Gamma-ray bursts come from neutron stars within the Milky Way.

Gamma-ray bursts are sudden flashes of extremely energetic radiation. They can last from a fraction of a second to several minutes and then disappear.

When satellites discovered them in the late 1960s, astronomers had almost no information about how far away they were. Neutron stars in the Milky Way offered plausible sources, so Galactic models dominated much of the early debate.

NASA’s Compton Gamma Ray Observatory changed the picture. Its BATSE instrument detected bursts all across the sky. If most came from ordinary objects in the Milky Way’s flat disk, they should have appeared concentrated along the bright band of the Galaxy. Instead, the distribution was almost uniform in every direction.

That was a serious problem for simple Galactic models, but it did not yet prove that the bursts were extremely distant. A huge spherical halo around the Milky Way could also look nearly uniform from Earth.

The decisive evidence came in 1997. Satellites began locating bursts quickly enough for telescopes to observe their fading afterglows in visible light and other wavelengths. Astronomers could then measure redshift, the stretching of light toward longer wavelengths as the universe expands, and identify host galaxies. The bursts were billions of light-years away.

That distance meant their energy output was enormous. Later observations connected long bursts with the deaths of massive stars and many short bursts with mergers involving neutron stars.

Some gamma-ray transients do occur inside the Milky Way, but the classical gamma-ray bursts at the center of the old distance debate are mostly cosmological.