Descriptive Astronomy - Openstax textbook
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| absorption spectrum | a series or pattern of dark lines superimposed on a continuous spectrum
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| blackbody | an idealized object that absorbs all electromagnetic energy that falls onto it
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| continuous spectrum | a spectrum of light composed of radiation of a continuous range of wavelengths or colors, rather than only certain discrete wavelengths
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| dispersion | separation of different wavelengths of white light through refraction of different amounts
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| 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
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| electromagnetic radiation | radiation consisting of waves propagated through regularly varying electric and magnetic fields and traveling at the speed of light
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| electromagnetic spectrum | the whole array or family of electromagnetic waves, from radio to gamma rays
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| emission spectrum | a series or pattern of bright lines superimposed on a continuous spectrum
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| 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
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| 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
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| excitation | the process of giving an atom or an ion an amount of energy greater than it has in its lowest energy (ground) state
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| frequency | the number of waves that cross a given point per unit time (in radiation)
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| gamma rays | photons (of electromagnetic radiation) of energy with wavelengths no longer than 0.01 nanometer; the most energetic form of electromagnetic radiation
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| ground state | the lowest energy state of an atom
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| infrared | 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
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| 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
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| ion | an atom that has become electrically charged by the addition or loss of one or more electrons
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| ionization | the process by which an atom gains or loses electrons
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| isotope | any of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons
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| microwave | electromagnetic radiation of wavelengths from 1 millimeter to 1 meter; longer than infrared but shorter than radio waves
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| nucleus (of an atom) | the massive part of an atom, composed mostly of protons and neutrons, and about which the electrons revolve
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| photon | a discrete unit (or “packet”) of electromagnetic energy
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| radial velocity | motion toward or away from the observer; the component of relative velocity that lies in the line of sight
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| radio waves | all electromagnetic waves longer than microwaves, including radar waves and AM radio waves
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| spectrometer | 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
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| 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
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| ultraviolet | electromagnetic radiation of wavelengths 10 to 400 nanometers; shorter than the shortest visible wavelengths
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| visible light | electromagnetic radiation with wavelengths of roughly 400–700 nanometers; visible to the human eye
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| wavelength | the distance from crest to crest or trough to trough in a wave
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| Wien’s law | formula that relates the temperature of a blackbody to the wavelength at which it emits the greatest intensity of radiation
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| X-rays | electromagnetic radiation with wavelengths between 0.01 nanometer and 20 nanometers; intermediate between those of ultraviolet radiation and gamma rays
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| adaptive optics | systems used with telescopes that can compensate for distortions in an image introduced by the atmosphere, thus resulting in sharper images
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| aperture | diameter of the primary lens or mirror of a telescope
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| charge-coupled device (CCD) | array of high-sensitivity electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens) to record an image or spectrum
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| chromatic aberration | distortion that causes an image to appear fuzzy when each wavelength coming into a transparent material focuses at a different spot
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| detector | device sensitive to electromagnetic radiation that makes a record of astronomical observations
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| eyepiece | magnifying lens used to view the image produced by the objective lens or primary mirror of a telescope
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| focus | (of telescope) point where the rays of light converged by a mirror or lens meet
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| interference | process in which waves mix together such that their crests and troughs can alternately reinforce and cancel one another
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| interferometer | instrument that combines electromagnetic radiation from one or more telescopes to obtain a resolution equivalent to what would be obtained with a single telescope with a diameter equal to the baseline separating the individual separate telescopes
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| interferometer array | combination of multiple radio dishes to, in effect, work like a large number of two-dish interferometers
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| prime focus | point in a telescope where the objective lens or primary mirror focuses the light
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| radar | technique of transmitting radio waves to an object and then detecting the radiation that the object reflects back to the transmitter; used to measure the distance to, and motion of, a target object or to form images of it
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| reflecting telescope | telescope in which the principal light collector is a concave mirror
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| refracting telescope | telescope in which the principal light collector is a lens or system of lenses
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| resolution | detail in an image; specifically, the smallest angular (or linear) features that can be distinguished
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| seeing | unsteadiness of Earth’s atmosphere, which blurs telescopic images; good seeing means the atmosphere is steady
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| telescope | instrument for collecting visible-light or other electromagnetic radiation
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| asteroid | a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets
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| comet | a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail
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| differentiation | gravitational separation of materials of different density into layers in the interior of a planet or moon
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| giant planet | any of the planets Jupiter, Saturn, Uranus, and Neptune in our solar system, or planets of roughly that mass and composition in other planetary systems
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| half-life | time required for half of the radioactive atoms in a sample to disintegrate
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| meteor | a small piece of solid matter that enters Earth’s atmosphere and burns up, popularly called a shooting star because it is seen as a small flash of light
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| meteorite | a portion of a meteor that survives passage through an atmosphere and strikes the ground
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| planetesimals | objects, from tens to hundreds of kilometers in diameter, that formed in the solar nebula as an intermediate step between tiny grains and the larger planetary objects we see today; the comets and some asteroids may be leftover planetesimals
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| radioactivity | process by which certain kinds of atomic nuclei decay naturally, with the spontaneous emission of subatomic particles and gamma rays
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| solar nebula | the cloud of gas and dust from which the solar system formed
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| terrestrial planet | any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon is included in the list
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| bar | a force of 100,000 Newtons acting on a surface area of 1 square meter; the average pressure of Earth’s atmosphere at sea level is 1.013 bars
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| basalt | igneous rock produced by the cooling of lava; makes up most of Earth’s oceanic crust and is found on other planets that have experienced extensive volcanic activity
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| convection | movement caused within a gas or liquid by the tendency of hotter, and therefore less dense material, to rise and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat
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| core | the central part of the planet; consists of higher density material
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| crust | the outer layer of a terrestrial planet
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| fault | in geology, a crack or break in the crust of a planet along which slippage or movement can take place, accompanied by seismic activity
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| granite | a type of igneous silicate rock that makes up most of Earth’s continental crust
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| greenhouse effect | the blanketing (absorption) of infrared radiation near the surface of a planet—for example, by CO2 in its atmosphere
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| greenhouse gas | a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range; on Earth, these atmospheric gases primarily include carbon dioxide, methane, and water vapor
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| igneous rock | rock produced by cooling from a molten state
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| magnetosphere | the region around a planet in which its intrinsic magnetic field dominates the interplanetary field carried by the solar wind; hence, the region within which charged particles can be trapped by the planetary magnetic field
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| mantle | the largest part of Earth’s interior; lies between the crust and the core
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| mass extinction | the sudden disappearance in the fossil record of a large number of species of life, to be replaced by fossils of new species in subsequent layers; mass extinctions are indicators of catastrophic changes in the environment, such as might be produced by a l
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| metamorphic rock | rock produced by physical and chemical alteration (without melting) under high temperature and pressure
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| ozone | (O3) a heavy molecule of oxygen that contains three atoms rather than the more normal two
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| photosynthesis | a complex sequence of chemical reactions through which some living things can use sunlight to manufacture products that store energy (such as carbohydrates), releasing oxygen as one by-product
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| plate tectonics | the motion of segments or plates of the outer layer of a planet over the underlying mantle
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| primitive rock | rock that has not experienced great heat or pressure and therefore remains representative of the original condensed materials from the solar nebula
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| rift zone | in geology, a place where the crust is being torn apart by internal forces generally associated with the injection of new material from the mantle and with the slow separation of tectonic plates
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| sedimentary rock | rock formed by the deposition and cementing of fine grains of material, such as pieces of igneous rock or the shells of living things
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| seismic wave | a vibration that travels through the interior of Earth or any other object; on Earth, these are generally caused by earthquakes
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| stratosphere | the layer of Earth’s atmosphere above the troposphere and below the ionosphere
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| subduction | the sideways and downward movement of the edge of a plate of Earth’s crust into the mantle beneath another plate
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| troposphere | the lowest level of Earth’s atmosphere, where most weather takes place
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| volcano | a place where material from a planet’s mantle erupts on its surface
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