Key Terms

absorption spectrum
a series or pattern of dark lines superimposed on a continuous spectrum
an idealized object that absorbs all electromagnetic energy that falls onto it
continuous spectrum
a spectrum of light composed of radiation of a continuous range of wavelengths or colors, rather than only certain discrete wavelengths
separation of different wavelengths of white light through refraction of different amounts
Doppler effect
the apparent change in wavelength or frequency of the radiation from a source due to its relative motion away from or toward the observer
electromagnetic radiation
radiation consisting of waves propagated through regularly varying electric and magnetic fields and traveling at the speed of light
electromagnetic spectrum
the whole array or family of electromagnetic waves, from radio to gamma rays
emission spectrum
a series or pattern of bright lines superimposed on a continuous spectrum
energy flux
the amount of energy passing through a unit area (for example, 1 square meter) per second; the units of flux are watts per square meter
energy level
a particular level, or amount, of energy possessed by an atom or ion above the energy it possesses in its least energetic state; also used to refer to the states of energy an electron can have in an atom
the process of giving an atom or an ion an amount of energy greater than it has in its lowest energy (ground) state
the number of waves that cross a given point per unit time (in radiation)
gamma rays
photons (of electromagnetic radiation) of energy with wavelengths no longer than 0.01 nanometer; the most energetic form of electromagnetic radiation
ground state
the lowest energy state of an atom
electromagnetic radiation of wavelength 103–106 nanometers; longer than the longest (red) wavelengths that can be perceived by the eye, but shorter than radio wavelengths
inverse square law
(for light) the amount of energy (light) flowing through a given area in a given time decreases in proportion to the square of the distance from the source of energy or light
an atom that has become electrically charged by the addition or loss of one or more electrons
the process by which an atom gains or loses electrons
any of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons
electromagnetic radiation of wavelengths from 1 millimeter to 1 meter; longer than infrared but shorter than radio waves
nucleus (of an atom)
the massive part of an atom, composed mostly of protons and neutrons, and about which the electrons revolve
a discrete unit (or “packet”) of electromagnetic energy
radial velocity
motion toward or away from the observer; the component of relative velocity that lies in the line of sight
radio waves
all electromagnetic waves longer than microwaves, including radar waves and AM radio waves
an instrument for obtaining a spectrum; in astronomy, usually attached to a telescope to record the spectrum of a star, galaxy, or other astronomical object
Stefan-Boltzmann law
a formula from which the rate at which a blackbody radiates energy can be computed; the total rate of energy emission from a unit area of a blackbody is proportional to the fourth power of its absolute temperature: F = σT4
electromagnetic radiation of wavelengths 10 to 400 nanometers; shorter than the shortest visible wavelengths
visible light
electromagnetic radiation with wavelengths of roughly 400–700 nanometers; visible to the human eye
the distance from crest to crest or trough to trough in a wave
Wien’s law
formula that relates the temperature of a blackbody to the wavelength at which it emits the greatest intensity of radiation
electromagnetic radiation with wavelengths between 0.01 nanometer and 20 nanometers; intermediate between those of ultraviolet radiation and gamma rays
This book was adapted from the following: Fraknoi, A., Morrison, D., & Wolff, S. C. (2016). Key Terms. In Astronomy. OpenStax. under a Creative Commons Attribution License 4.0
Access the entire book for free at


Icon for the Creative Commons Attribution 4.0 International License

PPSC AST 1120: Stellar Astronomy by OpenStax is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book