For the past decade, scientists have been a bit perplexed at a small subset of the supernovas they've witnessed. Some of these stellar explosions, while massive, only measure up to about one percent the luminosity* of traditional supernovas. These strange, "dim" supernovas have recently been found not to simply be weak versions of Type 1a, or traditional, supernovas. Instead, they seem to be a whole new kind of stellar explosion.
The new "runt" supernovas are classified as "Type 1ax." The similarity in classification to Type 1a supernovas is by design, as the two phenomena are somewhat related. Both types originate from binary star systems containing a white dwarf star. Type 1ax supernovas have an unique quirk in their making, however. It appears that, in these sorts of explosions, the white dwarf's companion star has somehow previously lost its outer hydrogen, leaving a layer of helium in its place.
The white dwarf star gradually steals gasses from its companion star, gathering up that outer layer of helium. It's unclear what happens next, but the result is an explosion which emanates from the white dwarf, but one that's not quite as catastrophic as what might have happened if the white dwarf star had sucked up a layer of hydrogen. Think popped balloon instead of Hindenburg disaster. While still a massive explosive event, Type 1ax supernovas are much less devastating than their bigger brethren, and the white dwarf stars that trigger these runt supernovas even have a chance at surviving the explosion. "The star will be battered and bruised, but it might live to see another day," says lead researcher Ryan Foley from the Harvard-Smithsonian Center for Astrophysics in an interview with Space.com.
Now that they're aware of them, astronomers are taking a good hard look at Type 1ax supernovas. They want to know how often stars can survive such explosions and in what manner. "We'd want to know things like how often a star loses half its stellar mass, or a tenth. Right now, we don't have the statistics to answer some of these questions," says Foley. The answers could even be lurking in our own interstellar back yard, and since astronomers now know what they're looking for, we're likely to get a lot more data on this fascinating phenomenon in the future.
*You can think of "luminosity" to mean "brightness," except that brightness refers to the amount of light that you see, while luminosity refers to the amount of light that something puts out. So, with something like a light bulb, it has a given luminosity that is always the same, but the brightness changes depending on how close you are to it. And if you know how luminous the bulb is, you can calculate your distance to it based on how bright it appears. Astronomers use this trick to estimate the distance to supernovas, stars, and other stellar phenomena.