Distant Star Explosion Chokes On Its Own Dust
While searching the skies for black holes using the Spitzer Space Telescope Deep Wide Field Survey, Ohio State University astronomers discovered a giant supernova that was smothered in its own dust. In this artist's rendering, an outer shell of gas and dust — which erupted from the star hundreds of years ago — obscures the supernova within. This event in a distant galaxy hints at one possible future for the brightest star system in our own Milky Way.
Credit: NASA/JPL-Caltech/R. Hurt

A giant star in a faraway galaxy has ended its life with a dust-shrouded whimper instead of the more typical bang, scientists say.

The star did explode in a supernova, but two huge dust clouds that cast off before the star died ultimately smothered the cosmic conflagration, researchers said. [Illustration of the dust-shrouded supernova.]

Astronomers have never-before observed such an event, but think they were likely more common in the universe's youth. And a similar dust squelching may await the brightest star system in our galaxy if it explodes, scientists said.

A supernova is the explosive end to a huge star around eight or more times the mass of our sun. One occurs when a massive star has exhausted its supply of fuel and can no longer sustain itself. Stars greater than 20 times the mass of the sun can collapse in on themselves to form stellar black holes.

Accidental discovery

It was black holes, of a more massive variety, that researchers were searching for when they discovered the fizzled supernova while poring over data from NASA's Spitzer Space Telescope.

The astronomers were searching for supermassive black holes at the centers of galaxies by identifying cosmic hot spots ? the heat signature radiated by the disks of material around such black hole giants.

One very hot spot, which appeared in August 2007, didn't match the signature of a black hole. It also burned hot, and then faded away quickly, disappearing entirely in March 2008.

"Over six months, it released more energy than our sun could produce in its entire lifetime," explained study team member Szymon Kozlowski, now of Warsaw University Observatory in Poland.

The fast flare-out, which occurred in a galaxy about 3 billion light-years from Earth, suggested the team had found a supernova. But these massive explosions typically release most of their energy as light, not heat.

The temperature of the mystery object was around 1,292 degrees Fahrenheit (700 degrees Celsius). The research team wondered what could absorb huge amounts of light energy and dissipate it as heat.

Their answer: dust, and a lot of it.

Two dusty shrouds

The scientists, led by astronomer researcher Christopher Kochanek of Ohio State University, worked backward to determine what kind of star could have spawned the supernova, and how the dust was able to muffle the explosion.

They found that the star was most likely a giant, at least 50 times more massive than our sun. Such massive stars typically belch clouds of material as they near the end of their existence.

At least two such ejection events were required to create the dust required for the events Kochanek and his team spotted. The first occurred about 300 years before the supernova exploded, and the other came just four years before the explosion. The dust and gas from both ejections then remained around the star in the form of two slowly expanding shells.

The younger inner shell ? created from the second dust ejection event ? would be very close to the star, while the outer shell would be much farther away.

"We think the outer shell must be nearly opaque, so it absorbed any light energy that made it through the inner shell and converted it to heat," Kochanek said.

Stars probably choked on their own dust much more often in the distant past, according to the researchers.

"These events are much more likely to happen in a small, low metallicity galaxy," said study team member Krzysztof Stanek, also of Ohio State. "Low metallicity" refers to a young galaxy that hasn?t been around long enough for its stars to fuse hydrogen and helium into heavier, more complex elements astronomers call metals.

The research is detailed in a recent online issue of the Astrophysical Journal.

Finding more dust-muffled supernovas

Astronomers will likely find more such supernovae in the near future, the researchers said, especially with the help of instruments such as the Wide-Field Infrared Survey Explorer, an infrared space telescope launched by NASA in December 2009.

"I would expect WISE to see 100 of these events in two years, now that we know what to look for," Kozlowski said.

Because of the unfavorable alignment of the star's host galaxy with Earth and our sun, astronomers were not able to see what the dust-shrouded supernova event looked like to the naked eye as it was happening.

But the researchers said the star may appear to brighten a decade or so from now, when the shockwave from its explosion reaches the inner dust shell and sends it crashing into the outer shell.

A similar light show could also occur closer to home, according to the scientists. Eta Carinae, the brightest star system in our Milky Way, would probably be dust-smothered if it exploded, they said.

?If Eta Carinae went supernova right now, this is what it would probably look like,? Kochanek said.

The two stars known to make up Eta Carinae are 7,500 light-years away, and they host a distinctive dust shell called the Homunculus Nebula, among other layers of dust. Astronomers believe that the nebula was created when the larger of the two stars underwent a massive eruption around 1840, and that future eruptions are likely.