Glossary
(Universe,G1-G18; An Introduction to Galaxies and Cosmology, 407-425)
baryonic matter- matter consisting of baryons (i.e. ordinary matter made of protons and neutrons)
Big Bang- An explosion of all space that took place roughly 13.7 billion years ago and that marks the beginning of the Universe.
blackbody- A hypothetical perfect radiator that absorbs and re-emits all radiation falling upon it.
cepheid variable star- a type of yellow, supergiant, pulsating star.
Chandrasekhar limit- The maximum mass of a white dwarf.
cluster of galaxies- A collection of galaxies containing a few to several thousand member galaxies.
cosmic microwave background (CMB)- An isotropic radiation field with a blackbody temperature of about 2.725 K that permeates the entire Universe.
cosmology- The study of the structure and evolution 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.
dark matter- Nonluminous matter that is the dominant form of matter in galaxies and throughout the Universe.
decoupled- no longer in thermal equilibrium with
deuterium bottleneck- The situation during the first 3 minutes after the Big Bang when deuterium (an isotope of hydrogen whose nucleus contains one proton and one neutron) inhibited the formation of heavier elements.
Doppler effect- The apparent change in wavelength of radiation due to relative motion between the source and the observer along the line of sight.
escape velocity- The velocity needed by an object to leave the gravitational field of a second object permanently and to escape into interplanetary space.
galaxy- A large assemblage of stars, nebulae, and interstellar gas and dust.
homogeneous- Having the same property in one region as in every other region.
Hubble Constant (H 0)- In the Hubble law, the constant of proportionality between the recessional velocities of remote galaxies and their distances. H 0 is equal to 22 km/s per million light years or 72 km/s per Mpc.
intrinsic brightness- the amount of radiation produced
isotropic- Having the same property in every direction.
last-scattering surface- The surface defined by the locations at which photons in the cosmic microwave background last underwent significant interaction with matter (with the exception of gravitational effects). This interaction was due primarily to electron scattering, and so last-scattering occurred at about the time of recombination.
light- Electromagnetic radiation in the visible spectrum.
light horizon- The boundary indicating how far light would have been able to travel.
luminosity- The rate at which electromagnetic radiation is emitted from a star or other object.
MACHOS- Massive Compact Halo Object- A dim star or low-mass black hole that may comprise part of the unseen dark matter.
microlensing- A phenomenon in which a compact object such as a MACHO acts as a gravitational lens, focusing the light from a distant star.
nuclear fusion- The process of combining elements to form more massive elements.
nucleosynthesis- The process of building up nuclei such as deuterium and helium from protons and neutrons.
parallax- The apparent displacement of an object due to the motion of the observer.
parsec (pc)- A unit of distance; 3.26 light-years.
Planck time- A time determined by a combination of physical constants that represents the earliest time in cosmic history at which currently established physical theory might be used to study the nature and evolution of the Universe.
primordial nucleosynthesis- The nuclear processes that were responsible for the initial formation of the nuclei of light elements (such as helium and lithium) in the early Universe. It is generally believed that primordial nucleosynthesis began when the age of the Universe was about 3 minutes, and that it continued for about thirty minutes.
recombination- A process in which an electron and an ion combine. The term recombination is also used to refer to the postulated episode in cosmic history in which the baryonic matter of the Universe made the transition from being predominantly plasma to predominantly neutral atoms. Recombination is believed to have occurred when the age of the universe was about 380,000 years.
redshift- The numerical quantity used to measure the shift in wavelength of a spectral line. The shifting to longer wavelengths of the light from remote galaxies; the Doppler shift of light from a receding source.
spectroscopy- The systematic study of spectra and spectral lines. Spectral lines are tremendously important in astronomy, because they provide reliable evidence about the chemical composition of distant objects.
standard candles- Any type of object whose luminosity is directly indicated by its observable properties, thus allowing its distance to be inferred from the difference between its apparent brightness and its true brightness.
supernova- A stellar outburst during which a star suddenly increases its brightness roughly a millionfold.
thermal equilibrium- A state in which there is a high level of interaction between matter and radiation which leads to the radiation having a black-body spectrum with a characteristic temperature that is the same as that of the matter.
Universe- The one known universe.
white dwarf- A low-mass star that has exhausted all its thermonuclear fuel and contracted to a size roughly equal to the size of the Earth.
WIMP (Weakly Interacting Massive Particle) - A hypothetical elementary particle that has a relatively high mass and which only interacts by the weak interaction and gravity. A large population of such particles might account for a significant amount of (non-baryonic) dark matter.
Comments: