Giant star goes supernova -- and is smothered by its own dust

Oct 12, 2010
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: Image courtesy of NASA/JPL-Caltech/R. Hurt.

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

Ohio State University researchers suspect that this odd event -- the first one of its kind ever viewed by astronomers – was more common early in the universe.

It also hints at what we would see if the brightest star system in our galaxy became a supernova.

In a paper published online in the Astrophysical Journal, Christopher Kochanek, a professor of astronomy at Ohio State, and his colleagues describe how the supernova appeared in late August 2007, as part of the Spitzer Space Telescope Deep Wide Field Survey.

The astronomers were searching the survey data for active galactic nuclei (AGN), super-massive black holes at the centers of . AGN radiate enormous amounts of heat as material is sucked into the black hole. In particular, the astronomers were searching for hot spots that varied in temperature, since these could provide evidence of changes in how the material was falling into the black hole.

Normally, astronomers wouldn't expect to find a supernova this way, explained then-Ohio State postdoctoral researcher Szymon Kozlowski. Supernovae release most of their energy as light, not heat.

But one very hot spot, which appeared in a galaxy some 3 trillion light years from Earth, didn't match the typical heat signal of an AGN. The visible spectrum of light emanating from the galaxy didn't show the presence of an AGN, either – the researchers confirmed that fact using the 10-meter Keck Telescope in Hawaii.

Enormous heat flared from the object for a little over six months, then faded away in early March 2008 – another clue that the object was a supernova.

"Over six months, it released more energy that our sun could produce in its entire lifetime," Kozlowski said.

The astronomers knew that if the source were a supernova, the extreme amount of energy it emitted would qualify it as a big one, or a "hypernova." The temperature of the object was around 1,000 Kelvin (about 700 degrees Celsius) -- only a little hotter than the surface of the planet Venus. They wondered -- what could absorb that much light energy and dissipate it as heat?

The answer: dust, and a lot of it.

Using what they learned from the Spitzer survey, the astronomers worked backward to determine what kind of star could have spawned the supernova, and how the dust was able to partly muffle the explosion. They calculated that the star was probably a giant, at least 50 times more massive than our sun. Such massive stars typically belch clouds of dust as they near the end of their existence.

This particular star must have had at least two such ejections, they determined – one about 300 years before the supernova, and one only about four years before it. The dust and gas from both ejections remained around the star, each in a slowly expanding shell. The inner shell – the one from four years ago – would be very close to the star, while the outer shell from 300 years ago 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," said Kochanek, who is also the Ohio Eminent Scholar in Observational Cosmology.

That's why the supernova showed up on the Spitzer survey as a hot dust cloud.

Krzysztof Stanek, professor of astronomy at Ohio State, said that stars probably choked on their own dust much more often in the distant past.

"These events are much more likely to happen in a small, low metallicity galaxy," he said -- meaning a young galaxy that hadn't been around long enough for its stars to fuse hydrogen and helium into the more complex chemicals that astronomers refer to as "metals."

Still, Kozlowski added that more such supernovae will likely be found by NASA's Wide-field Infrared Explorer (WISE), which was launched in December 2009. "I would expect WISE to see 100 of these events in two years, now that we know what to look for," he said.

Because of the alignment of the galaxy with Earth and our sun, astronomers were not able to see what the event looked like to the naked eye while it was happening. But Kochanek believes that we might see the star brighten a decade or so from now. That's how long it will take for the shockwave from the exploding star to reach the inner dust shell and slam it into the outer shell. Then we'll have something to see here on Earth.

We do have at least one chance to see a similar light show closer to home, though.

"If Eta Carinae went supernova right now, this is what it would probably look like," Kochanek said, referring to the brightest star system in our Milky Way Galaxy.

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

Explore further: Compact galaxy groups reveal details of their close encounters

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User comments : 16

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rjh405
4.9 / 5 (8) Oct 12, 2010
Undoubtedly the author meant the galaxy was 3 'billion' light years from Earth.
Squirrel
5 / 5 (2) Oct 12, 2010
Which implies it "recently ended its life" just before life first arose here on planet Earth.
EarthlingX
5 / 5 (2) Oct 12, 2010
'But one very hot spot, which appeared in a galaxy some 3 trillion light years from Earth, didn't match the typical heat signal of an AGN.'
Amazing, it was beyond 13.73 x 10^9 ly away and they could see it 8-o !

(Oops, i did not see comments before i commented)

Interesting article otherwise.
yyz
4.8 / 5 (6) Oct 12, 2010
"The astronomers knew that if the source were a supernova, the extreme amount of energy it emitted would qualify it as a big one, or a "hypernova.""

So could this event be classified as a possible-probable gamma-ray burst where the beam is not directed at Earth? We know of several GRBs where dust absorption, in the vicinity of the burst, has been observed and GRBs in general are thought to be generated by hypernovae.

"3 trillion light years from Earth"

rjh405 got it right, 'billion'.

yyz
5 / 5 (5) Oct 12, 2010
A preprint is available: http://arxiv.org/...62v2.pdf

[To answer my own question] it looks like a GRB event is ruled out in this case, due in part to the duration of the event (too long) and the SED is wrong for direct beamed emission. The total energy output was a ginormous 10^51 ergs, definitely hypernova territory.
Malyuta_Skuratov
2 / 5 (1) Oct 12, 2010
"3 trillion light years away"
WOW! ....Innumeracy is everywhere.
omatumr
1 / 5 (7) Oct 13, 2010
Congratulations, Professor Kochanek. This is an interesting observation.

If the universe is fragmentation from neutron repulsion, then giant explosions would probably be more common early in the universe. "Ohio State University researchers suspect that this odd event -- the first one of its kind ever viewed by astronomers – was more common early in the universe."

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo
www.youtube.com/w...e_Qk-q7M
omatumr
1 / 5 (9) Oct 13, 2010
Congratulations, Professor Kochanek. This is an interesting observation.

If the universe is fragmentation from neutron repulsion, then giant explosions would probably be more common early in the universe. "Ohio State University researchers suspect that this odd event -- the first one of its kind ever viewed by astronomers – was more common early in the universe."

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo

See: Scientific Genesis: The Origin of the Solar System

GSwift7
3.4 / 5 (5) Oct 13, 2010
"3 trillion light years away"

This is easily explained by budget cuts at NASA. Due to the second hand calculators they received from Congress, which had previously been used for budget calculations, they inadvertently added three extra zero's. The calculators, you see, are automatically programmed to produce 'inflation', so when used to calculate intergalactic distances, the calculators just assumed that the Universe itself should 'inflate' a bit more.
duurrk
not rated yet Oct 13, 2010
"But Kochanek believes that we might see the star brighten a decade or so from now. That's how long it will take for the shockwave from the exploding star to reach the inner dust shell and slam it into the outer shell. Then we'll have something to see here on Earth."

I guess I'm confused.. If the shockwave will take a couple decades, won't it take another 3 billion yrs until we can see the light which has to travel here? Or is the light seen using observatories real-time? Probably a dumb question.
GSwift7
4.3 / 5 (6) Oct 13, 2010
No problem Duurrk. What we are seeing right now, actually happened 3 billion years ago (+/-). What they are saying is that they expect that the star brightened 3 billion minus 10 years ago, so we should see that brightening in another 10 years. It's still an event that happened about 3 billion years ago. That's the part this article doesn't cover very clearly. Now that we are seeing more clearly into the parts of the universe which lie more than 3 billion years away, we should expect to observe more of these kinds of events. The part of the article which said something about this kind of thing being more common in the early universe alluded to this but didn't say it clearly.

You must understand that there's no such thing as viewing something in 'real time'. Even when looking at the sun, you can't see what's happening there until about 8 minutes after it actually happened. When looking at the other side of the room, the delay is very small, but still significant in quantum world.
duurrk
not rated yet Oct 13, 2010
Thank you, GSwift. I knew about the delay, but wasn't sure if (probably sounds ridiculous) there was any difference in the given delay when using equipment. I can understand why it would still be the same, and thank you for the clarification. I was under the impression they believed it was what was happening right now, and not 3b ly traveled-light-'right now'.
marraco
5 / 5 (3) Oct 13, 2010
We can't even take a "simultaneous" photograph of our own sun, since the center in the photograph is two seconds newer than the border.

In the case of a galaxy photo, the nearest point is hundred of thousands of years newer.
treemikey
1 / 5 (1) Oct 14, 2010
Good point marraco, makes you wonder what the speed of gravity is!
Journey
not rated yet Oct 17, 2010
Whoever wrote this article doesn't understand the speed of light and how we view it. Anything that is 3 billion light years away is, when we view it, how it looked 3 billion years ago. It's NOT how it is NOW. It could, speculatively, not even be there now at all.

The speed of light and our inability to view distant objects in "real time" inhibits our understanding of what our universe really is.... um, but of course it also can ADD to our understanding in viewing the past.
Graeme
5 / 5 (1) Oct 19, 2010
The distance was Z=0.19 which is 2.52 billion light years.

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