Astronomy All Chapters

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Chapter 1
astronomical unit (AU): Average distance from Earth to the sun; 1.5 x 108 km, or 93 x 106 mi. (p. 6)
galaxy: A large system of stars, star clusters, gas, dust, and nebulae orbiting a common center of mass. (p. 8)
light-year (ly): A unit of distance. The distance light travels in 1 year. (p. 7)
Milky Way: The hazy band of light that circles our sky. Produced by the glow of our galaxy. (p. 8)
Milky Way Galaxy: The spiral galaxy containing our sun. Visible in the night sky as the Milky Way. (p. 8)
planet: A small, nonluminous body formed by accretion in a disk around a protostar. (p. 6)
scientific notation: The system of recording very large or very small numbers by using powers of 10. (p. 6)
solar system: The sun and its planets, asteroids, comets, and so on. (p. 6)
spiral arm: Long spiral pattern of bright stars, star clusters, gas, and dust. Spiral arms extend from the center to the edge of the disk of spiral galaxies. (p. 8)
star: A globe of gas held together by its own gravity and supported by the internal pressure of its hot gases, which generate energy by nuclear fusion. (p. 6)
Chapter 2
angular diameter: The angle formed by lines extending from the observer to opposite sides of an object. (p. 21)
angular distance: The angle formed by lines extending from the observer to two locations. (p. 21)
apparent visual magnitude (mv): The brightness of a star as seen by human eyes on Earth. (p. 16)
asterism: A named grouping of stars that is not one of the recognized constellations. Examples are the Big Dipper and the Pleiades. (p. 15)
celestial equator: The imaginary line around the sky directly above Earth's equator. (p. 20)
celestial pole (north or south): One of the two points on the celestial sphere directly above Earth's poles. (p. 20)
celestial sphere: An imaginary sphere of very large radius surrounding Earth and to which the planets, stars, sun, and moon seem to be attached. (p. 20)
circumpolar constellation (north or south): A constellation so close to one of the celestial poles that it never sets or never rises as seen from a particular latitude. (p. 21)
constellation: One of the stellar patterns identified by name, usually of mythological gods, people, animals, or objects. Also, the region of the sky containing that star pattern. (p. 14)
east point: One of the four cardinal directions. The point on the horizon directly east. (p. 20)
horizon: The circular boundary between the sky and Earth. (p. 20)
magnitude scale: The astronomical brightness scale. The larger the number, the fainter the star. (p. 15)
minute of arc: An angular measure. One sixtieth of a degree. (p. 21)
model: See scientific model.
nadir: The point on the celestial sphere directly below the observer. The opposite of the zenith. (p. 20)
north celestial pole: The point on the celestial sphere directly above Earth's North Pole. (p. 20)
north point: One of the four cardinal directions. The point on the horizon directly north. (p. 20)
precession: The slow change in the direction of Earth's axis of rotation. One cycle takes nearly 26,000 years. (p. 18)
scientific model: A tentative description of a phenomenon for use as an aid to understanding. (p. 18)
second of arc: An angular measure. One sixtieth of a minute of arc. (p. 21)
south celestial pole: The point on the celestial sphere directly above Earth's South Pole. (p. 20)
south point: One of the four cardinal directions. The point on the horizon directly south. (p. 20)
west point: One of the four cardinal directions. The point on the horizon directly west. (p. 20)
zenith: The point on the sky directly above the observer. (p. 20)
Chapter 3
annular eclipse: A solar eclipse in which the solar photosphere appears around the edge of the moon in a bright ring, or annulus. The corona, chromosphere, and prominences cannot be seen. (p. 37)
aphelion: The orbital point of greatest distance from the sun. (p. 29)
apogee: The point farthest from Earth in the orbit of a body circling Earth. (p. 37)
autumnal equinox: The point on the celestial sphere where the sun crosses the celestial equator going southward. Also, the time when the sun reaches this point and autumn begins in the northern hemisphere - about September 22. (p. 28)
chromosphere: Bright gases just above the photosphere of the sun. (p. 36)
corona: On the sun, the faint outer atmosphere composed of low-density, high-temperature gas. (p. 37)
diamond-ring effect: During a total solar eclipse, the momentary appearance of a spot of photosphere at the edge of the moon, producing a brilliant glare set in the silvery ring of the corona. (p. 37)
ecliptic: The apparent path of the sun around the sky. (p. 26)
evening star: Any planet visible in the sky just after sunset. (p. 30)
lunar eclipse: The darkening of the moon when it moves through Earth's shadow. (p. 31)
Milankovitch hypothesis: Suggestion that Earth's climate is determined by slow periodic changes in the shape of its orbit, the angle of its axis, and precession. (p. 40)
morning star: Any planet visible in the sky just before sunrise. (p. 30)
node: The points where an object's orbit passes through the plane of Earth's orbit. (p. 39)
penumbra: The portion of a shadow that is only partially shaded. (p. 31)
perigee: The point closest to Earth in the orbit of a body circling Earth. (p. 37)
perihelion: The orbital point of closest approach to the sun. (p. 29)
photosphere: The bright visible surface of the sun. (p. 36)
prominence: Eruption on the solar surface. Visible during total solar eclipses. (p. 37)
revolution: Orbital motion about a point located outside the orbiting body. See also rotation. (p. 24)
rotation: Motion around an axis passing through the rotating body. See also revolution. (p. 24)
Saros cycle: An 18-year, 11-1/3-day period after which the pattern of lunar and solar eclipses repeats. (p. 39)
sidereal period: The time a celestial body takes to turn once on its axis or revolve once around its orbit relative to the stars. (p. 33)
solar eclipse: The event that occurs when the moon passes directly between Earth and the sun, blocking our view of the sun. (p. 35)
summer solstice: The point on the celestial sphere where the sun is at its most northerly point. Also, the time when the sun passes this point, about June 22, and summer begins in the northern hemisphere. (p. 28)
synodic period: The time a solar system body takes to orbit the sun once and return to the same orbital relationship with Earth. That is, orbital period referenced to Earth. (p. 33)
umbra: The region of a shadow that is totally shaded. (p. 31)
vernal equinox: The place on the celestial sphere where the sun crosses the celestial equator moving northward. Also, the time of year when the sun crosses this point, about March 21, and spring begins in the northern hemisphere. (p. 28)
winter solstice: The point on the celestial sphere where the sun is farthest south. Also the time of year when the sun passes this point, about December 22, and winter begins in the northern hemisphere. (p. 28)
zodiac: A band centered on the ecliptic and encircling the sky. (p. 27)
Chapter 4
center of mass: The balance point of a body or system of masses. The point about which a body or system of masses rotates in the absence of external forces. (p. 69)
circular velocity: The velocity an object needs to stay in orbit around another object. (p. 68)
closed orbit: An orbit that returns to the same starting point over and over. Either a circular orbit or an elliptical orbit. (p. 69)
deferent: In the Ptolemaic theory, the large circle around Earth along which the center of the epicycle was thought to move. (p. 51)
eccentricity, e: A number between 1 and 0 that describes the shape of an ellipse. The distance from one focus to the center of the ellipse divided by the semimajor axis. (p. 59)
ellipse: A closed curve around two points called the foci such that the total distance from one focus to the curve and back to the other focus remains constant. (p. 58)
epicycle: The small circle followed by a planet in the Ptolemaic theory. The center of the epicycle follows a larger circle (the deferent) around Earth. (p. 51)
equant: In the Ptolemaic theory, the point off center in the deferent from which the center of the epicycle appears to move uniformly. (p. 51)
escape velocity: The initial velocity an object needs to escape from the surface of a celestial body. (p. 69)
geocentric universe: A model universe with Earth at the center, such as the Ptolemaic universe. (p. 50)
geosynchronous satellite: A satellite that orbits eastward around Earth with a period of 24 hours and remains above the same spot on Earth's surface. (p. 68)
heliocentric universe: A model of the universe with the sun at the center, such as the Copernican universe. (p. 49)
hypothesis: A conjecture, subject to further tests, that accounts for a set of facts. (p. 59)
inverse square relation: A rule that the strength of an effect (such as gravity) decreases in proportion as the distance squared increases. (p. 66)
mass: A measure of the amount of matter making up an object. (p. 66)
natural law: A theory that is almost universally accepted as true. (p. 59)
neap tide: Ocean tide of low amplitude occurring at first- and third-quarter moon. (p. 71)
open orbit: An orbit that carries an object away, never to return to its starting point. (p. 69)
paradigm: A commonly accepted set of scientific ideas and assumptions. (p. 54)
parallax: The apparent change in position of an object due to a change in the location of the observer. Astronomical parallax is measured in seconds of arc. (p. 50)
retrograde motion: The apparent backward (westward) motion of planets as seen against the background of stars. (p. 50)
semimajor axis, a: Half of the longest diameter of an ellipse. (p. 58)
spring tide: Ocean tide of high amplitude that occurs at full and new moon. (p. 71)
theory: A system of assumptions and principles applicable to a wide range of phenomena that have been repeatedly verified. (p. 59)
uniform circular motion: The classical belief that the perfect heavens could only move by the combination of uniform motion along circular orbits. (p. 50)
Chapter 5
achromatic lens: A telescope lens composed of two lenses ground from different kinds of glass and designed to bring two selected colors to the same focus and correct for chromatic aberration. (p. 82)
active optics: Thin telescope mirrors that are controlled by computers to maintain proper shape as the telescope moves. (p. 89)
adaptive optics: A computer-controlled optical system used to partially correct for seeing in an astronomical telescope. (p. 89)
alt-azimuth mounting: A telescope mounting that allows the telescope to move in altitude (perpendicular to the horizon) and in azimuth (parallel to the horizon). See also equatorial mounting. (p. 89)
angstrom: A unit of distance. 1 angstrom = 10-10 m. Commonly used to measure the wavelength of light. (p. 79)
atmospheric window: Wavelength region in which our atmosphere is transparent - at visual, infrared, and radio wavelengths. (p. 80)
binding energy: The energy needed to pull an electron away from its atom. (p. 95)
black body radiation: Radiation emitted by a hypothetical perfect radiator. The spectrum is continuous, and the wavelength of maximum emission depends on the body's temperature. (p. 97)
Cassegrain focus: The optical design in which the secondary mirror reflects light back down the tube through a hole in the center of the objective mirror. (p. 88)
CCD: See charge-coupled device.
charge-coupled device (CCD): An electronic device consisting of a large array of light-sensitive elements used to record very faint images. (p. 90)
chromatic aberration: A distortion found in refracting telescopes because lenses focus different colors at slightly different distances. Images are consequently surrounded by color fringes. (p. 81)
comparison spectrum: A spectrum of known spectral lines used to identify unknown wavelengths in an object's spectrum. (p. 91)
diffraction fringe: Blurred fringe surrounding any image, caused by the wave properties of light. Because of this, no image detail smaller than the fringe can be seen. (p. 83)
electromagnetic radiation: Changing electric and magnetic fields that travel through space and transfer energy from one place to another; examples are light or radio waves. (p. 78)
equatorial mounting: A telescope mounting that allows motion parallel to and perpendicular to the celestial equator. (p. 89)
eyepiece: A short-focal-length lens used to enlarge the image in a telescope. The lens nearest the eye. (p. 80)
false-color image: A representation of graphical data with added or enhanced color to reveal detail. (p. 90)
focal length: The focal length of a lens is the distance from the lens to the point where it focuses parallel rays of light. (p. 80)
grating: A piece of material in which numerous microscopic parallel lines are scribed. Light encountering a grating is dispersed to form a spectrum. (p. 91)
interferometry: The observing technique in which separated telescopes combine to produce a virtual telescope with the resolution of a much-larger-diameter telescope. (p. 87)
light pollution: The illumination of the night sky by waste light from cities and outdoor lighting, which prevents the observation of faint objects. (p. 84)
light-gathering power: The ability of a telescope to collect light. Proportional to the area of the telescope's objective lens or mirror. (p. 82)
magnifying power: The ability of a telescope to make an image larger. (p. 84)
nanometer (nm): A unit of distance equaling one-billionth of a meter (10-9 m). (p. 79)
Newtonian focus: The optical design in which a diagonal mirror reflects light out the side of the telescope tube for easier access. (p. 88)
objective lens: In a refracting telescope, the long-focal-length lens that forms an image of the object viewed. The lens closest to the object. (p. 80)
objective mirror: In a reflecting telescope, the principal mirror (reflecting surface) that forms an image of the object viewed. (p. 80)
photon: A quantum of electromagnetic energy. Carries an amount of energy that depends inversely on its wavelength. (p. 78)
polar axis: In an equatorial telescope mounting, the axis that is parallel to Earth's axis. (p. 89)
primary lens: In a refracting telescope, the largest lens. (p. 80)
primary mirror: In a reflecting telescope, the largest mirror. (p. 80)
prime focus: The point at which the objective mirror forms an image in a reflecting telescope. (p. 88)
radio interferometer: Two or more radio telescopes that combine their signals to achieve the resolving power of a larger telescope. (p. 93)
reflecting telescope: A telescope that uses a concave mirror to focus light into an image. (p. 81)
refracting telescope: A telescope that forms images by bending (refracting) light with a lens. (p. 81)
resolving power: The ability of a telescope to reveal fine detail. Depends on the diameter of the telescope objective. (p. 83)
Schmidt-Cassegrain focus: The optical design that uses a thin corrector plate at the entrance to the telescope tube. A popular design for small telescopes. (p. 88)
secondary mirror: In a reflecting telescope, a mirror that directs the light from the primary mirror to a focal position. (p. 88)
seeing: Atmospheric conditions on a given night. When the atmosphere is unsteady, producing blurred images, the seeing is said to be poor. (p. 83)
sidereal drive: The motor and gears on a telescope that turn it westward to keep it pointed at a star. (p. 89)
spectrograph: A device that separates light by wavelengths to produce a spectrum. (p. 91)
wavelength: The distance between successive peaks or troughs of a wave. Usually represented by the Greek letter lambda. (p. 78)
Chapter 6
absolute zero: The theoretical lowest possible temperature at which a material contains no extractable heat energy. Zero on the Kelvin temperature scale. (p. 108)
absorption line: A dark line in a spectrum. Produced by the absence of photons absorbed by atoms or molecules. (p. 110)
absorption spectrum (dark-line spectrum): A spectrum that contains absorption lines. (p. 110)
Balmer series: A series of spectral lines produced by hydrogen in the near-ultraviolet and visible parts of the spectrum. The three longest-wavelength Balmer lines are visible to the human eye. (p. 111)
blue shift: A Doppler shift toward shorter wavelengths caused by a velocity of approach. (p. 105)
bright-line spectrum: See emission spectrum.
continuous spectrum: A spectrum in which there are no absorption or emission lines. (p. 110)
Coulomb force: The electrostatic force of repulsion or attraction between charged bodies. (p. 105)
dark-line spectrum: See absorption spectrum.
Doppler effect: The change in the wavelength of radiation due to relative radial motion of source and observer. (p. 115)
electron: Low-mass atomic particle carrying a negative charge. (p. 104)
emission line: A bright line in a spectrum caused by the emission of photons from atoms. (p. 110)
emission spectrum (bright-line spectrum): A spectrum containing emission lines. (p. 110)
energy level: One of a number of states an electron may occupy in an atom, depending on its binding energy. (p. 106)
excited atom: An atom in which an electron has moved from a lower to a higher energy level. (p. 106)
ground state: The lowest permitted electron energy level in an atom. (p. 107)
heat: Energy stored in a material as agitation among its particles. (p. 107)
ion: An atom that has lost or gained one or more electrons. (p. 105)
ionization: The process in which atoms lose or gain electrons. (p. 105)
isotopes: Atoms that have the same number of protons but a different number of neutrons. (p. 105)
joule (J): A unit of energy equivalent to a force of 1 newton acting over a distance of 1 m. One joule per second equals 1 watt of power. (p. 109)
Kelvin temperature scale: A temperature scale using Celsius degrees and based on zero at absolute zero. (p. 107)
Kirchhoff 's laws: A set of laws that describe the absorption and emission of light by matter. (p. 110)
L dwarf: A main-sequence star cooler than an M star. (p. 114)
Lyman series: Spectral lines in the ultraviolet spectrum of hydrogen produced by transitions whose lowest energy level is the ground state. (p. 111)
molecule: Two or more atoms bonded together. (p. 105)
neutron: An atomic particle with no charge and about the same mass as a proton. (p. 104)
nucleus (of an atom): The central core of an atom containing protons and neutrons. Carries a net positive charge. (p. 104)
Paschen series: Spectral lines in the infrared spectrum of hydrogen produced by transitions whose lowest energy level is the third. (p. 111)
permitted orbit: One of the energy levels in an atom that an electron may occupy. (p. 106)
proton: A positively charged atomic particle contained in the nucleus of an atom. The nucleus of a hydrogen atom. (p. 104)
quantum mechanics: The study of the behavior of atoms and atomic particles. (p. 105)
radial velocity (Vr): That component of an object's velocity directed away from or toward Earth. (p. 116)
redshift: A Doppler shift toward longer wavelengths caused by a velocity of recession. (p. 115)
spectral class or type: A star's position in the temperature classification system O, B, A, F, G, K, M. Based on the appearance of the star's spectrum. (p. 113)
spectral line: A line in a spectrum at a specific wavelength produced by the absorption or emission of light by certain atoms. (p. 104)
spectral sequence: The arrangement of spectral classes (O, B, A, F, G, K, M) ranging from hot to cool. (p. 113)
T dwarf: A very cool, low-mass star or brown dwarf located below the L stars on the main sequence. (p. 114)
temperature: A measure of the agitation among the atoms and molecules of a material. The intensity of heat. (p. 107)
transition: The movement of an electron from one atomic energy level to another. (p. 111)
wavelength of maximum intensity: The wavelength at which a perfect radiator emits the maximum amount of energy. Depends only on the object's temperature. (p. 108)
Chapter 7
active region: A magnetic region on the solar surface that includes sunspots, prominences, flares, etc. (p. 131)
aurora: The glowing light display that results when a planet's magnetic field guides charged particles toward the north and south magnetic poles, where they strike the upper atmosphere and excite atoms to emit photons. (p. 137)
Babcock model: A model of the sun's magnetic cycle in which the differential rotation of the sun winds up and tangles the solar magnetic field in a 22-year cycle. This is thought to be responsible for the 11-year sunspot cycle. (p. 133)
convection: Circulation in a fluid driven by heat. Hot material rises and cool material sinks. (p. 125)
coronal hole: An area of the solar surface that is dark at X-ray wavelengths. Thought to be associated with divergent magnetic fields and the source of the solar wind. (p. 137)
coronal mass ejection (CME): Matter ejected from the sun's corona in powerful surges guided by magnetic fields. (p. 137)
Coulomb barrier: The electrostatic force of repulsion between bodies of like charge. Commonly applied to atomic nuclei. (p. 138)
deuterium: An isotope of hydrogen in which the nucleus contains a proton and a neutron. (p. 139)
differential rotation: The rotation of a body in which different parts of the body have different periods of rotation. This is true of the sun, the Jovian planets, and the disk of the galaxy. (p. 132)
dynamo effect: The process by which a rotating, convecting body of conducting matter, such as Earth's core, can generate a magnetic field. (p. 132)
filament: A solar prominence seen from above silhouetted against the bright photosphere. (p. 126)
filtergram: A photograph (usually of the sun) taken in the light of a specific region of the spectrum - for example, an H-alpha filtergram. (p. 126)
flare: A violent eruption on the sun's surface. (p. 137)
granulation: The fine structure of bright grains covering the sun's surface. (p. 125)
helioseismology: The study of the interior of the sun by the analysis of its modes of vibration. (p. 128)
magnetic carpet: The network of small magnetic loops that covers the solar surface. (p. 127)
Maunder butterfly diagram: A graph showing the latitude of sunspots versus time. First plotted by W. W. Maunder in 1904. (p. 130)
Maunder minimum: A period of less numerous sunspots and other solar activity between 1645 and 1715. (p. 131)
neutrino: A neutral, massless atomic particle that travels at or nearly at the speed of light. (p. 139)
nuclear fission: Reactions that break the nuclei of atoms into fragments. (p. 135)
nuclear fusion: Reactions that join the nuclei of atoms to form more massive nuclei. (p. 135)
proton-proton chain: A series of three nuclear reactions that builds a helium atom by adding together protons. The main energy source in the sun. (p. 139)
reconnection: On the sun, the merging of magnetic fields to release energy in the form of flares. (p. 137)
solar wind: Rapidly moving atoms and ions that escape from the solar corona and blow outward through the solar system. (p. 127)
spicule: A small, flamelike projection in the chromosphere of the sun. (p. 126)
strong force: One of the four forces of nature. The strong force binds protons and neutrons together in atomic nuclei. (p. 135)
sunspot: Relatively dark spot on the sun that contains intense magnetic fields. (p. 124)
supergranule: Very large convective features in the sun's surface. (p. 127)
weak force: One of the four forces of nature. The weak force is responsible for some forms of radioactive decay. (p. 135)
Zeeman effect: The splitting of spectral lines into multiple components when the atoms are in a magnetic field. (p. 131)
Chapter 8
absolute visual magnitude (Mv): Intrinsic brightness of a star. The apparent visual magnitude the star would have if it were 10 pc away. (p. 149)
binary stars: Pairs of stars that orbit around their common center of mass. (p. 157)
bipolar flow: Jets of gas flowing away from a central object in opposite directions. Usually applied to protostars. (p. 167)
birth line: In the H-R diagram, the line above the main sequence where protostars first become visible. (p. 165)
Bok globule: Small, dark cloud only about 1 ly in diameter that contains 10 to 1000 solar masses of gas and dust. Believed to be related to star formation. (p. 166)
eclipsing binary system: A binary star system in which the stars eclipse each other. (p. 160)
flux: A measure of the flow of energy out of a surface. Usually applied to light. (p. 149)
giant: Large, cool, highly luminous star in the upper right of the H-R diagram. Typically 10 to 100 times the diameter of the sun. (p. 152)
H-R diagram: See Hertzsprung-Russell diagram.
Hertzsprung-Russell (H-R) diagram: A plot of the intrinsic brightness versus the surface temperature of stars. It separates the effects of temperature and surface area on stellar luminosity. Commonly plotted as absolute magnitude versus spectral type but also as luminosity versus surface temperature or color. (p. 151)
light curve: A graph of brightness versus time commonly used in analyzing variable stars and eclipsing binaries. (p. 160)
luminosity (L): The total amount of energy a star radiates in 1 second. (p. 149)
luminosity class: A category of stars of similar luminosity, determined by the widths of lines in their spectra. (p. 153)
main sequence: The region of the H-R diagram running from upper left to lower right, which includes roughly 90 percent of all stars. (p. 152)
mass-luminosity relation: The more massive a star is, the more luminous it is. (p. 162)
parsec (pc): The distance to a hypothetical star whose parallax is 1 second of arc. 1 pc = 206,265 AU = 3.26 ly. (p. 147)
red dwarf: A faint, cool, low-mass, main sequence star. (p. 153)
spectroscopic binary system: A star system in which the stars are too close together to be visible separately. We see a single point of light, and only by taking a spectrum can we determine that there are two stars. (p. 158)
spectroscopic parallax: The method of determining a star's distance by comparing its apparent magnitude with its absolute magnitude as estimated from its spectrum. (p. 154)
stellar parallax (p): A measure of stellar distance. See also parallax. (p. 147)
supergiant: Exceptionally luminous star whose diameter is 10 to 1000 times that of the sun. (p. 152)
visual binary system: A binary star system in which the two stars are separately visible in the telescope. (p. 156)
white dwarf: Dying star that has collapsed to the size of Earth and is slowly cooling off. At the lower left of the H-R diagram. (p. 153)
Chapter 9
association: Group of widely scattered stars (10 to 100) moving together through space. Not gravitationally bound into clusters. (p. 180)
brown dwarf: A star whose mass is too low to ignite nuclear fusion. Heated by contraction. (p. 192)
carbon-nitrogen-oxygen (CNO) cycle: A series of nuclear reactions that use carbon as a catalyst to combine four hydrogen atoms to make one helium atom plus energy. Effective in stars more massive than the sun. (p. 183)
CNO cycle: See carbon-nitrogen-oxygen cycle.
conservation of energy: One of the basic laws of stellar structure. The amount of energy flowing out of the top of a shell must equal the amount coming in at the bottom plus whatever energy is generated within the shell. (p. 187)
conservation of mass: One of the basic laws of stellar structure. The total mass of the star must equal the sum of the masses of the shells, and the mass must be distributed smoothly through the star. (p. 187)
dark nebula: A cloud of gas and dust seen silhouetted against a brighter nebula. (p. 177)
emission nebula: A cloud of glowing gas excited by ultraviolet radiation from hot stars. (p. 176)
energy transport: Flow of energy from hot regions to cooler regions by one of three methods: conduction, convection, or radiation. (p. 188)
evolutionary track: The path a star follows in the H-R diagram as it gradually changes its surface temperature and luminosity. (p. 180)
H II region: A region of ionized hydrogen around a hot star. (p. 176)
Herbig-Haro object: A small nebula that varies irregularly in brightness. Believed to be associated with star formation. (p. 185)
hydrostatic equilibrium: The balance between the weight of the material pressing downward on a layer in a star and the pressure in that layer. (p. 187)
infrared cirrus: Wispy network of cold dust clouds discovered by the Infrared Astronomy Satellite. (p. 178)
interstellar dust: Microscopic solid grains in the interstellar medium. (p. 174)
interstellar medium: The gas and dust distributed between the stars. (p. 174)

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