By Amina Khan
Los Angeles Times
At the heart of a strangely shaped planetary nebula, astronomers have discovered a pair of doomed white dwarf stars that will inevitably merge and explode in a dramatic supernova.
The imperiled pair, described in the journal Nature, sheds light on how different stellar systems can meet the same violent fate. The team led by researchers in Spain didn’t set out to look for these stars; they’d wanted to study why so many planetary nebulae end up with such odd shapes.
These beautiful, multicolored structures in the sky, such as the Cat’s Eye Nebula and the Helix Nebula, are formed when an aging star that has puffed up to red-giant size begins to blow off those outer layers, and ultraviolet radiation helps light up those shells of cast-off material. (Planetary nebula have nothing to do with planets; it’s a misnomer from the late 18th century that stuck.)
But why does all the stuff shed by a star end up with such an uneven form?
“The planetary nebula stage is the ultimate fate of stars with masses one to eight times that of the sun,” the study authors wrote. “The origin of their complex morphologies is poorly understood.”
While using the European Southern Observatory’s Very Large Telescope to examine one of these weirdly shaped nebulae, Henize 2-428, they found not a single star, but a pair of stars spinning around each other. They were roughly the same size and together have the mass of nearly 1.8 suns.
That lends support to the idea that the strange shapes of planetary nebulae are often caused by a pair of stars, not just one. But the scientists also observed the nebula using telescopes in the Canary Islands to look at the stars’ masses and their distance from each other, and they found something stranger. These stars were uncomfortably close to each other: Their orbital period is a mere 4.2 hours.
This is so close, in fact, that the stars’ gravity will pull them together, and within 700 million years, they will merge into one star. At that point, the combined star will be too big to withstand its own mass (according to the Chandrasekhar limit, a star that has reached the white dwarf stage can only support its own mass up to 1.4 solar masses) and the stellar twofer will go supernova — a far more violent event than the one that caused the planetary nebula.
This is fascinating because the type of supernova this particular star system would become — a Type 1a supernova — is thought to shine with such uniformity that these explosions were used as the “standard candles” that allowed scientists to discover the existence of dark energy, that strange repulsive force that is causing the universe to expand. It could be that while the brightness of these so-called “standard candles” is uniform, their origins are not.
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