Glossary of Terms

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Atmosphere: A gaseous envelopesurrounding a planet, or the visible layers of a star; also a unit of pressure(abbreviated atm) equal to the pressure of air at sea level on the Earth's surface.

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.

AU: Abbreviation for "astronomical unit," the distance between the Earth and the Sun (about 93 million miles)

Big Bang model: A theory of the evolutionof the universe that postulates its origin, in anevent called the Big Bang, from a hot, dense statethat rapidly expanded to cooler, less-dense states.

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.

Celestial sphere: An imaginary sphere of very large radius centered on the earth, on which the celestial bodies appear fastened and against which their motions are charted.

Chromosphere: The part of the Sun's atmosphere just above the photosphere; hotter and less dense than the photosphere; it creates the flash spectrum seen during eclipses.

Continuous spectrum: A spectrum showing emission at all wavelengths, unbroken by either absorption lines or emission lines.

Corona: The outermost region of the Sun's atmosphere, consisting of thin, ionized gases at a temperature of about 1,000,000 K.

Coronal holes: Regions in the Sun's corona that lack a concentration of high-temperature plasma; here magnetic field lines extend out into interplanetary space and mark the source of the solar wind.

Coronal interstellar gas: High-temperature interstellar plasma made visible by its X-ray emission.

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.

Cosmology: The study of the nature and evolution of the physical universe.

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.

Dark cloud: An interstellar cloud of as and dust that contains enough dust to blot out the light of stars behind it (as seen from the Earth).

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.

Density: The amount of mass per volume in an object or region of space.

Diffuse (bright) nebula: A cloud of ionized gas, mostly hydrogen, with an emission-line spectrum.

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 force: one ofthe four forces of nature; particles with electromagnetic charge eitherattract or repel each other depending upon whether the two charges are oppositeor identical.

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.

Electromagnetic spectrum: The range of all wavelengths of electromagnetic radiation.

Electron: A very lightweight, negatively charged subatomic particle.

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.

Emission-line spectrum: A spectrum containing only emission lines.

Empirical: Derived from experiment or observation.

Excitation: The process of raising an atom to a higher energy level.

Extinction: The dimming of light when it passes through some medium, such as the earth's atmosphere, or interstellar material.

Flash spectrum: The spectrum that appears immediately before the totality of a solar eclipse as the normal absorption spectrum is replaced briefly by the corona's own emission spectrum.

Flux: The amount of energy flowing through a givenarea in a given amount of time, usually expressed as counts per area persecond.

Frame of reference: A set of axes with respect to which the position or motion of something can be described, or physical laws can be formulated.

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.

Fusion: A nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy

Galactic equator: The great circle along the line of the Milky Way, marking the central plane of the Galaxy.

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.

Gamma ray: A very high energy photon with a wavelength shorter than that of X-rays.

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.

Gravitational focusing: The directing of the paths of small masses by a larger one so that their paths cross, which enhances their accretion onto the larger mass.

Gravitational lens: The bending effect of a large mass on light rays so that they form an image of the source of light.

Gravitational redshift: The change to longer wavelengths that marks the loss of energy by a photon that moves from a stronger to a weaker gravitational field.

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.

High-velocity stars: Stars in the Galaxy with velocities greater than 60 km/s relative to the Sun; they have orbits with high eccentricities, often at large angles with respect to the Galactic plane.

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.

Hubble time: Numerically the inverse of the Hubble constant; it represents, in order of magnitude, the age of the universe.

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

Interstellar extinction curve: The amount of extinction from interstellar dust as a function of wavelength.

Interstellar gas: Atoms, molecules, and ions in the interstellar medium.

Interstellar medium: All the gas and dust found between stars.

Ion: An atom that has become electrically charged by the gain or loss of one or more electrons.

Ionization: The process by which an atom loses or gains electrons.

Ionized gas: A gas that has been ionized so that it contains free electrons and charged particles; a plasma if it is electrically neutral overall.

Ionosphere: A layer of the Earth's atmosphere ranging from about 100 to 700 kilometers above the surface, where oxygen and nitrogen are ionized by sunlight, producing free electrons.

Isotope: Atoms with the same number of protons but different numbers of neutrons.

Isotropic: Having no preferred direction in space.

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

Large Magellanic Cloud (LMC): A small galaxy, irregular in shape, about 50 kpc from the Milky Way.

Lens: A curved piece of glass designed to bringlight rays to a focus.

Light-year: The distance light travels in one earth year, about 30,900,000,000,000 kilometers.

Local Group: A gravitationally bound group of about 20 galaxies to which our Milky Way Galaxy belongs.

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.

Magnetic field: The property of space having the potential of exerting magnetic forces on bodies within it.

Magnetic flux: The number of magneticfield lines passing through an area.

Magnetometer: A device to measure the strength of a magnetic field.

Magnetosphere: The region around a planet where particles from the solar wind are trapped by the planet's magnetic field.

Magnitude: An astronomical measurement of an object's brightness; larger magnitudes represent fainter objects.

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.

Metallic hydrogen: A state of hydrogen, reached at high pressures, where it is able to conduct electricity.

Microwave background radiation: A universal bath of low-energy photons having a blackbody spectrum with a temperature of 2.7 K.

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.

Molecular cloud: Large, dense, massive clouds in the plane of a spiral galaxy; they contain dust and a large fraction of gas in molecular form.

Molecule: A combination of two or more atoms bound together electrically; the smallest part of a compound that has the properties of that substances.

Momentum: The product of an object's mass and velocity.

Nanometer: 0.000000001 of a meter; common unit of wavelength measurement for light.

Nebula: (Latin for "cloud") A cloudof interstellar gas and dust.

Neutron: A subatomic particle with about the mass of a proton and no electric charge; one of the main constituents of an atomic nucleus; the union of a proton and an electron.

North magnetic pole: One of two points on a star or planet from which magnetic lines of force emanate and to which the north pole of a compass points.

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.

Nucleus: The massive central part of an atom, containing neutrons and protons, about which the electrons orbit.

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.

Period: The time interval for some regular event to take place; for example, the time required for one complete revolution of a body around another.

Photodissociation: The breakup of a molecule by the absorption of light with enough energy to break the molecular bonds.

Photometer: A light-sensitive detector placed at the focus of a telescope; it is used to make accurate measurements of small photon fluxes.

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.

Pickup ion: An atom from the interstellar medium which has been ionized by the Sun's radiation, and is then swept along with the solar wind.

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.

Plasma: A gas consisting of equal numbers of ionized atoms and electrons.

Prominences: Cool clouds of hydrogen gas above the sun's photosphere in the corona; they are shaped by the local magnetic fields of active regions.

Proton: A massive, positively charged elementary particle; one of the main constituents of the nucleus of an atom.

Radiation belts: In a planet's magnetosphere, regions with a high density of trapped solar wind particles.

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.

Reflection nebula: A bright cloud of gas and dust that is visible because of the reflection of starlight by the dust.

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.

Scattering (of light): The change in the paths of photons without absorption or change in wavelength.

Scientific model: A mental image ofhow the natural world works, based on physical, mathematical, and aestheticideas.

Second of arc: 1/3600 of a degree, or 1/60 of a minute of arc.

Solar core: Region of the Sun's interior where temperatures and densities are high enough for fusion reactions to take place.

Solar cosmic rays: Low energy cosmic rays generated in solar flares.

Solar flare: Sudden burst of electromagnetic energy and particles from a magnetic loop in an active region.

Solar wind: A stream of charged particles, mostly protons and electrons, that escapes into the Sun's outer atmosphere at high speeds and streams out into the solar system.

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.

Spiral galaxy: A galaxy with spiral arms; the presumed shape of our Milky Way galaxy.

Stellar nucleosynthesis: A process in which nuclear fusion builds up heavier nuclei while supplying the energy by which stars shine.

Sunspot: A temporary cool region in the Sun's photosphere, associated with an active region, with a magnetic field intensity of a few 0.1 T (tesla).

Supernova: A stupendous explosion of a massive star, which increases its brightness hundreds of millions of times in a few days.

Supernova remnant: Expanding gas cloud from the outer layers of a star blown off in a supernova explosion; detectable at radio wavelengths; moves through the interstellar medium at high speeds.

Temperature: A measure of the average random speeds of the microscopic particles in a substance.

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.

Thermal radiation: Electromagnetic radiation due to the fact that a body is hot; often characterized by a blackbody spectrum.

Thermosphere: A layer in the earth's atmosphere, above the mesosphere, heated by X-rays and ultraviolet radiation from the Sun.

Time: A measure of the flow of events.

Universe: The totality of all space and time; all that is, has been, and will be.

Wavelength: The distance between two successive peaks or troughs of a wave.

X-rays: High-energy electromagnetic radiation with a wavelength of about 10-10 meter.

Definitions taken from Introductory Astronomy & Astrophysics, Third Edition