A Star Named 'Nasty'

The star known as "Nasty" is an enduring mystery. A resident of our own Milky Way, the aging star is much more massive than our own Sun and operates in a peculiar way: Having quickly burned through its outer hydrogen layers, it's revealed a super-hot, super-bright helium-burning core. It's much, much brighter and hotter than our Sun.

Nasty is categorized as a Wolf-Rayet star, but it features a very significant deviation from other stars of its type. While fitting the category's characteristic spectra—which reveal large amounts of burning helium and relatively tiny amounts of hydrogen—Nasty isn't blasting jets of gas out into space like most of its peers. These jets, which reach velocities of up to 2,400 kilometers per second, are replaced in this case by an enormous gaseous disc nearly two trillion miles across that may be only a few thousand years old. Barely a blink in astro-time.

This disc is interesting because it suggests an alternate origin story for Wolf-Rayets. In the more commonly-accepted explanation, a Wolf-Rayet forms as the massive star begins to swell up near the end of its life, suddenly ejecting its hydrogen cloak. The new model, which is being advanced by a group of astronomers using the Hubble Space Telescope, suggests some interaction with another star, as might occur in a binary star system.

"We were excited to see this disk-like structure because it may be evidence for a Wolf-Rayet star forming from a binary interaction," offers Jon Mauerhan, an astronomer at the University of California, Berkeley and lead author of a new study describing the model, at HubbleSite. "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."

The team's scenario imagines a kind of stellar cannibalism, where, as one massive star begins to expand and shed its hydrogen layer, another star steps in and strips it away gravitationally. The disc itself is the result of cosmic sloppiness, where the feeding star manages to spill or lose some of its gaseous meal.

"What evolutionary path the star will take is uncertain, but it will definitely not be boring," Mauerhan says. "Nasty 1 could evolve into another Eta Carinae-type system. To make that transformation, the mass-gaining companion star could experience a giant eruption because of some instability related to the acquiring of matter from the newly formed Wolf-Rayet. Or, the Wolf-Rayet could explode as a supernova."

"A stellar merger is another potential outcome, depending on the orbital evolution of the system," the astronomer continues. "The future could be full of all kinds of exotic possibilities depending on whether it blows up or how long the mass transfer occurs, and how long it lives after the mass transfer ceases."

Oh, as for the name? It's derived from the star's catalog name, NaSt1.