totality of all matter and radiation and the space occupied by the same.

variable star - A star that varies in luminosity.

Last Modified: Mon, Nov 5 2001 8:00:00 pm PDT
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