Diamond Jubilee Debate Glossary

BATSE: Acronym for Burst and Transient Source Experiment on board the Compton Gamma-Ray Observatory. The BATSE experiment consists of 8 detectors operating together as an all sky monitor. It has accumulated more GRB data that all previous GRB experiments combined. Interpretation of this data, available starting in 1992, has fueled several different schools of thought on GRBs.

Cepheid: A pulsating variable star. This type of star undergoes a rhythmic pulsation as indicated by its regular pattern of changing brightness as a function of time. The period of pulsation has been demonstrated to be directly related to a Cepheid's intrinsic brightness making observations of these stars one of the most powerful tools for determining distance known to modern day astronomy. The existence of of this period-luminosity relationship was a point of contention during the 1920 Curtis-Shapley debate.

COMPTEL: Abbreviation for the imaging COMPton TELescope on board the Compton Gamma-Ray Observatory. COMPTEL has detected several GRBs. COMPTEL's positions for these GRBs are more accurate than BATSE's.

Compton: Short for the Compton Gamma Ray Observatory, the second of NASA's Great Observatories. The satellite was named for Nobel Laureate Arthur Holly Compton, a physicist famous for his work on understanding how high energy photons scatter in collisions with electrons. The Compton Observatory houses four gamma-ray instruments: BATSE, COMPTEL, EGRET, and OSSE.

cosmological distance: A distance far beyond the boundaries of our Galaxy. When viewing objects at cosmological distances, the curved nature of spacetime could become apparent. Possible cosmological effects include time dilation and red shift.

cyclotron lines: Absorption dips that have been reported in several GRB spectra. The abundance and even existence of cyclotron lines in GRB spectra is controversial.

EGRET: Acronym for Energetic Gamma Ray Experiment Telescope on board the Compton Gamma Ray Observatory. EGRET has detected photons as energetic as 9 GeV as late as 90 minutes after the onset of a GRB event.

galactic halo: A spherical region surrounding the center of a galaxy. This region may extend beyond the luminous boundaries of the galaxy and contain a significant fraction of the galaxy's mass. Compared to cosmological distances, objects in the halo of our galaxy would be very near by.

GRB: Gamma-Ray Burst. Plural is GRBs. A burst of gamma-rays from space lasting from a fraction of a second to many minutes. There is no clear scientific consensus as to their cause or even their distance. The later is the main topic of the `Diamond Jubilee' debate in 1995.

isotropy: Formal definition: identical in all directions (American Heritage Dictionary). The degree of isotropy of the distribution of GRB events on the sky is controversial.

keV, GeV, MeV: Respectively, acronyms for kilo-electron Volt (10^3 eV), Mega-electron Volt (10^6 eV), and Giga-electron Volt (10^9 eV). These are common units of energy used here to refer to the energy of gamma-ray photons. GRBs are known to be (temporarily) brighter than anything else in the entire gamma-ray sky at energies above about 10 keV.

neutron star: The imploded core of a massive star produced by a supernova explosion. (typical mass of 1.4 times the mass of the sun, radius of about 5 miles, density of a neutron.) According to astronomer and author Frank Shu, "A sugarcube of neutron-star stuff on Earth would weigh as much as all of humanity! This illustrates again how much of humanity is empty space." Neutron stars can be observed as pulsars.

OSSE: Acronym for Oriented Scintillation Spectroscopy Experiment on board the Compton Gamma Ray Observatory. OSSE has detected many GRBs with its shields at energies above BATSE's primary sensitivity.

parsec: 3.3 light years, or 3.1x10^18 cm. kiloparsec: 1000 parsecs.

pulsar: A rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, x-ray, and gamma-ray energies.

PVO: Pioneer Venus Orbiter. The Pioneer Venus Orbiter entered orbit around Venus on December 4, 1978 carrying 17 individual experiments, including a gamma-ray burst detector. The detector operated until August of 1993 when the orbiter's fuel ran out and entry into the Venusian atmosphere destroyed the spacecraft. The 15 year PVO data base currently contains the largest number of observations of bright GRBs.

red shift: The stretching of waves of electromagnetic radiation. The shift of a spectrum to longer wavelengths. A red shifted spectrum is a predicted effect of the cosmological paradigm. The existence and degree of a red shift in GRB spectra is controversial.

repetition: The recurrence of a GRBs from the same source. The existence and degree of repetition of GRBs is controversial.

SGR: Soft Gamma Repeater. Plural is SGRs. These are gamma-ray burst events which are thought to be distinct from classical GRBs by virtue of their characteristic soft spectra, short duration, and known repetitions of outbursts. Three SGR sources have been observed.

supernova: The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, supernova explosions can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant . Because of their similar appearance, the distinction between supernovae and the unrelated and intrinsically much fainter novae events were not appreciated at the time of the Curtis-Shapley debate.

time-dilation: Stretching of time produced by relativity. Time dilation is a predicted effect of the cosmological paradigm. The existence and degree of time-dilation effects in GRB data is controversial.

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