All About Space

’Partial supernova’ blasts white dwarf star across the Milky Way

- Words by Charles Q. Choi

Astrange white dwarf star hurtling through the Milky Way may be the survivor of a ‘partial supernova’, a new study finds.

White dwarfs are the cool, dim Earth-sized cores of dead stars that are left behind after average-sized stars have exhausted their fuel and shed their outer layers. Our Sun will one day become a white dwarf, as will more than 90 per cent of the stars in the Milky Way.

Scientists zeroed in on the white dwarf SDSS J1240+6710, located about 1,430 light years from Earth. Discovered in 2015, prior work found this white dwarf had an unusual atmosphere that seemed to possess neither hydrogen or helium, but instead was composed of a weird mix of oxygen, neon, magnesium and silicon. The Hubble Space Telescope was used to take a closer look at the white dwarf, and it identified carbon, sodium and aluminium in the object’s atmosphere. This mix sets this white dwarf apart from any other previously known.

In the research the scientists also found that the white dwarf was travelling at about 900,000 kilometres per hour (560,000 miles per hour) in the opposite direction of the way the galaxy is rotating. Moreover it had an especially low mass for a white dwarf – only about 40 per cent the mass of our Sun. “When we found this unusual white dwarf was really low in mass and moving really fast, that really triggered my curiosity into what happened to it in its past,” said Boris Gänsicke, an astrophysi­cist at the University of Warwick, UK.

What might explain all these strange details about this white dwarf? The researcher­s think that a thermonucl­ear explosion didn’t completely destroy the white dwarf, but rather a ‘partial supernova’ blasted what remained of the object across the Milky Way.

In the case of SDSS J1240+6710, the scientists noted that the elements seen in the white dwarf’s atmosphere could have all been produced in the first thermonucl­ear reactions of a supernova. However, there is a clear absence of what is known as the iron group of elements – iron, nickel, chromium and manganese.

“That’s what makes this white dwarf unique

– it did undergo nuclear burning, but stopped before it got to iron,” said Gänsicke. “When it had its supernova event, it was likely just brief, maybe a couple of hours.”

The explosion would have blasted SDSS J1240+6710 away from its companion, ripping matter off the small white dwarf and hurling it through deep space at the speed at which it orbited its partner.

The scenario — the researcher­s said — would help to explain the white dwarf’s speed, puny size and bizarre atmosphere.

 ??  ?? The strange white dwarf doesn’t contain any iron like most postsupern­ova stars do
The strange white dwarf doesn’t contain any iron like most postsupern­ova stars do

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