Astronomers say they've learned some new things about a star that behaves like no other in our home Milky Way galaxy, a star so weird they've given it the nickname Nasty 1.
That's a bit of word play on the catalog designation NaSt1 given to the star, one of a class known as Wolf-Rayet stars, rapidly evolving ones many times as massive as our sun.
Such stars quickly lose their outer layers of hydrogen, exposing their extremely bright and super-hot helium-burning cores.
Although NaSt1 has been known for decades, it has been a continuing subject of study because it doesn't conform to the normal configuration of a Wolf-Rayet star, say astronomers using the Hubble Space Telescope to study it.
They had been expecting to see twin lobes of gas streaming away on opposite sides of the star - typical Wolf-Rayet behavior - but instead have detected a pancake-shape nebula of gas almost 2 trillion miles wide surrounding it, they report in the Monthly Notices of the Royal Astronomical Society.
The disk may be the result of a companion star "snacking" on the outermost layer of NaSt1 during what may be a brief transitory stage in its evolution, the researchers suggest.
"We were excited to see this disk-like structure because it may be evidence for a Wolf-Rayet star forming from a binary interaction," says study leader Jon Mauerhan from the University of California, Berkeley. "There are very few examples in the galaxy of this process in action because this phase is short-lived, perhaps lasting only a hundred thousand years, while the timescale over which a resulting disk is visible could be only ten thousand years or less."
Scientists have debated just how a Wolf-Rayet star forms; one scenario holds that as massive stars consume their hydrogen they swell up, causing their outer hydrogen layers to be less strongly bound to the star and making them susceptible to gravitational stripping - a kind of stellar cannibalism - by a close companion star.
Another proposal suggests they form as a massive star throws off its own hydrogen envelope as part of a strong stellar wind consisting of streaming charged particles.
However, binary interaction models involving a companion star are gaining favor because observation has shown 70 percent of massive stars that could be candidates to become Wolf-Rayet stars are members of double-star systems, astronomers say.
"Mass exchange in binary systems seems to be vital to account for Wolf-Rayet stars and the supernovae they make, and catching binary stars in this short-lived phase will help us understand this process," says Nathan Smith from the University of Arizona in Tucson.
That exchange of mass between the stars isn't completely efficient, astronomers suggest, so a portion of the stripped gas can spill out into surrounding space in the ongoing gravitational tug-of-war between the two stars, coming together in a disk that surrounds the binary system.
"That's what we think is happening in Nasty 1," Mauerhan says. "We think there is a Wolf-Rayet star buried inside the nebula, and we think the nebula is being created by this mass-transfer process. So this type of sloppy stellar cannibalism actually makes Nasty 1 a rather fitting nickname."
