It took 21 years but scientists have finally found the companion star for SN 1993J, a rare kind of supernova. This confirms the theory that the supernova is part of a double-star system where one star is responsible for the explosion of the other.

In SN 1993J's case, its companion star was so hot that its so-called continuum emission, the glow of a star, was mostly in ultraviolet light. UV light can't readily be observed from Earth because of the planet's atmosphere so the companion star remained undiscovered until now. With the help of the Hubble Space Telescope, scientists were able to spot the companion star's continuum glow even after light from other stars was removed.

According to estimates, one supernova explodes somewhere in the universe every second. However, astronomers don't completely understand how stars go off. What they do know is that in double-star or binary systems, a primary star must have a companion star to help it lose its hydrogen envelope. University of California at Berkeley professor of astronomy Alex Filippenko likens the search for a companion star to a crime scene, saying finding SN 1993J's partner was like identifying the robber.

SN 1993J is part of galaxy M81, which is around 11 million light-years from Earth towards the direction of the Ursa Major constellation. It was in 1993 when light from the supernova was first discovered. Based on the characteristics of its explosion, SN 1993J was classified as a rare kind of supernova known as Type llb. Since then, scientists have been looking for its companion star but glare from SN 1993J's residual glow kept it out of sight.

Observations from the W.M. Keck Observatory in Mauna Kea, Hawaii in 2004 yielded circumstantial evidence about special absorption features that could have possibly come from the suspected companion. But again, researchers couldn't be certain because the field of view was so crowded.

To find the companion star, images from two Hubble instruments used for collecting UV light and ground-based data for optical light were combined. A multi-wavelength spectrum was then constructed and matched to the predicted glow from the companion star.

By finding the companion star to SN 1993J, this study also affirmed the classification the supernova was given.

Lead researcher for the study was Ori Fox from UC Berkeley, working alongside Filippenko and Azalee Bostroem from the Space Telescope Science Institute. Results from the study were published in the Astrophysical Journal.