Finding the Source of Low-Hydrogen Supernovae

by | December 20, 2023, 12:05 PM | Stars & Nebulae

Visualization of a binary star experiencing mass transfer. Credit: Ylva Götberg

One factor upping the difficulty of understanding our universe is… the complexity of our universe. It sometimes feels like every time we think we’re getting a handle on how things work, some new dataset comes along and says, “But can you explain this?” as it reveals new objects or old objects in new numbers. 

For instance, we thought we had a pretty good handle on stellar evolution and what kinds of star deaths we should be able to see. But then the universe said, “Here are some supernovae that don’t have enough hydrogen, explain this!” 

A typical star starts and ends its life with an outer atmosphere made primarily of hydrogen. Stars with eight solar masses or less will generally puff off this atmosphere and form a planetary nebula as the core settles into a white dwarf. Larger stars will explode, sending their hydrogen-rich atmosphere outwards with fury as the core collapses into a neutron star, black hole, or energy-filled nothing. To get supernovae without hydrogen… well, that requires some gravitational help from a friend.

In binary star systems, a star can expand out until its outer atmosphere gets close enough to its neighbor that the neighbor can gravitationally strip off the hydrogen. This will create one star with no hydrogen atmosphere and one with too much of an atmosphere; both stars will now experience an evolution unlike anything our single-star models could predict. 

And this binary star solution seemed to work… in computers. In the actual universe, researchers have struggled to find systems with the stars needed to explain all the observed hydrogen-poor supernovae. 

In a new paper in the journal Science that is led by Maria Drout, researchers observed 25 “stars” in the Large and Small Magellanic Clouds — a pair of nearby galaxies easily seen from the southern hemisphere. They found that eight of these 24 “stars” were binary systems in which one of the two stars is a hydrogen-poor core left behind when the other star stole its atmosphere. These stars – hiding in the light of their parasitic companion – should exist in the needed numbers to explain all those hydrogen-poor supernovae. Once again, the universe has said, “Explain this,” and science has had, with a bit of searching, a good-looking answer.

Go science.