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Final Glossarys (Ch 11-15)

Terms in this set (52)

Big Bang model
Theoretical description of the development of the universe from an extremely hot, dense state to its current cold, low-denseity state.
Steady-state model
Theory in which the universe (on average) does not change with time. (unsupported by data)
Cyclic model of the universe
Theory in which the universe undergoes multiple episodes of expansion that start with a state of extremely high temperature and density.
Dark night sky paradox (Olber's)
{rpblem with models in which the universe is infinite in space and has always existed: the night sky should be bright from distant starlight.
Cosmological principle (Copernician)
Assumption that the universe has the same appearance, on average, as observed from any place inside it.
Cosmological constant (symbol-lambda)
A term in Einstein's equations describing the space-time of the universe. It corresponds to a "dark energy" that promotes expansion.
Parsec (pc)
Distance unit used for stars and galaxies. 1 __ = distance of a star with a parallax of 1 arcsecond.
Light-year (lt-yr)
The distance traveled by light in one year.
Blackbody
An opaque object. Its luminosity depends strongly upon temperature and size, and its spectrum (intensity v. wavelength) has a characteristic shape.
Hubble Law
The relation between velocity or redshift and distance of galaxies beyond the Local Group. Can be used to determine distances to remote objects. Led to the conclusion that space is expanding.
Hubble's Constant (symbol-H_0)
Slope of the Hubble Law. Its value at the current epoch of the universe is about 70 km/s/Mpc.
Epoch
Time after the expansion of the universe began when the universe was at a particular stage in its development.
Planck epoch
Very early stage of the universe when all the forces of nature were unified. This period ended when the universe was only ~10^-43 s old and had a temp. of ~10^32 K.
Inflation
Hypothesized period of the very early universe when space expanded by about 10^50 times during a very brief peiod following a sudden drop in the potential energy of space.
"Primordial Soup"
Nickname often given to the early universe, which was a mixture of many different kinds of particles and with "lumps" of slightly higher-than-average densities.
Nucleosynthesis
Nuclear fusion of protons and neutrons to form nuclei of atoms other than simple Hydrogen, which is only a proton.
Cosmic Microwave Background (CMB) radiation
Radio and infrared photons that arrive from each direction in the sky with a very nearly uniform brightness and the spectrum of a blackbody. Thought to be the light released when the universe first became transparent, but has since increased in wavelengths because of the expansion of space.
Dark matter
Material (probably particles not yet discovered) that does not emit light but whose gravitational influence is apparent in galaxies and clusters of galaxies.
Dark energy
Nickname given to some agent (e.g., the Cosmological constant) that counteracts gravity to promote expansion of the universe.
Critical density
The mass-energy per unit volume that would be required for the universe to decelerate gradually toward a steady state because of gravity, if there were no accelerating agent.
Omega (symbol-Omega_0)
The actual current density of the universe divided by the critical density.
Redshift (symbol-z, no units)
A measurement of the increase in wavelength caused by the expansion of the universe. Similar to the Doppler shift. 1+__ = by how many times the universe has spread out since the light we now rceive left the object being observed.
Type Ia Supernova
A class of exploding stars with the important characteristic that the luminosity at maximum brightness is the same from one to the other, except from minor differences that can be determined from the spectrum. Very useful as a standard candle to measure distances to galaxies since these are extremely luminous.
Multiverse
Hypothetical realm of existance containing many universes.
Galaxy
A gravitationally bound system of millions, billions, or even trillions of stars.
Spiral galaxy
System of stars with a general spiral pattern.
Elliptical galaxy
System of stars with a general circular or elliptical pattern.
Milky Way Galaxy
The spiral galaxy to which our solar system belongs.
Cluster of galaxies
A gravitationally bound system of hundreds of thousands of galaxies.
Supercluster of galaxies
A collection of clusters of galaxies grouped together. Thought to be the largest non-random structures in the universe.
Cosmic web
Nickname given to the pattern of superclusters in the universe consisting of interwoven filaments.
Void
A large region containtaing low density of galaxies.
Space-time
The framework of the 4-dimensional macroscopic universe, consisting of three dimensions in space and one in time.
Black hole
Object that has shrunk to a radius smaller than the Schwarzschild radius corresponding to its mass.
Schwarzschild radius
Radius of a black hole (2GM/c^2).
Event horizon
Boundary that defines the outer edge of a black hole. For a non-rotating black hole, is a distnce = to Schwarzschild radius of the black hole.
Nucleus of the galaxy
The central region of a galaxy.
Bulge of a galaxy
A structure in the shape of a sphere or flattened sphere that surrounds the nucleus of a galaxy. The structure contains a substantial fraction of the stars of the galaxy.
Active galaxy nucleus (AGN)
The center of a galaxy that produces a very high luminosity.
Quasar
The active nucleus of a galaxy that prodfuces extremely high luminosities of electromagnetic radiation and displays highly energetic phenomena. Thought to be powered by matter falling into a supermassive black hole, which converts gravitational potential energy to thermal and kinetic energy.
Jet
A high-speed beam of particles and magentic field. Produced by quasars and less luminous AGN. Flow from nucelus at speeds near c in the most extreme cases, can extend beyond boundary into inter-galactic space.
"Dark ages"
{eriod in the development of the universe between when atoms formed and when galaxies formed. There was very little visible light in the universe during this time.
Merging
Process by which the mutual gravitational attraction of galaxies passing close to each other causes them to combine into a signle, larger galaxy.
Gravitational collapse
Process by which the attraction of the mass of different regions causes them all to fall toward the center of the mass distribution.
Gas pressure
Force per unit area, directed outward, exterted by a gas. Depends directly on the density and temperature of the gas.
Dust
Fine grains of many molecules that, in space, block starlight by reflection and absorption. Absorbs visible and ultraviolet light.
Nuclear fusion
Process by which light atomic nuceli combine to form more complex nuceli. Mass is converted to energy. Main source of energy in a normal star.
Supernova
The explosion of a star. Most or all of the star is ejected into space. The luminosity of the expanding remneant is extremely high during the first several weeks after the explosion.
Angular Momentum
The equivalent of momentum for objects or particles that are spinning or moving along curved trajectories. For an object of mass m in a circular orbit of radius r at a speed v, = mvr. Conserved.
Axis of rotation
Imaginary line about which an object revolves or rotates. It is perpendicular to the equitorial plane.
Protostar
A newly formed star that obtains its energy mainly from the heat of the gravitational contraction that formed it. This is a stage prior to when the nuclear fusion at the core of the object produces the energy, at which point the objectis a normal star.
Star
A massive object composed of gas that radiates light. Its energy source is nuclear fusion in its core.
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