Astronomers using NASA’s Hubble Space Telescope have discovered a companion star to a rare type of supernova, which had been hidden for 21 years behind the supernova’s glare, according to the Space Telescope Science Institute. The new observation confirms the theory that the explosion originated in a double-star system where one star fueled the mass-loss from an aging, primary star.

Astronomers were able to estimate the surviving star’s luminosity and mass, which provide insight into the conditions that preceded the explosion.

Below is a an artist’s rendering of what it looks like:

“A binary system is likely required to lose the majority of the primary star’s hydrogen envelope prior to the explosion. The problem is that, to date, direct observations of the predicted binary companion star have been difficult to obtain since it is so faint relative to the supernova itself,” said lead researcher Ori Fox of the University of California (UC) at Berkeley about the new discovery.

Astronomers estimate that a supernova goes off once every second somewhere in the universe.

Yet scientists still don’t fully understand how stars explode. Finding a companion star provides an important “smoking gun” , with new clues to the variety of supernovae in the universe. “This is like a crime scene, and we finally identified the robber,” said team member Alex Filippenko, professor of astronomy at UC Berkeley. “The companion star stole a bunch of hydrogen before the primary star exploded.”