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the dilemma posed by the fact that the combined average mass density of the universe is very nearly equal to the critical density
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
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
an expansion of all space that took roughly 13.7 billion years ago that marks the beginning of the 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
the assumption that the universe is homogeneous and isotropic on the largest scale
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
a universe with negative curvature, so that its geometry is analogous to a saddle-shaped hyperbolic (saddle) surface
a galaxy that is emitting exceptionally large amounts of energy: a seyfert, radio galaxy, or quasar
double radio source
an extragalactic radio source charecterized by two large regions of radio emission, typically located on either side of an active galaxy
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
a massive object that reflects light rays from a remote source, forming an image much as an ordinary lens does
the empirical relationship stating that the redshifts of remote galaxies are directly proportional to their distances from earth
a flattened rotating galaxy with pinwheel like spiral arms winding outward from the galaxys nucleus
nonluminous matter that is the dominant form of matter in galaxies and throughout the universe
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
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
lanes of interstellar gas, dust and young stars that wind outward in a plane from the central region of a galaxy
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