An image of supernova explosion AT2018cow and its host galaxy, CGCG 137-068.

A distant galaxy (centre) includes a mysterious object (one of blue dots in lower right quadrant) that burst into view in 2018. Credit: R. Margutti/W. M. Keck Observatory

Astronomy and astrophysics

The explosive birth of a celestial ‘Cow’

Evidence points to a supernova as the source of a dazzling object that telescopes spotted in 2018.

A mysterious and tremendously bright astronomical object spotted last year was probably a massive star’s explosion.

The immense flash, designated AT2018cow and nicknamed ‘the Cow’, appeared in the sky in 2018 and then faded from view over several months. The Cow belongs to a class of objects that appear and dim much faster than most supernovae, the brilliant explosions of massive stars at the ends of their lives.

The Cow’s origins are unclear: previous observations of the region around it did not find a type of gas that is a marker of a massive star. Some astronomers think the Cow’s unusually quick initial burst of light was caused not by a supernova, but by a white dwarf star being torn apart in a collision with another celestial body.

A team led by Kana Morokuma-Matsui at the University of Tokyo analysed molecular gas — the stuff from which stars are born — in the Cow’s host galaxy. The researchers found that the galaxy is currently forming new stars, including massive stars that can become supernovae. This lends credence to the supernova scenario.