Glossary of Terms

²⁶Al (26-aluminum)

A radioactive isotope of aluminum that has a half-life of about a million years. When it decays, it produces gamma-rays. It is produced inside massive stars before they supernova, and so its detection is an indication of where in our galaxy supernovae have exploded.

absorption line

Colors missing in a continuous spectrum because of absorption of those photons by some intervening material.

afterglow

In gamma-ray bursts, emission seen after a GRB, which can be seen in X-rays, optical, and radio. Afterglow can last for days or even weeks.

aperture

The opening in a telescope that admits light.

arcminute

An angular unit of measure; 60 arcminutes make up one degree.

arcsec

An angular unit of measure; 60 arcseconds makes up an arcminute, or 3600 arcseconds makes up one degree.

autonomous

Able to act independently. In the case of Swift, this means that the spacecraft can repoint itself without ground controllers feeding it commands.

background

In astronomy, a background is the "noise" seen in any detector. A familiar example is the static picked up by a cell phone. The static is inevitable, but is not part of the original message sent by the caller.

black hole

One possible end point of a star's life. It is so dense that not even light can escape it's surface.

Burst Alert Telescope (BAT)

The gamma-ray detector aboard Swift. BAT will watch the sky for sudden bursts of gamma-rays.

Burst And Transient Source Experiment (BATSE)

One of four experiments aboard the Compton Gamma-Ray Observatory. BATSE observed nearly the entire sky for transient sources, and could position these sources more accurately than earlier experiments.

Cadmium-Zinc-Tellurium (CZT)

A semiconductor used in the detection of gamma-rays.

Charged Coupled Device (CCD)

Device which detects incoming photons by storing charge produced by the interaction of the photon and detector material, then transferring that charge sequentially to an amplifier and detector.

coded aperture mask

A method of determining the direction of a gamma-ray in a gamma-ray telescope. Consists of a plate of material constructed of tiles with a random half-open, half-closed pattern, which causes incoming gamma-rays to cast a shadow-pattern on the detector which can be used to determine the direction of the gamma-rays.

cosmological distance

Distances corresponding to a time when the age of the universe was a small fraction of its current age.

count rate

In astronomy, the number of photons registered by a detector each second.

electromagnetic spectrum

The range of all wavelengths (energies/frequencies) of electromagnetic radiation.

emission line

An energy peak in a continuous spectrum caused by the emission of those photons by the de-excitation of electrons in atoms.

erg

A measure of energy. It is equivalent to 10^-7 Joules, or is about enough energy to light a 60 Watt light bulb for about 2 nanoseconds (2 billionths of a second).

extragalactic

Outside of the Milky Way Galaxy.

field-of-view

In astronomy, the angular area that a telescope can 'see'.

filter (U, B, V)

In optical astronomy, a filter is used to limit the wavelength of light allowed into a telescope's detector. The U filter centers on 360 nanometers, the B filter on 440 nanometers, and the V filter on 550 nanometers.

filter wheel

In optical astronomy, a wheel holding several different filters mounted on a telescope.

flux

The rate that energy crosses a given surface (real or imaginary).

gamma ray

The region of the electromagnetic spectrum defined by radiation energies above 1 million eV (or wavelengths less than 0.001 nanometers); this waveband represents the highest energy band in the electromagnetic spectrum.

gamma-ray burst (GRB)

A brief, but brilliant, burst of gamma-rays coming from a random point in the sky about once per day.

grazing incidence

In terms of mirrors, this refers to shining light onto the mirror at a very shallow angle - nearly parallel to the surface of the mirror; this is the only way to focus X-rays with a mirror

grism

A specialized prism for taking spectra.

hard X-ray

X-rays on the harder, or more energetic, end of the spectrum, loosely defined as X-rays with energies above 10 keV

High Energy Astronomy Observatory (HEAO-1 A4; August 1977 - January 1979)

HEAO-1, the first of three High Energy Astronomical Observatories. A4 was the Hard X-Ray/Low Energy Gamma Ray Experiment aboard HEAO-1, which completed an all-sky hard X-ray survey.

http://mamacass.ucsd.edu:8080/instdev/heao1-a4/

hypernova

An explosion about 100 times more powerful than a supernova explosion, caused by the collapse of a very massive star (mass greater than about 40 Suns).

Imaging Compton Telescope (COMPTEL)

One of four experiments aboard the Compton Gamma-Ray Observatory. COMPTEL produced an all-sky map of gamma-ray emission line, ²⁶Al, which showed us the locations of nucleosynthesis and massive stars throughout the Galaxy

intensity

For gamma-ray bursts, a measure of the strength of a burst. It is measured as the number of photons crossing a given surface (i.e. the detector) per second.

light year

A measure of the distance light travels in one year; equal to about 9.5 x 10¹² kilometers, or 5.9 x 10¹² miles.

light curve

A graph of an object's changing brightness over time.

localization

For gamma-ray bursts, the area of the sky from which the burst occurred. The smaller the area a telescope is able to pinpoint a burst to, the better the localization.

Milky Way Galaxy

The galaxy that the Sun and Solar System (and Earth) are in.

nanometer (nm)

A unit of distance that is one billionth of a meter (10^-9 m).

narrow-field

In astronomy, a relatively small field-of-view for a telescope.

neutron star

One of the possible end-points of a star. A neutron star is very dense, with the mass of about 1.4 Suns contained in a sphere with a radius of about 10 km.

Oort cloud

An spherical envelope of small bodies theorized to surround our Solar System at 0.1 to 1 lightyear, and proposed to be the source of the longest-period comets.

optical radiation

The narrow part of the electromagnetic spectrum that can be picked up by human eyes, specifically the region with wavelengths ranging from 400 to 700 nanometers or with energies of 2 - 3 eV.

radiation

In astronomy, energy sent out from a source as light (photons).

radio waves

The lowest energy portion of the electromagnetic spectrum, with wavelengths longer than a meter or energies of below one millionth of an eV (below 10^-6 eV).

redshift

The shift in energy (or wavelength) of a photon emitted when moving away from an observer; in cosmology, the redshift is a measure of the distance from the observer of the source emitting the photons.

slew

In astronomy, to turn a telescope about a fixed point or axis.

solar system

The Sun and the objects orbiting it, including the planets, asteroids and comets.

spectrum

The amount of energy given off by an object at measured energies (plural, spectra)

supernova

An explosion caused by the death of a massive star. At its peak energy output, a supernova can outshine a galaxy.

transient

In astronomy, a source that suddenly changes, such as appearing, disappearing or drastically brightening or dimming.

trigger

In astronomy, a pre-defined set of conditions met by a detector, initiating a set of observations or data-collection. The trigger conditions vary by detector and mission.

ultraviolet light

The portion of the electromagnetic spectrum from about 5 - 400 nanometers; this is the form of light responsible for a suntan.

Ultraviolet/Optical Telescope (UVOT)

The ultraviolet and optical telescope aboard Swift, designed to observe GRBs and their afterglow.

waveband

A region of the electromagnetic spectrum defined by a range of energies (or, equivalently, frequencies or wavelengths).

wavelength

The distance between two successive peaks or troughs of a wave; for light, this uniquely specifies the energy and frequency of the light.

X-ray

The region of the electromagnetic spectrum from about 0.001 - 1 nanometers (1 keV - 1 MeV).

X-Ray Telescope (XRT)

The X-ray telescope aboard Swift, designed to observe GRBs and their afterglow.

This page was last modified on Friday, 28-Mar-2008 09:12:51 PDT.