Merging White Dwarf Stars March to Supernova Doom

Astronomers have discovered a stellar rarity deep inside an oddly-shaped nebula — two white dwarf stars on the verge of a cataclysmic merging event.

During a survey to try to understand the peculiar shapes of nebulae in our galaxy, astronomers using the European Southern Observatory’s Very Large Telescope (VLT) in Chile zoomed in on the planetary nebula Henize 2-428, which is lop-sided. Planetary nebulae are formed during the later stages of a star’s life after it has run out of hydrogen in its core. This violent phase sheds the star’s outermost layers through powerful stellar winds.

But some distant planetary nebula have strange asymmetries — i.e. the cloud of dust and plasma expand into unexpected, non-uniform shapes. This is a puzzle to astronomers; if the nebula was created by a single dying star, surely all the material should be ballooning out as the same speed in all directions?

Now part of Henize 2-428′s asymmetry mystery has been solved. Embedded deep inside the cloud there’s not a single star, but two stars — a fact that has led to an explosive realization.

“Further observations made with telescopes in the Canary Islands allowed us to determine the orbit of the two stars and deduce both the masses of the two stars and their separation. This was when the biggest surprise was revealed,” said Romano Corradi, a researcher at the Instituto de Astrofísica de Canarias (Tenerife, IAC), and co-author of a paper published in the journal Nature on Monday (Feb. 9).

Both stars are white dwarfs, tiny yet dense stars each with a mass slightly less than the sun. By the researchers’ calculations, the stars orbit every 4 hours and they are slowly spiraling into one another. In around 700 million years, the pair will merge.

Not only have astronomers now explained why this particular nebula has a weird shape, they’ve also uncovered a gravitational wave factory — energy is gradually being lost as both stars rip around one another, causing their orbital distance to decrease.

But what will happen when both stars collide and merge? Their combined masses will be 1.8 times the mass of the sun, well above the mass threshold (known as the Chandrasekhar limit) that will cause an object to collapse in on itself. In other words, supernova!

Read more at Discovery News