anisotropy Irregularities on a sky map which may indicate cosmic structures.
anthropic principle A philosophical construct based on the requirement that the physical parameters of our universe must be compatible with the existence of intelligent observers.
apparent brightness The brightness of an object as seen from Earth.
apparent magnitude The standard measure for the apparent brightness of astronomical objects; the scale was originally set by Arab and central Asian astronomers. Smaller numbers designate brighter objects.
baryon acoustic oscillations The sound waves propagating in the early universe are like the waves coming out of an organ pipe, and measuring their properties on the sky can determine cosmic parameters in the same way that we can tell the length of the organ pipe by the depth of the sound.
baryonic matter Ordinary matter containing protons and neutrons and the ordinary chemical elements; not dark matter.
beta decay A type of radioactive decay in which an atom emits a beta particle (e.g., an electron).
big bang model The theory that describes how the universe was formed 13.7 billion years ago in an explosion from an extraordinarily dense initial state. The hot big bang model, now standard, also presumes that the early state of the universe was extraordinarily hot, and the observed cosmic background radiation shows the ashes of this primeval fireball.
Big Crunch The opposite of the Big Bang, in which the universe would end if gravity were strong enough to reverse the expansion of space and cause a re-collapse.
black hole An object that has collapsed to such a small volume that the gravitational field created does not allow light to escape. Supermassive black holes (from a million to many billion times more massive than the Sun) reside at the center of most massive galaxies.
boundary conditions In solving equations, the conditions that apply at the boundary between the volume studied and the rest of the world, so the boundary condition for water sloshing in a bottle is that the water never leaves the bottle.
Cosmic background radiation (CBR) The Cosmic Background Radiation field is the radiation at our cosmic epoch seen primarily as radio waves that is the remnant of the very hot radiation field left over from the hot big bang explosion.
CCD Charge-coupled device, a detector similar to what is in digital cameras that, when used in astronomy, greatly increases the power of telescopes to see distant objects.
celestial sphere The imaginary sphere on which objects in the sky appear to move while the Earth rotates.
Cepheids A special kind of variable star that helps astronomers measure cosmic distances because of the precise relation between its period of oscillation and its intrinsic luminosity.
closed universe If the density of the universe is greater than the critical density, the geometry of space-time is such that a circle has a circumference less than π times its diameter and, as on a sphere, one can travel in one direction but come around to where one started.
CNO cycle The reactions that occur in high-mass stars that fuse hydrogen into helium using carbon and nitrogen as catalysts in the reactions.
Cosmic Background Explorer (COBE) The satellite launched in 1989 that detected the fluctuations in the cosmic background radiation field which reflected the primordial perturbations that had seeded the growth of galaxies and all cosmic structure.
cosmic rays Atomic nuclei and various subatomic particles that travel through space at nearly the speed of light.
cosmological constant The term in Einstein’s field equations of general relativity that represents a repulsive force or energy (dark energy) that pushes matter from other matter and causes the expansion of the universe to accelerate.
cosmology The study of the nature, overall structure, and evolution of the universe.
covariant equations Equations written in a special form so that they will be true for all observers regardless of the velocity at which they are moving.
critical mass density The precise mass density that lies between a universe that re-collapses and one that expands forever.
cyclic model A current version by Steinhardt and Turok of a model universe that could have repeated collapse and expansion phases.
dark ages The period after the radiation and gas from the Big Bang had cooled down due to expansion and before stars and galaxies had begun to form.
dark energy Mysterious energy that is a property of space that causes the expansion of the universe to accelerate.
dark matter Mysterious matter that dominates the total mass of the universe. It neither emits nor absorbs light, but we infer its existence from the gravitational forces it exerts on objects that we can see, and it responds to gravitational forces in the same fashion as normal matter.
deceleration parameter A measurable, dimensionless number that expresses the rate at which gravity is causing the expansion of the universe to slow.
dipole variation The cosmic background radiation is slightly brighter in the octant of the sky toward which we are moving and slightly fainter in the opposite one.
disk The flat, more or less circular portion of a spiral galaxy that contains the spiral arms, that will look elongated if seen edge on.
Doppler shift The shift in wavelength and frequency of photons emitted from an object that is moving toward or away from the observer.
electromagnetism One of the four fundamental forces; it governs interactions between atoms and molecules.
elliptical galaxy Smooth galaxies with a rounded distribution of stars that typically contain orange-red, older star populations as compared to spiral galaxies.
energy density The average energy per unit volume in the entire universe.
entropy The degree of disorder in a physical system.
epicycle Epicycles were the extra “wheels on wheels” that astronomers in antiquity needed to explain the apparent motions of the planets on the sky before the sun-centered solar system was understood.
epoch of reionization Era in the early universe when the first stars were formed.
era of recombination Time in the early universe when the hot soup of protons and electrons cooled enough to combine into atoms of hydrogen.
escape velocity The velocity an object would have to achieve to escape from the gravitational field of a massive body.
Euclidean geometry The geometry that we learn in high school that is valid on a flat plane or piece of paper.
extragalactic Outside of our Milky Way galaxy.
filaments Large, cosmic structures that consist of numerous individual galaxies in a gravitationally connected linear structure.
flat universe The geometry of space-time is flat and a circle has a circumference exactly π times its diameter as it has on a plane. On a sphere the circumference is less than π times its diameter, and on a saddle it is more. In a universe without a cosmological constant, the flat universe has exactly the critical density.
flux Another term for the apparent brightness of an object, the unit usually being energy passing through a square centimeter per second.
general theory of relativity Einstein’s theory (1915) that extends special relativity to include cases of accelerating observers. Rather than thinking of gravity as a force, Einstein proposed that gravity is the curvature of space-time.
globular cluster Round stellar system containing roughly one hundred thousand stars each. They reside primarily in the spherical component surrounding galaxies and contain very old stars.
Goldilocks problem The situation where cosmic parameters seem to have been finely tuned to be “just right” to allow for the universe that we see today.
Grand unified theory (GUT) A concept that three of the prime forces of physics (electromagnetism, the strong force, and the weak force) can be combined into a single theory at sufficiently high energies.
gravitational lens An object creating a significant gravitational field that causes light to bend. The image we detect will be magnified and often distorted.
gravitational redshift The spectral shift of light toward the red caused by its losing energy as it climbs out of a gravitational field as a photon moves toward an observer.
great wall A large sheet-like structure composed of numerous galaxies.
halo The roughly spheroidal region enveloping a galaxy. While it may contain stars, most of its mass is in the form of dark matter.
heat death The idea that a universe that lasted forever would ultimately run out of fuel and everything would cool down to a very low temperature.
Higgs boson Hypothetical elementary particle that naturally should exist in the standard model of particle physics and whose tentative discovery was announced by the Large Hadron Collider team on July 4, 2012.
Hubble constant A number, H0, that represents the current rate at which the universe is expanding; when multiplied by the distance to the object in mega-parsecs it gives its velocity of recession in km/s.
Hubble law The law which states that the farther away a galaxy is from the Milky Way, the faster it moves away from us, the velocity being proportional to the distance.
hydrostatic equilibrium Describes a state in which the inward gravitational forces are exactly balanced by the outward pressure forces, that, in turn, arise from the hot interior of a star.
inflation The idea that the universe had a period of exponential growth in its earliest phases.
interstellar medium The gas and dust that are found between the stars.
intrinsic luminosity The total amount of energy emitted from a star each second.
inverse square law (gravitation) The law, due to Isaac Newton, which states that the gravitational attraction between two objects is proportional to their masses and inversely proportional to the square of the distance between them.
ionization The process by which electrons are stripped from an atom.
irregular galaxy Galaxies whose shapes are neither especially elliptical nor spiral.
island universe A term coined by Immanuel Kant to refer to the idea that the spiral nebulae are galaxies similar to our own Milky Way.
isotopes Atoms of the same element that differ in the number of neutrons they contain. Different isotopes of the same element weigh different amounts, and can have different nuclear reactions, but they have essentially the same chemical properties.
isotropic The same in all directions. Looking around on a very foggy day, the brightness of the light in all directions is isotropic.
Jeans instability An explanation of gravitational instabilities (discovered by James Jeans) that cause massive structures to grow from very small initial fluctuations.
Jeans mass The critical mass for which an interstellar cloud will begin a runaway gravitational collapse.
LCDM Abbreviation for Lambda-Cold Dark Matter, which is the standard model of big bang cosmology that incorporates both dark matter and dark energy.
LHC Large Hadron Collider, the world’s largest and highest energy elementary particle accelerator, first activated in 2008 with hopes to find the Higgs particle and also perhaps to detect dark matter particles.
light-year The distance a beam of light (moving at a speed of 300,000 km/s) can travel in one year, equal to 9.5 × 1012 km.
local group The group of galaxies that contains our own Milky Way galaxy, Andromeda (M31), M33, and numerous smaller systems.
mass density The average mass per unit volume of all the matter in the universe.
mass-to-light ratio The mass of an object divided by its luminosity usually given in solar units.
Messier catalog A catalog of extended objects in the sky studied by Charles Messier (published in 1771) and denoted by a capital M followed by a number, e.g., M81.
microwave background radiation The remnant radiation that radio telescopes detect from a time just 380,000 years after the Big Bang, when the universe first became transparent.
molecular clouds Dense clouds of gas in interstellar space that are made up primarily of hydrogen molecules. Star formation occurs in molecular clouds.
nebula Presently defined as an extended cloud of dust and gas that is often seen glowing from absorbed or reflected starlight. Before galaxies were understood as comprising numerous stars too close together to be identified individually, they were often called nebulae.
nebular origin of the solar system The theory that describes how our solar system formed out of a rotating cloud of dust and gas over 4 billion years ago.
neutrino A very low-mass elementary particle.
neutron star A type of star, approximately the mass of the Sun, but only about 10 km in size, made almost entirely of neutrons as dense as the nucleus of an atom. Probably made in supernova explosions.
nuclear fission The process in which a large atomic nucleus splits into smaller nuclei with enormous release of energy, as in an atomic bomb.
nuclear fusion The process by which small atomic nuclei are joined together to create a larger nucleus.
open universe If the density of the universe is less than the critical density, the geometry of space-time is such that a circle has a circumference greater than π times its diameter. An open universe can never re-collapse.
parallax The apparent shift in an object’s position in the sky due to viewing it from different positions.
parsec (pc) The distance an observer would be from our solar system to see our orbit around the Sun as being one arc second in angle on the sky. One parsec is several times farther than the distance light can travel in a year and is roughly equal to 30 trillion kilometers.
perihelion The point in an object’s orbit at which it is closest to the Sun.
period The time it takes for a variable star to complete one full cycle in luminosity.
perturbation A small change in a system or part of a system caused by an external force such as gravity.
phase transition The abrupt transition from one state of matter to another, e.g., the transition from water to ice.
photon An individual particle of light.
Planck era The very early era when the universe was younger than 5.4 × 10-44 seconds, and our current theories of gravity and quantum mechanics come into conflict.
plasma An ionized gas in which the electrons have been separated from the atoms to which they were bound.
population I stars Young, typically bright blue stars in the spiral arms of galaxies.
population II stars Older stars in the central region of spiral galaxies and globular clusters typically fainter and redder than population I stars.
precession The gradual wobble than an object experiences as it rotates.
principle of relativity The idea that the fundamental laws of physics must be the same for all observers moving at a constant velocity with respect to each other.
proton-proton cycle The reactions that occur in low-mass stars that fuse hydrogen into helium.
pulsar A neutron star that rotates rapidly such that we detect periodic pulses of radiation from it, as when a beam from a lighthouse passes by us.
quantum gravity At attempt to unify quantum mechanics and gravity into a single model.
quantum mechanics A description of physical phenomena at the smallest scales where discreteness and randomness are essential properties. It describes the interactions of atoms, subatomic particles, and photons.
quasars The especially luminous centers of some galaxies that are powered by mass from the galaxy falling into the supermassive black hole at the galactic center.
quintessence A possible force field that causes the expansion of the universe to accelerate: a variety of dark energy.
radiometer Instrument for measuring the strength of radio signals.
relativity See special theory of relativity, general theory of relativity
rotation curve The change in the rate of rotation of orbiting bodies with increasing distance from the center of the system. In the solar system, the outer planets rotate around the Sun at a slower rate than the inner ones.
SDSS Sloan Digital Sky Survey was an automated survey of the northern sky (starting in 2000) using CCDs to catalog and measure the properties of one million galaxies and one hundred thousand quasars, the most comprehensive astronomical database ever assembled.
space-time continuum The mathematical combination of the three dimensions of space and the one dimension of time.
special theory of relativity Einstein’s theory (1905) that describes the idea that all motion is relative and that the speed of light in empty space is the same for all observers in non-accelerating frames of reference.
spectral lines Bright or dark lines that appear in graphs of an object’s distribution of photons at various wavelengths or frequencies. By studying these lines, astronomers can identify the chemical composition of the objects they see and the velocity of the object with respect to the observer.
spectroscope A prism-like device that spreads out light like a rainbow into its different colors.
spiral galaxy A galaxy composed of a large, flat disk of stars, often with a bulge at the center. Spiral arms within the disk are filled with younger, blue star-forming regions.
standard candle An object that is useful for measuring cosmic distances because its true luminosity is known.
static universe The model universe proposed by Einstein in 1917 in which gravity is exactly balanced by the cosmological constant and nothing moves. It was proposed before Hubble’s results were widely known, and is unstable.
steady state model A theory that held that the universe is eternal, having neither a beginning nor an end, appearing the same at all times and places. Matter must be created everywhere continuously in this cosmological model.
string theory An attempt to unify quantum mechanics and general relativity into a single model.
strong force One of the four fundamental forces; it holds the nucleus together by overcoming electrostatic repulsion.
supernova A star that explodes at the end of its lifetime, either totally disrupting or leaving a condensed object like a black hole or neutron star.
tensor In general relativity, a four-by-four array of numbers used to describe quantities in space-time.
thermal blackbody radiation The spectrum of radiation produced by an ideal, opaque object of a definite temperature. The hotter the object, the more the light is shifted from the red to the blue or ultraviolet.
Type Ia and Type II supernovae The first is a special kind of supernova that always emits nearly identical luminosities when it explodes, thus allowing astronomers to use it as a standard candle to measure cosmic distances. They arise from relatively low mass stars. The second is from the explosion of massive stars, and they are quite variable in their brightness.
universal gravitation Isaac Newton’s realization that all clumps of matter, from atoms to clusters of galaxies, attract each other.
variable star A star whose brightness oscillates in a regular way.
vector An array of three numbers showing both quantity and direction. Velocity is a vector.
virial theorem A statistical method for relating the total kinetic energy of a system of orbiting bodies to its total potential energy if it is to stay in equilibrium and neither collapse nor expand.
weak force One of the four fundamental forces; it is responsible for radioactive decay.
weakly interacting massive particle (WIMP) A popular candidate for a dark matter elementary particle.
Wilkinson Microwave Anisotropy Probe WMAP, launched in 2001, that has provided the most precise analysis of the cosmic background radiation and has measured the cosmic parameters with sufficient precision to confirm the LCDM standard cosmic model.