Imagine the Universe!
Imagine Home  |   Teachers' Corner   |  

VI. Glossary

Absolute Magnitude - apparent magnitude a star would have if placed at a distance of 10 parsecs from Earth

Afterglow - The electromagnetic radiation emitted after removal of a source of energy, especially: the glow of an incandescent material as it cools

Apparent Magnitude - a measure of observed light flux received from an object at the Earth

Arc Minutes - a unit of measurement used for very small angles; there are 60 arc minutes in one degree

Arc Seconds - a unit of measurement used for very small angles; there are 60 arc seconds in one arc minute

Black Hole - region in space where the escape velocity is equal to, or greater than, the speed of light; thus, nothing (including radiation) can escape from it

Cosmological — Related to cosmology, the study of the physical universe considered as a totality of phenomena in time and space. Cosmology is the astrophysical study of the history, structure, and constituent dynamics of the Universe.

Cosmological Distance — A distance that is a large fraction of the size of the observable universe.

Counterpart — An object which serves as the complement of another object or event. Related to GRBs, it is the object which astronomers can detect in other parts of the electromagnetic spectrum which is somehow related to the GRB. For example, the optical transient observed in the host galaxy of the GRB is the optical counterpart of the GRB.

Electromagnetic Radiation - radiation consisting of periodically varying electric and magnetic fields that vibrate perpendicular to each other and travel through space at the speed of light

Electromagnetic Spectrum - the full range of electromagnetic radiation spread out by wavelength, it consists of gamma-rays, X-rays, ultraviolet rays, optical light, infrared radiation, microwaves, and radio waves. Wavelength, energy, frequency, or temperature can classify these electromagnetic waves.

GRB — Gamma-Ray Burst

Hypernova — (Hypernovae is the plural) Possibly the most powerful explosions in our Universe since the Big Bang. Hypernovae are even more powerful than supernovae explosions, the spectacular bursts released at the deaths of massive stars. Still considered a hypothetical notion, a hypernova may emit 100 times more energy than a supernova, and represents a popular explanation for gamma-ray bursts. What causes a hypernova remains unknown, although astronomers have proposed that they happen when a very large and rapidly rotating star collapses. Another possibility is that the explosions aren’t so bright after all, except for a pair of narrow flashlight-like beams of gamma-rays. The word "hypernova" was coined by Bohdan Paczynski, a professor of astrophysics at Princeton University in 1997.

km - kilometer = 0.6 mile.

Light-Year - the distance light travels in one Earth year, equal to 9.46 x 1012km

Log-Log Plot - A log plot portrays each 10 to 1 change as a fixed linear displacement. A log-log plot does this on both the X and Y axes. Logarithmically scaled plots are extremely useful in science at showing two important aspects of a data set.  First, the log plot expands the resolution of the data at the lower end of the scale to portray data that would be difficult to see on a linear plot.  The log scale never reaches zero, so data points that are 1 millionth of the peak still receive equal treatment.  On a linear plot, points near zero simply disappear. The second advantage of the log plot is that percentage difference is represented by the same linear displacement everywhere on the graph.  On a linear plot, 0.09 is much closer to 0.10 than 9 is to 10, although both sets of numbers differ by exactly 10 percent.  On a log plot, 0.09 and 0.10 are the same distance apart as 9 and 10, 900 and 1000, and even 90 billion and 100 billion.  This makes it much easier to determine a spectral match on a log plot than a linear plot.

Luminosity - the rate of radiation of electromagnetic energy into space by a star or other object

Magnitude - The units used to describe brightness of astronomical objects. The smaller the numerical value, the brighter the object. The human eye can detect stars to 6th or 7th magnitude on a dark, clear night far from city lights; in suburbs or cities, stars may only be visible to mag 2 or 3 or 4, due to light pollution. The brightest star, Sirius, shines at visual magnitude -1.5. Jupiter can get about as bright as visual magnitude -3 and Venus as bright as -4. The full Moon is near magnitude -13, and the Sun near mag —26. The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2.5 times in brightness; a change of 5 magnitudes is defined as a change of exactly 100 times in brightness.

Neutron Star - the final stage of existence for stars born three to seven times more massive than our Sun. Neutron stars are produced by supernova explosions, which blow away most of the material of the star and leave behind a roughly 1.4 solar mass core. In these cores, material is so highly compressed that all the protons, electrons, and neutrons are piled together, breaking down the normal structure of an atom. These remnant cores are called neutron stars.

Parsec - unit of distance often used by astronomers, equal to 3.2616 light-years (a kiloparsec is equal to 1,000 parsecs)

Photon - a unit of electromagnetic energy associated with a specific wavelength or frequency

Progenitor — originator. Related to GRBs, the progenitor of the event is the object (or objects) which underwent some sort of catastrophic occurrence which resulted in the burst of gamma-ray emission.

Redshift - An increase in the wavelength of radiation emitted by a celestial body as a consequence of the Doppler effect.

Shockwave - 1. A large-amplitude compression wave, as that produced by an explosion or by supersonic motion of a body in a medium. 2. A violent disruption, disturbance, or reaction.

Speed of Light - the ultimate speed limit in the Universe: 300,000 kilometers/second.

Star - a self-luminous sphere of gas

Stellar Spectroscopy - breaking down the electromagnetic radiation from a star in order to study the different wavelengths individually

Supernova - (Supernovae is the plural) A supernova is one of Nature's grandest spectacles. Most commonly, it occurs when a star runs out of nuclear fuel. Its core collapses leaving the star's outer layers unsupported. They fall inward and the result is a gigantic explosion that for a day or so can outshine all the stars in a galaxy.

Universal Time (UT, or UTC) - A measure of time used by astronomers; UT conforms (within a close approximation) to the mean daily (apparent) motion of the sun. UT is determined from observations of the diurnal (daily) motions of the stars for an observer on the earth. UT is usually used for astronomical observations, while Terrestrial Dynamical Time (TDT, or simply TT) is used in orbital and ephemeris computations that involve geocentric computations. Coordinated Universal Time (UTC) is that used for broadcast time signals (available via shortwave radio, for example), and it is within a second of UT.

Back Index Next

Download a pdf version.

If words seem to be missing from the articles, please read this.

Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Project Leader: Dr. Barbara Mattson
Curator: Meredith Gibb
Responsible NASA Official: Phil Newman
All material on this site has been created and updated between 1997-2014.
This page last updated: Thursday, 21-Nov-2002 12:11:31 EST