Glossary

absorption spectrum - Dark lines superimposed on a continuous spectrum.

absolute magnitude - A measure of the brightness a star would have if it were to be placed at a standard distance of 10 parsecs (32.6 light years) from the sun.

apparent magnitude - The brightness of a star (or any other celestial object) as seen from the earth; an astronomical measure of an object's flux.

Big Bang Model - A picture of the evolution of the universe that postulates its origin in an event called the Big Bang, from a hot, dense state that rapidly expanded to cooler, less dense states.

cepheid variable stars - Stars that vary in brightness as a result of regular variation in size and temperature; a class of variable stars for which the star Delta Cephei is the prototype.

cosmic microwave background - Radiation with a blackbody spectrum at a temperature of about 2.7 K permeating the universe; the remains of the primeval fireball in which the universe was created and an observational confirmation of the Big Bang Model.

Doppler effect - 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 (redshift) or decreases (blueshift) according to whether the motion is away from or toward the observer.

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 differ in wavelength or frequency, which relates to the energy.

electromagnetic spectrum - The range of all wavelengths of electromagnetic radiation.

Hubble constant - The proportionality constant relating velocity and distance in the Hubble law; the value, now around 75 km/s/Mpc (20 km/s/Mly), changes with time as the universe expands.

inverse-square law - The decrease of the intensity or flux of light with the inverse square of the distance from the source.

light year (ly) - the distance light travels in a year, about 30,900,000,000,000 kilometers.

magnitude - An astronomical of an object's brightness; larger magnitudes represent fainter objects.

photometry - Measurement of the intensity of light.

photon - A discrete amount of light energy, the energy of a photon is related to the frequency of the light.

primeval fireball - The hot, dense beginning of the universe in the Big Bang model, when most of the energy was in the form of high-energy light.

radial velocity - The component of relative velocity that lies along the line of sight.

redshift - An increase in the wavelength of the radiation received from a receding celestial body as a consequence of the Doppler effect; a shift toward the long-wavelength (red) end of the spectrum.

standard candle - An astronomical object of known luminosity used to estimate distances to galaxies.