A gamma-ray burst is a highly luminous flash associated with an explosion in a distant galaxy and producing gamma rays, the most energetic form of electromagnetic radiation, and often followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio).

Astronomer Mark Metzger determined that GRB 970508 occurred at least 6 billion light years from Earth; this was the first measurement of the distance to a gamma-ray burst.

Although the possibility of multiple types of GRBs meant that the two theories were not mutually exclusive, the distance measurement unequivocally placed the source of the GRB outside the Milky Way, effectively ending the debate.

[2] The initial burst is often followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio).

[5][6] It was also detected by Ulysses, a robotic space probe designed to study the Sun,[7] and by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory.

[6] After a rough position of the burst had been determined, Enrico Costa of the BeppoSAX team contacted astronomer Dale Frail at the National Radio Astronomy Observatory's Very Large Array.

Mark Metzger, a colleague of Djorgovski at the Caltech observatory, conducted a more extensive analysis of the data, but was also unable to identify any new light sources.

[10] After discovering the burst's X-ray afterglow, the BeppoSAX team provided a more accurate localization, and what Metzger had assumed to be a variable star was still present in this smaller error box.

[12] He then sent the data to Metzger, who after identifying a system of absorption lines associated with magnesium and iron determined a redshift of z = 0.8349 ± 0.0002,[13][14][15] indicating that light from the burst had been absorbed by matter roughly 6 billion light-years from Earth.

The fluctuations did not occur simultaneously along all of the observed wavelengths, which Jeremy Goodman of Princeton University explained as being the result of the radio waves being bent by interstellar plasma in the Milky Way.

[27] This implied that the gamma-ray and kinetic energy of the burst's ejecta were comparable, effectively ruling out those GRB models which are relatively inefficient at producing gamma rays.

The distance measurement and the calculations of the burst's total energy release unequivocally supported the latter theory, effectively ending the debate.

[32] The afterglow eventually disappeared, revealing the burst's host, an actively star-forming dwarf galaxy with an apparent magnitude of V = 25.4 ± 0.15.