A ring
The outermost ring of Saturn visible from Earth; it is located just beyond Cassini’s division.
absolute magnitude
The apparent magnitude that a star would have if it were 10 parsecs from Earth.
absorption line
A dark line in a continuous spectrum created when photons of a certain energy are absorbed by atoms or molecules.
absorption line spectrum
Dark lines superimposed on a continuous spectrum.
acceleration
A change in the direction or magnitude of a velocity.
accretion
The gradual accumulation of matter by an astronomical body, usually caused by gravity.
accretion disk
An orbiting disk of matter spiraling in toward a star or black hole.
active galactic nuclei (AGN)
Supermassive black holes in the cores of some galaxies that emit particles and radiation which, when viewed from different angles, create Seyfert galaxies, radio galaxies, double radio sources, BL Lacertae objects, and quasars.
active galaxy
A very luminous galaxy, often containing an active galactic nucleus.
active optics
A system that adjusts a reflecting telescope in response to changes in temperature and shape of the mount; it helps optimize an image.
adaptive optics
Primary telescope mirrors that are continuously and automatically adjusted to compensate for the distortion of starlight due to the motion of Earth’s atmosphere.
AGB star
See asymptotic giant branch (AGB) star.
albedo
The fraction of sunlight that a planet, asteroid, or satellite scatters directly back into space.
amino acids
A class of chemical compounds that are the building blocks of proteins.
angle
The opening between two straight lines that meet at a point.
angular diameter (angular size)
The arc angle across an object.
angular momentum
A measure of how much energy an object has stored in its rotation and/or revolution.
angular resolution (resolution)
The angular size of the smallest detail of an astronomical object that can be distinguished with a telescope.
annular eclipse
An eclipse of the Sun in which the Moon is too distant to cover the Sun completely so that a ring of sunlight is seen around the Moon at mideclipse.
anorthosite
A light-colored rock found throughout the lunar highlands and in some very old mountains on Earth.
aphelion
The point in its orbit where a planet or other solar system body is farthest from the Sun.
Apollo asteroid
An asteroid that is sometimes closer to the Sun than Earth is.
apparent magnitude
A measure of the brightness of light from a star or other object as seen from Earth.
arc angle
The measurement of the angle between two objects or two parts of the same object.
asterism
The pattern of relatively bright stars associated with a constellation.
asteroid
Any of the rocky objects larger than about ten meters in diameter (and not classified as a planet or moon) that orbits the Sun.
asteroid belt
A 1½-astronomical-unit-wide region between the orbits of Mars and Jupiter in which most of the asteroids closer to the Sun than Neptune are found.
astrobiology
The study of life in the universe.
astronomical unit (AU)
The average distance between the Earth and the Sun: 1.5 × 108 km = 93 million mi.
asymptotic giant branch (AGB) star
A red giant star that has completed core helium fusion and has reexpanded for a second time.
atom
The smallest particle of an element that has the properties characterizing that element.
atomic number
The number of protons in the nucleus of an atom.
autumnal equinox
The intersection of the ecliptic and the celestial equator where the Sun crosses the equator moving from north to south. The beginning of autumn (around September 23).
average density
The mass of an object divided by its volume.
B ring
The brightest of the three rings of Saturn visible from Earth; it lies just inside the Cassini division.
barred spiral galaxy
A spiral galaxy in which the spiral arms begin from the ends of a bar running through the nuclear bulge.
belt asteroid
An asteroid whose orbit lies in the asteroid belt.
belts (of Jupiter)
Dark, reddish bands in Jupiter’s cloud cover.
Big Bang
An explosion that took place about 13.8 billion years ago, creating all space, time, matter, and energy in which the universe emerged.
binary star
Two stars revolving about each other; a double star.
birth line
A line on the Hertzsprung-Russell diagram corresponding to where stars with different masses transform from protostars to pre–main-sequence stars.
BL Lacertae (BL Lac) object
A type of active galaxy; a blazar.
black hole
An object whose gravity is so strong that the escape velocity from it exceeds the speed of light.
blackbody
A hypothetical perfect radiator that absorbs and reemits all radiation falling upon it.
blackbody curve
The curve obtained when the intensity of radiation from a blackbody at a particular temperature is plotted against wavelength.
blazar
A BL Lacertae object.
blueshift
A shift of all spectral features toward shorter wavelengths; the Doppler shift of light from an approaching source.
Bok globule
A small, roundish dark nebula in which stars are forming.
brown dwarf
Any of the planetlike bodies with less than 0.08 M⊙ and more than about 13 MJupiter; such bodies do not have enough mass to sustain fusion in their cores.
C ring
The faint, inner portion of Saturn’s main ring system.
caldera
The crater at the summit of a volcano.
capture theory
The idea that the Moon was created at a different location in the solar system and subsequently captured by Earth’s gravity.
carbonaceous chondrites
A class of extremely ancient, carbon-rich meteorites.
Cassegrain focus
An optical arrangement in a reflecting telescope in which light rays are reflected by a secondary mirror through a hole in the primary mirror.
Cassini division
A prominent gap between Saturn’s A and B rings discovered in 1675 by J. D. Cassini.
celestial equator
A great circle on the celestial sphere 90° from the celestial poles.
celestial sphere
A hypothetical sphere of very large radius centered on the observer; the apparent sphere of the night sky.
center of mass
The point around which a rigid system is perfectly balanced in a gravitational field; also, the point in space around which mutually orbiting bodies have elliptical orbits.
central bulge
A flattened sphere of stars centered on a spiral galaxy’s nucleus, extending out to the vicinity of the spiral arms.
Cepheid variable
One of two types of yellow, supergiant, pulsating stars.
Cerenkov radiation
Radiation produced by particles traveling through a substance faster than light can.
Chandrasekhar limit
The maximum mass of a white dwarf, about 1.4 M⊙.
charge-coupled device (CCD)
A type of solid-state silicon wafer designed to detect photons.
chondrites
Stony meteorites that have never been melted or otherwise altered since they formed. They contain a variety of debris, including spherical droplets called chondrules.
chromatic aberration
An optical property whereby different colors of light passing through a lens are focused at different distances from it.
chromosphere
The layer in the solar atmosphere between the photosphere and the corona.
circumpolar stars
All the stars that never set at a given latitude. In the northern hemisphere, these are all the stars between Polaris and the northern horizon.
close binary
A binary star whose members are separated by a few stellar diameters.
closed universe
A universe that contains enough matter to cause it to recollapse. It is finite in extent and has no “outside.”
cluster (of galaxies)
A collection of a few hundred to a few thousand galaxies bound by their mutual gravitational attraction.
cocreation theory
The theory that the Moon formed simultaneously with Earth and in orbit around it.
collision-ejection theory
The theory that the Moon was created by the impact of a planet-sized object with Earth; presently considered the most plausible theory of the Moon’s formation.
coma (of a comet)
The nearly spherical volume of diffuse gas surrounding the nucleus of a comet near the Sun.
comet
A small body of ice and dust in orbit about the Sun. While passing near the Sun, a comet’s vaporized ices give rise to a coma, tails, and a hydrogen envelope.
configuration (of a planet)
A particular geometric arrangement of Earth, a planet, and the Sun.
confinement
The moment shortly after the Big Bang when quarks bound together to form particles like protons and neutrons.
conjunction
The alignment of two bodies in the solar system so that they appear in the same part of the sky as seen from Earth.
conservation of angular momentum
The law of physics stating that the total amount of angular momentum in an isolated system remains constant.
conservation of linear momentum
If the sum of the external forces on a system remains zero, the total linear momentum of the system remains constant.
constellation
Any of the 88 contiguous regions that cover the entire celestial sphere, including all the objects in each region; also, a configuration of stars often named after an object, a person, or an animal.
contact binary
A close binary system in which both stars fill or overflow their Roche lobes.
continental drift
The gradual movement of the continents over the surface of Earth due to plate tectonics.
continuous spectrum (continuum)
A spectrum of light over a range of wavelengths without any spectral lines.
convection
The transfer of energy by moving currents of fluid or gas containing that energy.
convective zone
A layer in a star where energy is transported outward by means of convection; also known as the convective envelope or convection zone.
core
The central portion of any astronomical object.
core (of the Earth)
The central solid iron region and the liquid iron region surrounding it inside the Earth.
core (of the Sun)
The central quarter of the Sun’s radius, in which hydrogen is fused into helium, releasing the energy that enables the Sun to shine.
core helium fusion
The fusion of helium to form carbon and oxygen at the center of a star.
corona
The outermost layer of the Sun’s atmosphere.
coronal hole
A dark region of the Sun’s inner corona as seen at X-ray wavelengths.
coronal mass ejection
Large volumes of high-energy gas released from the Sun’s corona.
cosmic censorship
The belief that the only connection between a black hole and the universe is the black hole’s event horizon.
cosmic light horizon
A sphere, centered on Earth, whose radius equals the distance traveled by light since the Big Bang.
cosmic microwave background
Photons from every part of the sky with a blackbody spectrum at 2.73 K; the cooled-off radiation from the primordial fireball that originally filled all space.
cosmic rays (primary cosmic rays)
High-speed particles traveling through space.
cosmic ray shower
(see also the related secondary cosmic rays) Groups of particles from Earth’s atmosphere propelled Earthward by the impact of a primary cosmic ray.
cosmological constant
A number sometimes inserted in the equations of general relativity that represents a pressure that opposes gravity throughout the universe.
cosmological redshift
An increase in wavelength from distant galaxies and quasars caused by the expansion of the universe.
cosmology
The study of the formation, organization, and evolution of the universe.
coudé focus
A reflecting telescope in which a series of mirrors direct light out the side of the telescope to a remote focus.
crater
A circular depression on a celestial body caused by the impact of a meteoroid, asteroid, or comet or by a volcano.
crust
The solid surface layer of some astronomical bodies, including the terrestrial planets, the moons, the asteroids, and some stellar remnants.
dark ages
The age of the universe between the time of decoupling and the first burst of star formation.
dark energy
A repulsive gravitational effect that is causing the universe to accelerate outward.
dark matter
The as-yet-unidentified matter in the universe that is underluminous and probably quite different from ordinary matter.
dark nebula
A cloud of interstellar gas and dust that obscures the light of more distant stars.
declination (dec)
The coordinate on the celestial sphere exactly analogous to latitude on Earth; measured north and south of the celestial equator.
decoupling
The epoch in the early universe when electrons and ions first combined to create stable atoms; the time when electromagnetic radiation ceased to dominate over matter.
degree
(°) A unit of angular measure (360° in a circle) or a temperature measure (in units of Kelvins, centigrade, or Fahrenheit).
dense core
Any of the regions of interstellar gas clouds that are slightly denser than normal and destined to collapse to form one or a few stars.
detached binary
A binary system in which the surfaces of both stars are inside their Roche lobes.
differential rotation
The rotation of a nonrigid object in which parts at different latitudes or different radial distances move at different speeds.
diffraction grating
An optical device consisting of closely spaced lines ruled on a piece of glass or plastic that is used like a prism to disperse light into a spectrum.
direct motion
The daily, eastward (leftward) apparent motion of a planet against the background stars as seen from Earth.
disk (of a galaxy)
A flattened assemblage of stars, gas, and dust in a spiral galaxy.
distance modulus
The difference between the apparent and absolute magnitudes of an object.
diurnal motion
Cyclic motion with a 1-day period.
Doppler shift
The change in wavelength of radiation due to relative motion between the source and the observer along the line of sight.
double-radio source
An extragalactic radio source characterized by two large lobes of radio emission, often located on either side of an active galaxy.
Drake equation
A mathematical equation used to estimate the number of extraterrestrial civilizations that may exist in our Galaxy.
dust devil
Whirlwind found in dry or desert areas on both Earth and Mars.
dust tail
A comet tail composed of dust particles escaping from the comet’s nucleus.
dwarf planet
A celestial body that is in orbit around the Sun and has sufficient mass for its self-gravity to pull the body into a nearly spherical shape, but does not have enough gravity to clear its orbital neighborhood of all the small debris orbiting there.
dynamo theory
The generation of a magnetic field by circulating electric charges.
eclipse path
The track of the tip of the Moon’s shadow along Earth’s surface during a total or annular solar eclipse.
eclipsing binary
A double star system in which stars periodically pass in front of each other as seen from Earth.
ecliptic
The annual path of the Sun on the celestial sphere; the plane of Earth’s orbit around the Sun.
Einstein cross
The appearance of four images of the same galaxy or quasar due to gravitational lensing by an intervening galaxy.
Einstein ring
The circular or arc-shaped image of a distant galaxy or quasar created by gravitational lensing by an intervening galaxy.
ejecta blanket
The ring of material surrounding a crater that was ejected during the crater-forming impact.
electromagnetic force
The interaction between charged particles, the second of four fundamental forces in nature.
electromagnetic radiation
Radiation consisting of oscillating electric and magnetic fields, namely gamma rays, X rays, visible light, ultraviolet and infrared radiation, and radio waves.
electromagnetic spectrum
The entire array of electromagnetic radiation.
electron
A negatively charged subatomic particle usually found in orbit about the nucleus of an atom.
electron degeneracy pressure
A powerful pressure produced by repulsion of closely packed (degenerate) electrons.
element
A substance that cannot be decomposed by chemical means into simpler substances. Every atom of the same element contains the same number of protons.
ellipse
A closed curve obtained by cutting completely through a circular cone with a plane; the shape of planetary orbits.
elliptical galaxy
A galaxy with an elliptical shape, little interstellar matter, and no spiral arms.
elongation
The angle between a planet and the Sun as seen from Earth.
emission line
A bright line of electromagnetic radiation.
emission line spectrum
A spectrum that contains only bright emission lines.
emission nebula
A glowing gaseous nebula whose light comes from fluorescence caused by a nearby star.
Encke division
A thin gap in Saturn’s A ring, possibly first seen by J. F. Encke in 1838.
energy flux
The amount of energy emitted from each square meter of an object’s surface per second.
equinox
Either of the two days of the year when the Sun crosses the celestial equator and is therefore directly over Earth’s equator; see also autumnal equinox and vernal equinox.
era of recombination
The time, roughly 400,000 years after the Big Bang, when the universe became transparent.
ergoregion
The region of space immediately outside the event horizon of a rotating black hole where it is impossible to remain at rest.
event horizon
The location around a black hole where the escape velocity equals the speed of light; the boundary of a black hole.
evolutionary track
On the Hertzsprung-Russell diagram, the path followed by a point representing an evolving star.
excited state
The orbit of an electron with energy greater than the lowest energy orbit (or state) available to that election.
expanding universe
The motion of the superclusters of galaxies away from each other.
eyepiece lens
A convex lens used to view the image produced at the focus of a telescope.
F ring
A thin ring just beyond the outer edge of Saturn’s main ring system.
filament
A dark curve seen above the Sun’s photosphere that is the top view of a solar prominence.
fission theory
The theory that the Moon formed from matter flung from the young Earth because the planet was rotating extremely fast when it formed.
focal length
The distance from a lens or concave mirror to where converging light rays meet.
focal plane
The plane at the focal length of a lens or concave mirror on which an extended object is focused.
focal point
The place at the focal length where light rays from a point object (that is, one that is too distant or tiny to resolve) are converged by a lens or concave mirror.
focus (plural foci) (of an ellipse)
The two points inside an ellipse the sum of whose distances from any point on the ellipse is constant.
force
That which can change the momentum of an object; equivalently, that which can accelerate an object.
frequency
The number of peaks or troughs of a wave that pass a fixed point each second. Equivalently, the number of complete vibrations or oscillations per second.
galactic cannibalism
A collision between two galaxies of unequal mass and size in which the smaller galaxy is absorbed by the larger galaxy.
galactic merger
A collision and subsequent merger of two roughly equal-sized galaxies.
galactic nucleus
The center of a galaxy; the center of the Milky Way Galaxy.
galaxy
A large assemblage of stars, gas, and dust bound together by their mutual gravitational attraction.
Galilean moon (Galilean satellite)
Any one of the four large moons of Jupiter (Callisto, Europa, Ganymede, Io) that is visible from Earth through a small telescope.
gamma ray
The most energetic form of electromagnetic radiation.
gamma-ray burst
A short burst of gamma rays; the sources of the bursts are outside our Galaxy.
gas (ion) tail
The relatively straight tail of a comet produced by the solar wind acting on ions in a comet’s coma.
giant molecular cloud
A large interstellar cloud of cool gas and dust in a galaxy.
giant star
A star whose diameter is roughly 10 to 100 times that of the Sun.
glitch
A sudden speedup in the period of a pulsar.
globular cluster
A large spherical cluster of gravitationally bound stars usually found in the outlying regions of a galaxy.
Grand Unified Theory (GUT)
A theory that unifies the strong, weak, and electromagnetic forces, as occurred early in the life of the universe.
granules
Lightly colored convection features about 1000 km in diameter seen constantly in the solar photosphere.
gravitation
The tendency of all matter and energy to attract all other matter.
gravitational lensing
The distortion of the appearance of an object by a source of gravity between it and the observer.
gravitational radiation
See gravitational waves.
gravitational redshift
The redshift of photons leaving the gravitational field of any massive object, such as a star or black hole.
gravitational waves
Ripples in the overall geometry of space produced by nonspherical moving objects.
Great Dark Spot
A large, dark, oval-shaped storm that used to be in Neptune’s southern hemisphere.
Great Red Spot
A large, red-orange, oval-shaped storm in Jupiter’s southern hemisphere.
greenhouse effect
The trapping of infrared radiation near a planet’s surface by the planet’s atmosphere.
ground state
The lowest energy level of an atom.
H II region
A region of ionized hydrogen in interstellar space.
habitable zone
The region around any star wherein water can exist in liquid form and, hence, life as we know it can conceivably exist.
halo (of a galaxy)
A spherical distribution of globular clusters, isolated stars, and dark matter that surrounds a galaxy.
Hawking process
The formation of real particles from virtual ones just outside a black hole’s event horizon; the means by which black holes evaporate.
head-tail source
A radio galaxy with both a point radio source and an extended radio-bright lobe of gas jetting out from the point source.
heliocentric cosmology
A theory of the formation and evolution of the solar system with the Sun at the center.
helioseismology
The study of vibrations of the solar surface.
helium flash
The explosive ignition of helium fusion in the core of a low-mass, giant star.
helium shell flash
The explosive ignition of helium fusion in a thin shell surrounding the core of a low-mass star.
helium shell fusion
Helium fusion that occurs in a thin shell surrounding the core of a star.
Hertzsprung-Russell (H-R) diagram
A plot of the absolute magnitude or luminosity of stars versus their surface temperatures or spectral classes.
highlands
Heavily cratered, mountainous regions of the lunar surface.
homogeneity
The property of the universe being smooth or uniform as measured over suitably large distance intervals.
horizon problem
The difficulty in explaining why seemingly disconnected regions of the universe have the same temperature.
horizontal-branch stars
A group of post–helium-flash stars near the main sequence on the Hertzsprung-Russell diagram of a typical globular cluster.
hot-spot volcanism
The creation of volcanoes on a planet’s surface caused by a reservoir of hot magma in the planet’s mantle under a thin part of the crust.
Hubble classification
A system of classifying galaxies according to their appearance into one of four broad categories: spirals, barred spirals, ellipticals, and irregulars.
Hubble constant (H0)
The constant of proportionality in the relation between the recessional velocities of remote galaxies and their distances; from this number, the age of the universe can be determined.
Hubble flow
The recession of the galaxies caused by the expansion of the universe.
Hubble law
The relationship that states that the speeds of remote galaxies moving away from us are directly proportional to their distances from Earth.
hydrocarbon
A molecule based on hydrogen and carbon.
hydrogen envelope
An extremely large, tenuous sphere of hydrogen gas surrounding the head of a comet.
hydrogen fusion
The thermonuclear fusion of hydrogen to produce helium.
hydrogen shell fusion
Hydrogen fusion that occurs in a thin shell surrounding the core of a star.
hydrostatic equilibrium
A balance between the weight of a layer in a star and the pressure that supports it.
hyperbola
An open curve obtained by cutting a cone with a plane.
impact breccia
A rock consisting of various fragments cemented together by the impact of a meteoroid.
impact crater
A crater on the surface of a planet or moon produced by the impact of an asteroid, meteoroid, or comet.
inferior conjunction
The configuration when Mercury or Venus is directly between the Sun and Earth.
inferior planet
Those planets closer to the Sun than the Earth, namely Mercury and Venus.
inflation
A sudden expansion of the young universe.
inflationary epoch
A brief period shortly after the Big Bang during which the size of the universe increased much more rapidly than it does today.
infrared radiation
Electromagnetic radiation of a wavelength longer than visible light but shorter than radio waves.
initial mass function
The numbers of stars on the main sequence at all different masses.
instability strip
A region on the Hertzsprung-Russell diagram occupied by pulsating stars.
interferometry
A method of increasing resolving power by combining electromagnetic radiation collected by two or more telescopes.
intergalactic gas
Gas located between the galaxies within a cluster of galaxies.
intermediate-mass black hole
A black hole with a mass between a few hundred and few thousand solar masses.
interstellar extinction
The dimming of starlight as it passes through the interstellar medium.
interstellar medium
Interstellar gas and dust.
interstellar reddening
The reddening of starlight passing through the interstellar medium resulting from the scattering of short wavelength light more than long wavelength light.
inverse-square law
The gravitational attraction between two objects is inversely proportional to the square of their separation; also, the apparent brightness of a light source is inversely proportional to the square of its distance from an observer.
ion
An atom that has become electrically charged due to the loss or addition of one or more electrons.
ionization
The process by which an atom loses or gains electrons.
ionosphere (thermosphere)
Region of Earth’s atmosphere, above the mesosphere, in which sunlight ionizes many atoms.
iron meteorite
A meteorite composed primarily of iron with an admixture of nickel; also called an iron.
irregular cluster (of galaxies)
An unevenly distributed group of galaxies bound together by their mutual gravitational attraction.
irregular galaxy
An asymmetrical galaxy having neither spiral arms nor an elliptical shape.
isotopes
Atoms that all have the same number of protons (atomic number) but different numbers of neutrons. Their nuclear properties often differ greatly.
isotropy
The fact that the average number of galaxies at different distances from Earth is the same in all directions; also, the fact that the temperature of the cosmic microwave background is essentially the same in all directions.
isotropy problem
See horizon problem.
Jeans instability
The condition under which gravitational forces overcome thermal forces to cause part of an interstellar cloud to collapse and form stars and planets.
Jeans unstable
A volume of gas collapsing due to the Jeans instability.
Kepler’s laws
Three statements, formulated by Johannes Kepler, that describe the paths of planets and other objects orbiting the Sun, and moons orbiting planets.
Kerr black hole
Any rotating, uncharged black hole.
kinetic energy
The energy an object has as a result of its motion.
Kirchhoff’s laws
Three statements formulated by Gustav Kirchhoff describing what physical conditions produce each type of spectra.
Kirkwood gaps
Gaps in the spacing of asteroid orbits discovered by Daniel Kirkwood that are caused by gravitational attractions of planets.
Kuiper belt
A doughnut-shaped ring of space around the Sun beyond Neptune and centered on the ecliptic that contains many frozen comet nuclei and other debris, some of which are occasionally deflected toward the inner solar system.
Kuiper belt object (KBO)
Space debris orbiting the Sun beyond Neptune in the Kuiper belt. The debris includes small solar system bodies like Pluto, as well as myriad small rocky and icy objects, including many comet nuclei.
law of equal areas
(Kepler’s second law) The physical law that a line joining a planet and the Sun sweeps out equal areas in equal intervals of time.
law of inertia
(Newton’s first law of motion) The physical law that an object will stay at rest or move at a constant speed in a fixed direction unless acted upon by an outside force.
law of universal gravitation
Newton’s law of gravitation, which describes how the gravitational force between two bodies depends on their masses and separation.
lenticular galaxy
A disk-shaped galaxy without spiral arms.
light curve
A graph that displays variations in the brightness of a star or other astronomical object over time.
light-gathering power
A measure of how much light a telescope intercepts and brings to a focus.
light-year (ly)
The distance that light travels through a vacuum in 1 year.
lighthouse model
The explanation that a pulsar pulses by rotating and funneling energy outward via magnetic fields that are not aligned with a rotating black hole’s rotation axis.
limb (of the Sun)
The apparent edge of the Sun as seen in the sky.
limb darkening
The phenomenon whereby the Sun is darker near its limb than near the center of its disk.
line of nodes
The line along which the plane of the Moon’s orbit intersects the plane of the ecliptic.
liquid metallic hydrogen
A metallike form of hydrogen that is produced under extreme pressure.
Local Group
The cluster of about 40 galaxies of which our own Galaxy is a member.
long-period comet
A comet that takes tens of thousands of years or more to orbit the Sun once.
luminosity
The rate at which electromagnetic radiation is emitted from a star or other object.
luminosity class
The classification of a star of a given spectral type according to its luminosity and density; the classes are supergiant, bright giant, giant, subgiant, and main sequence.
lunar eclipse
An eclipse during which Earth blocks sunlight that would have struck the Moon.
lunar phases
The names given to the apparent shapes of the Moon as seen from Earth.
magnetar
Especially hot, rapidly rotating neutron stars whose motion helps generate extra-strong magnetic fields.
magnetic dynamo
A theory that explains phenomena of the solar cycle as a result of periodic winding and unwinding of the Sun’s magnetic field in the solar atmosphere.
magnification
The number of times larger in angular diameter an object appears through a telescope than when it is seen by the naked eye.
main sequence
A grouping of stars on the Hertzsprung-Russell diagram extending diagonally across the graph from the hottest, brightest stars to the dimmest, coolest stars.
main-sequence star
A star, fusing hydrogen to helium in its core, whose surface temperature and luminosity place it on the main sequence on the Hertzsprung-Russell diagram.
mantle
That portion of a terrestrial planet located between its crust and core.
mare (plural maria)
Latin for “sea”; a large, relatively crater-free plain on the Moon.
mare basalt
Dark, solidified lava that covers the lunar maria.
mascons
Regions of high-density matter near the surface of the Moon.
mass
A measure of the total amount of material in an object.
mass-luminosity relation
The direct relationship between the masses and luminosities of main-sequence stars.
matter-dominated universe
A universe in which the radiation field that fills all space is unable to prevent the existence of neutral atoms.
mesosphere
The layer in Earth’s atmosphere above the stratosphere.
metals
In astronomy, all elements except hydrogen and helium.
meteor
The streak of light seen when any space debris vaporizes in Earth’s atmosphere; a “shooting star.”
meteor shower
Frequent meteors that appear to originate from a common point in the sky.
meteorite
A fragment of space debris that has survived passage through Earth’s atmosphere.
meteoroid
A small rock in interplanetary space.
microlensing
The gravitational focusing of light from a distant star by a closer object to give a brighter image of the star.
Milky Way Galaxy
The barred spiral galaxy in which our solar system resides.
missing mass
See dark matter.
model
A hypothesis that has withstood observational or experimental tests.
molecular clouds
Nebulae that are often embedded in much larger bodies of gas and dust.
molecule
Two or more atoms bonded together.
moment of inertia
A measure of the inertial resistance of an object to changes in the object’s rotational motion about its rotation axis.
momentum
A measure of the inertia of an object; an object’s mass multiplied by its velocity.
moons (natural satellites)
Bodies that orbit larger objects, which in turn orbit stars.
neap tide
The least change from high to low tide during a day; it occurs during the first and third quarter phases of the Moon.
nebula
(plural nebulae) A cloud of interstellar gas and dust.
neutrino
A subatomic particle, with no electric charge and little mass, that is important in many nuclear reactions and in supernovae.
neutron
A nuclear particle with no electric charge and with a mass nearly equal to that of the proton.
neutron degeneracy pressure
A powerful pressure produced by degenerate neutrons.
neutron star
A very compact, dense stellar remnant composed almost entirely of neutrons.
Newton’s laws of motion
Newton’s equations that describe the motion of matter as a result of forces acting on it.
Newtonian reflector
An optical arrangement in a reflecting telescope in which a small, flat mirror reflects converging light rays to a focus on one side of the telescope tube.
Nice model
A theory that describes the formation of the planets and other objects orbiting the Sun, with Jupiter and the other giant planets forming first, followed by the inner planets, asteroid belt, Kuiper belt, and Oort comet cloud.
north celestial pole
The location on the celestial sphere directly above Earth’s northern rotation pole.
northern lights (aurora borealis)
Light radiated by atoms and ions in Earth’s upper atmosphere due to high-energy particles from the Sun and seen mostly in the northern polar regions.
northern waste (northern devastation)
Relatively young, crater-free terrain surrounding the North Pole of Mars.
nova (plural novae)
A star in a binary system that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by a factor of between 104 and 106.
nucleosynthesis
The formation, by fusion, of higher mass elements from lower mass ones.
nucleus (of a comet)
A collection of ices and dust that constitute the solid part of a comet.
nucleus (of an atom)
The massive part of an atom, composed of protons and neutrons; electrons surround a nucleus.
OB association
An unbound group of very young, massive stars predominantly of spectral types O and B.
OBAFGKM sequence
The sequence of stellar spectral classifications from hottest to coolest stars.
objective lens
The principal lens of a refracting telescope.
Occam’s razor
The principle of choosing the simplest scientific theory that correctly explains any phenomenon.
occultation
The eclipsing of an astronomical object other than the Moon or Sun by another astronomical body.
Oort cloud
A thick spherical shell of the solar system beyond the Kuiper belt where most comets are believed to spend most of their lives.
open cluster
A loosely bound group of young stars in the disk of a galaxy; a galactic cluster.
open universe
A universe that lacks the mass necessary to ever stop expanding.
opposition
The configuration of a planet when it is at an elongation of 180° and thus appears opposite the Sun in the sky.
optical double
A pair of stars that appear to be near each other but are unbound and at very different distances from Earth.
orbital inclination
The tilt or angle of an object’s orbital plane around the Sun compared to the ecliptic.
organic molecule
A carbon-based compound.
overcontact binary
A close binary system in which the two stars share a common atmosphere.
ozone layer
The lower stratosphere, where most of the ozone in the air exists.
pair production
The creation of a particle and an antiparticle from energetic photons.
parabola
An open curve formed by cutting a circular cone at an angle parallel to the sides of the cone.
parallax
The apparent displacement of an object relative to more distant objects caused by viewing it from different locations.
parsec (pc)
A unit of distance equal to 3.26 light-years.
partial eclipse
A lunar or solar eclipse in which the eclipsed object does not appear completely covered.
Pauli exclusion principle
A principle of quantum mechanics that states that two identical particles cannot simultaneously have the same position and momentum.
peculiar galaxy (pec)
Any Hubble class of galaxy that appears to be blowing apart.
penumbra
The portion of a shadow in which only part of the light source is covered by the shadowmaking body.
penumbral eclipse
A lunar eclipse in which the Moon passes only through Earth’s penumbra.
perihelion
The point in its orbit where a planet or other body is nearest the Sun.
period-luminosity relation
A relationship between the period and average luminosity of a pulsating star.
periodic table
A listing of the chemical elements according to their properties; created by D. Mendeleev.
photodisintegration
The breakup of nuclei in the core of a massive star due to the effects of energetic gamma rays.
photometry
The measurement of light intensities.
photon
A discrete unit of electromagnetic energy.
photosphere
The region in the solar atmosphere from which most of the visible light escapes into space.
pixel
A contraction of the term “picture element”; usually refers to one square of a grid into which the light-sensitive component of a charge-coupled device is divided.
plage
A bright spot on the Sun believed to be associated with an emerging magnetic field.
Planck era
Time from the Big Bang until the Planck time (10−43 s).
Planck time
The earliest time, about 10−43 s after the Big Bang, for which science has equations describing the universe. At that time all four forces in nature today (gravity, electromagnetism, weak, and strong) behaved as one force.
Planck’s law
The relationship between the energy carried by a photon and its wavelength.
planet
An object orbiting a star that is held together by its own gravitational force in a nearly spherical shape, that is able to clear its neighborhood of debris, and is not the moon (or satellite) of a larger orbiting body.
planetary differentiation
The process early in the life of each planet whereby denser elements sank inward and lighter ones rose.
planetary nebula
A luminous shell of gas ejected from an old, low-mass star.
planetesimal
Primordial asteroidlike object from which the planets accreted.
plasma
A hot, ionized gas.
plate tectonics
The motions of large segments (plates) of Earth’s surface caused by convective motions in the underlying mantle.
polymer
A long molecule composed of many smaller molecules.
poor cluster (of galaxies)
A cluster of galaxies with only a few members.
Population I star
A star, such as the Sun, whose spectrum exhibits spectral lines of many elements heavier than helium; a metal-rich star.
Population II star
A star whose spectrum exhibits comparatively few spectral lines of elements heavier than helium; a metal-poor star.
positron
An electron with a positive rather than negative electric charge; an antielectron.
potential energy
The energy stored in an object as a result of its location in space.
pre–main-sequence star
The stage of star formation just before the main sequence; it involves slow contraction of the young star.
precession (of Earth)
A slow, conical motion of Earth’s axis of rotation caused by the gravitational pull of the Moon and Sun on Earth’s equatorial bulge.
precession of the equinoxes
The slow westward motion of the equinoxes along the ecliptic because of Earth’s precession.
primary cosmic rays (cosmic rays)
High-speed particles traveling through space.
primary mirror
The large, concave, light-gathering mirror in a reflecting telescope, analogous to the objective lens on a refracting telescope.
prime focus
The point in a reflecting telescope where the primary mirror focuses light.
primordial black hole
A relatively low-mass black hole hypothetically formed at the beginning of the universe.
primordial fireball
The extremely hot gas that filled the universe immediately following the Big Bang.
primordial nucleosynthesis
The transformation by fusion of protons and electrons into hydrogen isotopes, helium, and some lithium in the first few minutes of the existence of the universe.
prograde motion
The normal motion of a planet or other solar system body from right to left (that is, eastward) among the background stars.
prograde orbit
An orbit of a moon or satellite around a planet that is in the same direction as the planet’s rotation.
prominence
Flamelike protrusion seen near the limb of the Sun and extending into the solar corona. The side view of a filament.
proper motion
The change in the location of a star on the celestial sphere.
proton
A heavy, positively charged nuclear particle.
protoplanetary disk (proplyd)
A disk of material encircling a protostar or a newborn star.
protostar
The earliest stage of a star’s life before fusion commences and when gas is rapidly falling onto it.
protosun
The Sun prior to the time when hydrogen fusion began in its core.
pulsar
A pulsating source associated with a rapidly rotating neutron star with an off-axis magnetic field.
quantum mechanics
The branch of physics dealing with the structure and behavior of atoms and their interactions with each other and with light.
quark
A particle that is a building block of the heavy nuclear particles such as protons and neutrons.
quasar (quasi-stellar radio source)
A starlike object with a very large redshift.
quasi-stellar object (QSO)
A quasar.
quintessence
One of the proposed explanations of the dark energy causing the universe to accelerate outward.
radial velocity
That portion of an object’s velocity parallel to the line of sight.
radial-velocity curve
A plot showing the variation of radial velocity with time for a binary star or variable star.
radiation (photon) pressure
The transfer of momentum carried by radiation to an object on which the radiation falls.
radiation-dominated universe
The time at the beginning of the universe when the electromagnetic radiation prevented ions and electrons from combining to make neutral atoms.
radiative zone
A region inside a star where energy is transported outward by the movement of photons through a gas from a hot location to a cooler one.
radio galaxy
A galaxy that emits an unusually large amount of radio waves.
radio lobes
Vast regions of radio emission on opposite sides of a radio galaxy.
radio telescope
A telescope designed to detect radio waves.
radio wave
Long-wavelength electromagnetic radiation.
radioactive
Unstable atomic nuclei that naturally decompose by spontaneously emitting particles.
red dwarf
A low-mass main-sequence star.
red giant
A large, cool star of high luminosity.
redshift
The shifting to longer wavelengths of the light from remote galaxies and quasars; the Doppler shift of light from any receding source.
reflecting telescope (reflector)
A telescope in which the principal light-gathering component is a concave mirror.
reflection
The rebounding of light rays off a smooth surface.
reflection nebula
A comparatively dense cloud of gas and dust in interstellar space that is illuminated by a star between it and Earth.
refracting telescope (refractor)
A telescope in which the principal light-gathering component is a lens.
refraction
The bending of light rays when they pass from one transparent medium to another.
refractor
See refracting telescope.
regolith
The powdery, lifeless material on the surface of a moon or planet.
regular cluster (of galaxies)
An evenly distributed group of galaxies bound together by mutual gravitational attraction.
resonance
The large response of an object to a small periodic gravitational tug from another object.
retrograde motion
The occasional backward (that is, westward) apparent motion of a planet against the background stars as seen from Earth. Retrograde motion is an optical illusion.
retrograde orbit
The orbit of a moon or satellite around a planet that is in the direction opposite to the planet’s rotation.
retrograde rotation
The rotation of a planet, dwarf planet, or small solar system body opposite to its direction of revolution around the Sun. Of the planets and dwarf planets, only Pluto, Uranus, and Venus have retrograde rotation.
revolution
The orbit of one body about another.
rich cluster (of galaxies)
A cluster of galaxies with many members.
right ascension (r.a.)
The celestial coordinate analogous to longitude on Earth and measured around the celestial equator from the vernal equinox.
rille
A winding crack or depression in the lunar surface caused by the collapse of a solidified lava tube.
ringlet
Any one of numerous, closely spaced, thin bands of particles in planetary ring systems.
Roche limit
The shortest distance from a planet or other object at which a second object can be held together by its own gravitational forces.
Roche lobe
The teardrop-shaped regions around each star in a binary star system inside of which gas is gravitationally bound to that star.
rotation
The spinning of a body about an axis passing through it.
rotation curve (of a galaxy)
A graph showing how the orbital speed of material in a galaxy depends on the distance from the galaxy’s center.
RR Lyrae variable
A type of pulsating star with a period less than 1 day.
Sagittarius A
The strong radio source associated with the nucleus of the Milky Way Galaxy.
scarp
Cliffs on Mercury, our Moon, Iapetus, Rhea, and possibly other solar system bodies that formed when these bodies first cooled and shrank.
Schmidt corrector plate
A specially shaped lens used with spherical mirrors that corrects for spherical aberration and provides an especially wide field of view.
Schwarzschild black hole
Any nonrotating, uncharged black hole.
Schwarzschild radius
The distance from the center to the event horizon in any black hole.
scientific method
The method of doing science based on observation, experimentation, and the formulation of hypotheses (theories) that can be tested.
scientific notation
The style of writing large and small numbers using powers of ten.
scientific theory
An idea about the natural world that is subject to verification and refinement.
seafloor spreading
The process whereby magma upwelling along rifts in the ocean floor causes adjacent segments of Earth’s crust to separate.
secondary cosmic rays (cosmic ray shower)
Particles from Earth’s atmosphere given high speeds Earthward by primary cosmic rays from space.
secondary mirror
A relatively small mirror used in reflecting telescopes to guide the light out the side or bottom of the telescope.
seeing disk
The size that a star appears to have on a photographic or charge-coupled-device image as a result of the changing refraction of the starlight passing through Earth’s atmosphere.
seismic waves
Vibrations traveling through or around an astronomical body usually associated with earthquakelike phenomena.
seismograph
A device used to record and measure seismic waves, such as those produced by earthquakes.
semidetached binary
A close binary system in which one star fills or is overflowing its Roche lobe.
semimajor axis (of an ellipse)
Half of the longest dimension of an ellipse.
SETI
The search for extraterrestrial intelligence.
Seyfert galaxy
A spiral galaxy with a bright nucleus whose spectrum exhibits emission lines.
Shapley–Curtis debate
An inconclusive debate between Harlow Shapley and Heber Curtis in 1920 about whether certain nebulae (now known to be galaxies) are outside the Milky Way.
shepherd satellite (moon)
A small satellite whose gravitational tug is responsible for maintaining a sharply defined ring of matter around a planet such as Saturn or Uranus.
short-period comet
A comet that orbits the Sun in the vicinity of the planets, thereby reappearing with tails every 200 years or less.
sidereal month
The period of the Moon’s revolution about Earth measured with respect to the Moon’s location among the stars; 27⅓ Earth days.
sidereal period
The orbital period of one object about another measured with respect to the stars.
singularity
A place of infinite curvature of spacetime in a black hole.
small solar system bodies (SSSBs)
All objects in the solar system that are not planets, dwarf planets, or moons.
snow line
The distance from the Sun beyond which ices stayed frozen in the early solar system.
solar corona
The Sun’s outer atmosphere.
solar cycle
A 22-year cycle during which the Sun’s magnetic field reverses its polarity twice.
solar day
From noontime to the next noontime; for Earth it is 24 hours.
solar eclipse
An eclipse during which the Moon blocks the Sun.
solar flare
A violent eruption on the Sun’s surface.
solar luminosity (L⊙)
The total energy emitted by the Sun each second.
solar model
A set of equations that describe the internal structure and energy generation of the Sun.
solar nebula
The cloud of gas and dust from which the Sun and the rest of the solar system formed.
solar system
The Sun and everything that orbits it, including planets, their moons, dwarf planets, and small solar system bodies.
solar wind
An outward flow of particles (mostly electrons and protons) from the Sun.
south celestial pole
The location on the celestial sphere directly above Earth’s south rotation pole.
southern highlands
Older, cratered terrain in the Martian southern hemisphere.
southern lights (aurora australis)
Light radiated by atoms and ions in Earth’s upper atmosphere due to high-energy particles from the Sun; seen mostly in the southern polar regions.
spacetime
The concept from special relativity that space and time are both essential in describing the position, motion, and action of any object or event.
spectral analysis
The identification of chemicals by the appearance of their spectra.
spectral type
A classification of stars according to the appearance of their spectra.
spectrograph
A device for photographing a spectrum.
spectroscope
A device for directly viewing a spectrum.
spectroscopic binary
A double star whose binary nature can be deduced from the periodic Doppler shifting of lines in its spectrum.
spectroscopic parallax
A method of determining a star’s distance from Earth by measuring its surface temperature, luminosity, and apparent magnitude.
spectrum (plural spectra)
The result of electromagnetic radiation passing through a prism or grating so that different wavelengths are separated.
spherical aberration
An optical property whereby different portions of a spherical lens or spherical, concave mirror have slightly different focal lengths, thereby producing a fuzzy image.
spicule
A narrow jet of rising gas in the solar chromosphere.
spin (of an electron or proton)
A small, well-defined amount of angular momentum possessed by electrons, protons, and other particles.
spiral arms
Lanes of interstellar gas, dust, and young stars that wind outward in a plane from the central regions of some galaxies.
spiral density wave
A spiral-shaped pressure wave that orbits the disk of a spiral galaxy and induces new star formation.
spiral galaxy
A flattened, rotating galaxy with pinwheel-like spiral arms winding outward from the galaxy’s nuclear bulge.
spoke
A moving dark region of Saturn’s rings.
spring tide
The greatest daily difference between high tide and low tide, occurring when the Moon is new or full.
stable Lagrange points
Locations in the solar system where the gravitational forces from the Sun and a planet keep space debris trapped.
standard candle
An object whose known luminosity can be used to deduce the distance to a galaxy.
starburst galaxy
A galaxy undergoing an exceptionally high rate of star formation.
Stefan-Boltzmann law
The relationship stating that an object emits energy at a rate proportional to the fourth power of its temperature, in Kelvins.
stellar evolution
The changes in size, luminosity, temperature, and chemical composition that occur as a star ages.
stellar parallax
The apparent shift in a nearby star’s position on the celestial sphere resulting from Earth’s orbit around the Sun.
stellar spectroscopy
The study of the properties of stars encoded in their spectra.
stony meteorite
A meteorite composed of rock with very little iron; also called a stone.
stony-iron meteorite
A meteorite composed of roughly equal amounts of rock and iron.
stratosphere
The second layer in Earth’s atmosphere, directly above the troposphere.
strong nuclear force
The force that binds protons and neutrons together in nuclei.
summer solstice
The point on the ecliptic where the Sun is farthest north of the celestial equator; the day with the largest number of daylight hours in the northern hemisphere, around June 21.
sunspot
A temporary cool region in the solar photosphere created by protruding magnetic fields.
sunspot maximum
The time during the solar cycle when the number of sunspots is greatest.
sunspot minimum
The time during the solar cycle when the number of sunspots is minimum.
supercluster (of galaxies)
A gravitationally bound collection of many clusters of galaxies.
supergiant
A star of very high luminosity.
supergranule
A large convective cell in the Sun’s chromosphere containing many granules.
superior conjunction
The configuration when Mercury or Venus is on the opposite side of the Sun from Earth.
superior planet
A planet farther from the Sun than the Earth, namely Mars, Jupiter, Saturn, Uranus, and Neptune.
supermassive black hole
A black hole whose mass exceeds 1000 solar masses.
supernova remnant
A nebula left over after a supernova detonates.
superstring theories
A set of theories that hope to describe the nature of spacetime and matter at a more fundamental level than is presently possible.
synchronous rotation
The condition when a moon’s rotation rate and revolution rate are equal or when a planet’s rotation rate equals its moon’s revolution rate.
synchrotron radiation
The radiation emitted by charged particles moving through a magnetic field; nonthermal radiation.
synodic month (lunar month)
The period of revolution of the Moon with respect to the Sun; the length of one cycle of lunar phases; 29½ Earth days.
synodic period
The interval between successive occurrences of the same configuration of a planet as seen from Earth.
T Tauri stars
Young, variable pre–main-sequence stars associated with interstellar matter that show erratic changes in luminosity.
terminator
The line dividing day and night on the surface of any body orbiting the Sun; the line of sunset or sunrise.
terrestrial planet
Any of the planets Mercury, Venus, Earth, or Mars; a planet with a composition and density similar to that of Earth.
Theories of Everything
Theories under development that comprehensively explain all four fundamental forces in nature.
theory
See scientific theory.
theory of general relativity
A description of spacetime formulated by Einstein explaining how gravity affects the geometry of space and the flow of time.
theory of special relativity
A description of mechanics and electromagnetic theory formulated by Einstein according to which measurements of distance, time, and mass are affected by the observer’s motion.
thermonuclear fusion
A reaction in which the nuclei of atoms are fused together at a high temperature.
3-to-2 spin-orbit coupling
The rotation of Mercury, which makes three complete rotations on its axis for every two complete orbits around the Sun.
time zone
One of 24 divisions of Earth’s surface separated by 15° along lines of constant longitude (with allowances for some political boundaries).
total eclipse
A solar eclipse during which the Sun is completely hidden by the Moon, or a lunar eclipse during which the Moon is completely immersed in Earth’s umbra.
trailing-arm spiral galaxy
A galaxy with its spiral arms pointing away from the direction of rotation, characteristic of all but one known spiral galaxies.
trans-Neptunian objects (TNOs)
Objects orbiting the Sun beyond the orbit of Neptune.
transition (of an electron)
The change in energy and orbit of an electron around an atom or molecule.
transition zone
Region between the Sun’s chromosphere and corona where the temperature skyrockets to about 1 million K.
transverse velocity
The portion of an object’s velocity perpendicular to our line of sight to it.
Trojan asteroid
One of several asteroids at stable Lagrange points that share Jupiter’s orbit about the Sun.
troposphere
The lowest level of Earth’s atmosphere.
Tully-Fisher relation
A correlation between the width of the 21-centimeter line of a spiral galaxy and its absolute magnitude.
turnoff point
The location of the brightest mainsequence stars on the Hertzsprung-Russell diagram of a globular cluster.
21-cm radio radiation
Radio emission from a hydrogen atom caused by the flip of the electron’s spin orientation.
twinkling
The apparent change in a star’s brightness, position, or color due to the motion of gases in the Earth’s atmosphere.
Type I Cepheid
A Population I Cepheid variable star found in the disks of spiral galaxies.
Type Ia supernova
A supernova occurring after a white dwarf accretes enough mass from a companion star to exceed the Chandrasekhar limit.
Type II Cepheid
A Population II Cepheid variable star found in elliptical galaxies and in the halos of disk galaxies that is 1.5 magnitudes dimmer than a Type I Cepheid.
Type II supernova
A supernova occurring after a massive star’s core is converted to iron.
ultraviolet (UV) radiation
Electromagnetic radiation of wavelengths shorter than those of visible light but longer than those of X rays.
umbra
The central, completely dark portion of a shadow.
universal constant of gravitation
The constant of proportionality in Newton’s law of gravitation, usually denoted G.
universe
All space along with all the matter and radiation in space.
Van Allen radiation belts
Two flattened, doughnut-shaped regions around Earth where many charged particles (mostly protons and electrons) are trapped by Earth’s magnetic field.
variable star
A star whose luminosity varies.
velocity
A quantity that specifies both direction and speed of an object.
vernal equinox
The point on the ecliptic where the Sun crosses the celestial equator from south to north; the beginning of spring, around March 21.
very-long-baseline interferometry (VLBI)
A method of connecting widely separated radio telescopes to make observations of very high resolution.
virtual particle
A particle or an antiparticle, that are created simultaneously in pairs and which quickly disappear without a trace.
visual binary
A double star in which the two components can be resolved through a telescope.
water hole
The part of the electromagnetic spectrum at a few thousand megahertz where there is very little background noise from space.
wavelength (λ)
The distance between two successive peaks in a wave.
weak nuclear force
A nuclear interaction involved in certain kinds of radioactive decay.
weight
The force with which a body presses down on the surface of a world such as Earth.
white dwarf
A low-mass stellar remnant that has exhausted all its thermonuclear fuel and contracted to a size roughly equal to the size of Earth.
Widmanstätten patterns
Crystalline structure seen inside iron meteorites.
Wien’s law
The relationship that the dominant wavelength of radiation emitted by a blackbody varies inversely with its temperature.
winter solstice
The point on the ecliptic where the Sun is farthest south of the celestial equator; fewest hours of daylight in the northern hemisphere, around December 22.
Wolf-Rayet stars
Rotating stars of at least 20 M⊙ with strong magnetic fields and stellar winds.
work
Change in an object’s energy as a result of a force being applied to it.
wormhole
A hypothetical connecting passage between black holes and other places in the universe.
X rays
Electromagnetic radiation whose wavelength is between that of ultraviolet light and gamma rays.
X-ray burster
A neutron star in a binary star system that accretes mass, undergoes thermonuclear fusion on its surface, and therefore emits short bursts of X rays.
Zeeman effect
A splitting of spectral lines in the presence of a magnetic field.
zenith
The point on the celestial sphere directly overhead.
zero-age main sequence (ZAMS)
The positions of stars on the Hertzsprung-Russell diagram that have just begun to fuse hydrogen in their cores.
zodiac
A band of 13 constellations around the sky through which the Sun appears to move throughout the year.
zones (on Jupiter)
Light-colored bands in Jupiter’s cloud cover.