Atmospheric reddening: Preferential scattering of blue light over red by air particles so that an object appears redder than it actually is.
Atom: The smallest particle of an element that exhibits the chemical properties of the element.
Blackbody spectrum: The continuous spectrum emitted by a blackbody; the flux at each wavelength is given by Planck's law.
Black Hole: A mass that has collapsed to such a degree that the escape velocity from its surface is greater than the speed of light, so that light is trapped by the intense gravitational field.
Blue shift: A decrease in the wavelength of the radiation emitted by an approaching celestial body as a consequence of the Doppler effect; a shift toward the short-wavelength (blue) end of the spectrum.
Celestial equator: An imaginary projectionof the earth's pole onto the celestial sphere;a point about which the apparent daily rotation of the stars takes place.
Cosmic blackbody microwave radiation: Radiation with a blackbody spectrum at a temperature of about 2.7 K permeating the universe; believed to be the remains of the primeval fireball in which the Universe was created.
Cosmic rays: Charged atomic particles moving in space with very high energies (the particles travel close to the speed of light); most originate beyond the solar system, but some of low energy are produced in solar flares.
Crab nebula: A supernova remnant, located in the constellation Taurus, produced by the supernova explosion visible from earth in 1054 CE; a pulsar in the nebula marks the neutron-star corpse of the exploded star.
Critical density: In cosmology, the density that marks the transition from an open to a closed universe; the density that provides enough gravity to just bring the expansion to a stop after infinite time.
Degenerate electron gas: Anionized gas in which nucleiand electrons are packed together as much as possible,filling all possible low energy states, so that the perfect gas law relatingpressure, temperature, and density no longer applies.
Degenerate neutron gas: Matter made up of neutrons packed together as tightly as possible.
Doppler shift: A change in the wavelength of waves from a source reaching an observer when the source and the observer are moving with respect to each other along the line of sight; the wavelength increases (red shift) or decreases (blue shift) according to whether the motion is away from or toward the observer, respectively.
Ecliptic: From the Earth, the apparent yearly path on the celestial sphere of the Sun with respect to the stars; also, the plane of the earth's orbit.
Electromagnetic radiation: a self-propagating electric and magnetic wave, such as light, radio, ultraviolet, or infrared radiation; all types travel at the same speed and can be differentiated by wavelength or frequency.
Element: A substance that is made of atoms with the same chemical properties, and which cannot be decomposed chemically into simpler substances.
Emission (bright) lines: Light of specific wavelengths or colors emitted by atoms; sharp energy peaks in a spectrum caused by downward electron transitions from a discrete quantum state to another discrete state.
Excitation: The process of raising an atom to a higher energy level.
Frequency: The number of waves that pass a particular point in some time interval (usually a second); usually given in units of hertz, one cycle per second.
Galactic latitude: The angular distancenorth of south of the galactic equator.
Galactic longitude: The angular distance along the galactic equator from a zero point in the direction of the galactic center.
Galaxy: A huge assembly of stars (between one hundred and one million), plus gas and dust, which is held together by gravity; the Galaxy, our own galaxy, containing the Sun.
Giant molecular clouds: Large interstellar clouds, with sizes up to tens of parsecs and containing 100,000 solar masses of material; found in the spiral arms of the Galaxy, giant molecular clouds are the sites of massive star formation.
Gravitation: In Newtonian terms, a force between masses that is characterized by their acceleration toward each other; the magnitude of the force depends directly on the product of the masses and inversely on the square of the distance between them; in Einstein's terms, the curvature of space-time.
Hertz: A physical unit of frequency equal to 1 cycle/s.
High-velocity clouds: Clouds ofgas associated with the Galaxy, moving at speeds ofhundreds of kilometers per second.
Hubble's Law: A description of the expansion of the Universe, such that the more distant a galaxy lies from us, the faster it is moving away; the relation, v = Hd, between the expansion velocity (v),and the distance (d) of a galaxy, where H is the Hubble constant.
Hubble constant: The proportionality constant relating velocity and distance in the Hubble law; the value, now around 75 km/s/Mpc, changes with time as the Universe expands.
Infrared telescope: A telescope, optimized for use in the infrared part of the spectrum, fitted with an infrared detector.
Intergalactic medium: the gasand dust found between the galaxies.
Interstellar dust: Small (micrometers in diameter) solid particles in the interstellar medium
Kelvin temperature scale: A measure of temperature with its zero point located at "absolute zero," the point at which the random motion of atoms and molecules ceases. Each degree has the same change as in Celcius, and zero degrees Celcius is at 273.16 K
Local Supercluster: The supercluster of galaxies in which the Local Group is located; spread over 10,000,000 pc, it contains the Virgo and Coma clusters.
Luminosity: The total rate at which radiative energy is given off by a celestial body, over all wavelengths; the Sun's luminosity is about 4x1026 watts.
Magnetometer: A device to measure the strength of a magnetic field.
Mass: A measure of an object's resistance to change in its motion (inertial mass); a measure of the strength of gravitational force an object can produce (gravitational mass).
Megaparsec: One million parsecs (Mpc).
Mesosphere: Region of the Earth's atmosphere between 50 and 100 km where the temperature falls rapidly.
Milky Way: The band of light that encircles the sky, caused by the bending of light from many stars lying near the plane of the Galaxy; also sometimes used to refer to the Galaxy in which the Sun belongs.
Nanometer: 0.000000001 of a meter; common unit of wavelength measurement for light.
Nova: (Latin, "new") A star that has a sudden outburst of energy, temporarily increasing its brightness by hundreds to thousands of times; now believed to be the outburst of a degenerate star in a binary system; also used in the past to refer to some stellar outbursts that modern astronomers now call supernovas.
Nucleosynthesis: The chain of thermonuclear fusion processes by which hydrogen is converted to helium, helium to carbon, and so on through all the elements of the periodic table.
Observable universe: The parts of the Universe that can be detected by the light they emit.
Opacity: The property of a substance that hinders(by absorption or scattering) light passing throughit; opposite of transparency.
Orion Nebula: A hot cloud of ionized gas that is a nearby region of recent star formation, located in the sword of the constellation of Orion; also catalogued as Messier 42 (M42).
Parallax: The change in an object's apparent position when viewed from two different locations; specifically, half the angular shift of a star's apparent position as seen from opposite ends of the Earth's orbit.
Parsec (pc): The distance an object would haveto be from the earth so that its heliocentric parallaxwould be 1 second of arc; equal to 3.26 light years; a kiloparsec is 1000parsecs.
Photodissociation: The breakup of a molecule by the absorption of light with enough energy to break the molecular bonds.
Photon: A discrete amount of light energy; the energy of a photon is related to the frequency f of the light be the relation E = hf, where h is Planck's constant.
Photosphere: The visible surface of the Sun; the region of the solar atmosphere from which visible light escapes into space.
Planck curve: The continuous spectrum of a blackbody radiator.
Planck's constant: The number that relates the energy and frequency of light; it has a value of 6.63 x 10-34 joule seconds.
Planetary nebula: A thick shell of gas ejected from and moving out from an extremely hot star; thought to be the outer layers of a red giant star thrown out into space, the core of which eventually becomes a white dwarf.
Radiation: Usually refers to electromagneticwaves, such as light, radio, infrared, X-rays, ultraviolet; also sometimesused to refer to atomic particles of high energy, such as electrons(beta-radiation), helium nuclei (alpha-radiation),and so on.
Reddening: The preferential scattering or absorption of blue light by small particles, allowing more red light to pass directly through.
Red shift: An increase in the wavelength of the radiation received from a receding celestial body as a consequence of the Doppler effect; a shift towards the long-wavelength (red) end of the spectrum.
Relativistic Doppler shift: Wavelength shift from the radial velocity of a source as calculated in special relativity, so that very large red shifts do not imply that the source moves faster than light.
Relativity: Two theories proposed by A. Einstein; the special theory describes the motion of nonaccelerated objects, and general relativity is a theory of gravitation.
Solar flare: Sudden burst of electromagnetic energy and particles from a magnetic loop in an active region.
Space-time: A 4-dimensional universe with space and time unified; a continuous system of one time coordinate and three space coordinates by which events can be located and described.
Spectral line: A particular wavelength of light corresponding to an energy transition in an atom.
Spectroscope: An instrument for examining spectra; also a spectrometer or spectrograph if the spectrum is recorded and measured.
Spectroscopy: The analysis of light by separating it by wavelengths (colors, in visible light).
Spectrum: The array of colors or wavelengths obtained when light is dispersed, as by a prism; the amount of energy given off by an object at every different wavelength.
Stellar nucleosynthesis: A process in which nuclear fusion builds up heavier nuclei while supplying the energy by which stars shine.
Tesla: In the SI system, a unit of measure of magnetic flux.
Theory: An idea in science which is supported by numerous pieces of evidence, and which has thus far withstood the rigors of testing by other scientists. It is different from a hypothesis, which is an idea based on observations but which has not yet been tested, or does not have further evidence supporting it. Theory in the scientific sense is very different from the colloquial definition, where a theory is essentially a guess. Examples include the Big Bang theory, the theory of gravitation, and the theory of evolution.
Universe: The totality of all space and time; all that is, has been, and will be.