Astronomer Ylva Getberg tracks hydrogen-poor supernova source stars. He made an amazing discovery. Supernova explosions are usually so powerful that they shine as brightly as an entire galaxy. These stars are usually rich in hydrogen. But there are also supernovae that contain very little hydrogen, which baffles astronomers.

There are many more low-hydrogen supernovae than can be explained by current models. So far, researchers have not been able to identify the protostar. It was as if these supernovae appeared out of nowhere. Astronomer Ylva Getberg, together with Maria Draut of the University of Toronto, solved this cosmological puzzle. Low-hydrogen supernovae occur when a star explodes and loses its hydrogen-rich shell. This occurs naturally in one-third of all massive stars when the outer skin is stripped from the binary. The stars Drout and Getberg were looking for turned out to be a couple. The formation of such double stars is not new in astronomy. The brightest star, Sirius A, and its fainter companion, Sirius B, belong to such a binary system. In a binary system, the stars orbit each other until the thick, hydrogen-rich shell of the more massive star expands. The companion star can attract the hydrogen-rich shell of its older brother and suck it out, so to speak. What remains is a compact helium core that is about two to eight times more massive than the Sun and 10 times hotter than the Sun’s surface. A star like this has never been observed before.

But the so-called star of Wolf Rayet was found. It also consists of only helium nuclei, but it has between 10 and 265 solar masses. Therefore, it is too massive to be considered a low-hydrogen supernova. Some do not have enough mass to meet the criteria. Before Getberg and Draut’s work, only one star had been discovered that met the expected criteria for mass and composition. However, because stars that evolve in this way have very long lifetimes, many of them must be scattered throughout the observable universe. So did researchers simply not notice this star? Using UV photometry and optical spectroscopy, they identified a population of 25 stars, consistent with expectations for intermediate-mass helium stars. These stars are located in two well-studied neighboring galaxies: the Large Magellanic Cloud and the Small Magellanic Cloud. The emission spectra of these stars suggest that their outer layers have been removed. It also has strong helium properties. Both indicate that these stars may be precursors to hydrogen-poor supernovae. The study identified a missing population of intermediate-mass helium stars that are predicted to be hydrogen-poor supernova precursors. These stars will always be there, and there will probably be many more. The study was published in the journal Science.