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matter consisting of antiparticles such as antiprotons, antineutrons, and positrons

flatness problem

the dilemma posed by the fact that the combined average mass density of the universe is very nearly equal to the critical density

grand unified theory

a theory that describes and explains the four physical forces


a sudden expansion of space

isotropy problem

the dilemma posed by the fact that the cosmic microwave background is isotropic; also called the horizon problem


the process of building up nuclei such as deuterium and helium from protons and neutrons

pair production

the creation of a particle and its antiparticle from energy

population III star

one of the first stars to form after the big bang, composed of only hydrogen, helium and tiny amounts of lithium and beryllium; a "zeroth-generation" star

big bang

an expansion of all space that took roughly 13.7 billion years ago that marks the beginning of the universe

closed universe

a universe with positive curvature, so that its geometry is analogous to that of the surface of a sphere

cosmic microwave background

an isotropic radiation field with a blackbody temperature of about 2.725 K that permeates the entire universe

cosmological principle

the assumption that the universe is homogeneous and isotropic on the largest scale

cosmological redshift

a redshift that is caused by the expansion of the universe

dark energy

a form of energy that appears to pervade the universe and causes the expansion of the universe to accelerate but has no discernible gravitational effect

flat universe

space that is not curved; space with zero curvature

observable universe

the portion of the universe inside our cosmic light horizon

olber's paradox

the dilemma associated with the fact that the night sky is dark

open universe

a universe with negative curvature, so that its geometry is analogous to a saddle-shaped hyperbolic (saddle) surface

active galactic nucleus

the center of an active galaxy

active galaxy

a galaxy that is emitting exceptionally large amounts of energy: a seyfert, radio galaxy, or quasar


a type of active galaxy whose nucleus has a featureless spectrum

double radio source

an extragalactic radio source charecterized by two large regions of radio emission, typically located on either side of an active galaxy


a very luminous object with a very large redshift and a star-like appearance

radio galaxy

a galaxy that emits an unusually large amount of radio waves

radio lobes

a region near an active galaxy from which significant radio radiation is emitted

seyfert galaxy

a spiral galaxy with a bright nucleus whose spectrum exhibits emission lines

barred spiral galaxy

a spiral galxy in which the spiral arms begin from the ends of a bar running through the nucleus rather than from the nucleus itself

clusters of galaxies

a collection of galaxies containing a few to several thousand member galaxies

distance ladder

the sequence of techniques used to determine the distances to very distant galaxies

elliptical galaxy

a galaxy with an elliptical shape and no conspicuous interstellar material

gravitational lens

a massive object that reflects light rays from a remote source, forming an image much as an ordinary lens does

hubble flow

the recessional motion of distant galaxies caused by the expansion of the universe

hubble law

the empirical relationship stating that the redshifts of remote galaxies are directly proportional to their distances from earth

irregular galaxy

an asymmetrical galaxy having neither spiral arms nor an elliptical shape

lenticular galaxy

a galaxy with a central bulge and a disk but no spiral structure; an SO galaxy

local group

a cluster of about 40 galaxies of which the milky way is a member

spiral galaxy

a flattened rotating galaxy with pinwheel like spiral arms winding outward from the galaxys nucleus

central bulge

a spherical distribution of stars around the nucleus of a spiral galaxy

dark matter

nonluminous matter that is the dominant form of matter in galaxies and throughout the universe

density wave

in a spiral galaxy, a localized region in which matter piles up as it orbits the center of the galaxy

disk of a galaxy

the disk-shaped distribution of population I stars that dominates the appearance of a spiral galaxy


a spherical distribution of globular clusters and population II stars that surround a spiral galaxy

Massive compact halo object (MACHO)

a dim star or low-mass black hole that may comprise part of the unseen dark matter

rotation curve

a plot of the orbital speeds of stars and nebulae in a galaxy versus distance from the center of the galaxy

self-propagating star formation

the process by which the formation of stars in one location in a galaxy stimulates the formation of stars in a neighboring location

spiral arm

lanes of interstellar gas, dust and young stars that wind outward in a plane from the central region of a galaxy

weakly interacting massive particle (WIMP)

a hypothetical massive particle that may comprise part of the unseen dark matter

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