White dwarfs are the remaining cores of red giants after these huge stars have died and shed their outer layers, cooling over the course of billions of years.
Now one of them is crossing our galaxy at high speed due to an explosion. Specifically, the so-called SDSS J1240 + 6710 was a binary star that survived its supernova explosion, which sent it and its companion through the Milky Way in opposite directions.
SDSS J1240 + 6710
Scientists theorize that the supernova disrupted the white dwarf’s orbit with its companion star when it abruptly ejected a large proportion of its mass.
The scientists were able to measure the speed of the white dwarf and found that it travels at 900,000 kilometers per hour. It also has a particularly low mass for a white dwarf, only 40% the mass of our Sun, which would be consistent with the loss of mass from a partial supernova.
SDSS J1240 + 6710 was discovered in 2015, and it appeared to contain neither hydrogen nor helium – it was made up of an unusual mix of oxygen, neon, magnesium, and silicon . As Boris Gaensicke from the Department of Physics at the University of Warwick explains:
This star is unique in that it has all the key characteristics of a white dwarf, but it has very high speed and unusual abundances that make no sense when combined with its low mass. It has a chemical composition that is the trace of nuclear combustion, a low mass and a very high velocity: all these facts imply that it must have come from some kind of close binary system and must have undergone a thermonuclear ignition. It would have been a type of supernova, but of a type that we have not seen before.
The research has been published in Monthly Notices of the Royal Astronomical Society and has been funded by the Leverhulme Trust and Science and Technology Facilities Council (STFC). As Gaensicke adds:
The study of thermonuclear supernovae is a huge field and it takes a great deal of observational effort to find supernovae in other galaxies. The difficulty is that you see the star when it explodes, but it is very difficult to know what the properties of the star were before it exploded. We are now discovering that there are different types of white dwarfs that survive supernovae under different conditions and, using the compositions, masses, and velocities they have, we can determine what type of supernova they have undergone. There is clearly a whole zoo out there. Studying the survivors of supernovae in our Milky Way will also help us understand the supernovae we see in other galaxies.