NASA / ESA/JPL-Caltech / UCLA / CXC / SAO
This image combines data from four different space telescopes to create a multi-wavelength view of all that remains of the oldest documented example of a supernova, called RCW 86. The Chinese witnessed the event in 185 A.D., documenting a mysterious "guest star" that remained in the sky for eight months.
updated 10/25/2011 1:52:23 PM ET 2011-10-25T17:52:23

Two NASA space telescopes have helped solve some of the most enduring mysteries of the first documented report of star explosion — an ancient supernova spotted nearly 2,000 years ago, scientists say.

In 185 A.D., Chinese astronomers witnessed what they called a mysterious "guest star" that appeared in the sky and lingered for about eight months. It wasn't until the 1960s that scientists determined that this cosmic object was the first documented observation of a supernova that signaled the violent death of a distant star.

Now, infrared views of the supernova from NASA's Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE) reveal that the star explosion detonated inside a region of space that was relatively free of gas and dust. This allowed the star's explosion to travel out much farther and faster than expected, researchers said.

"This supernova remnant got really big, really fast," Brian Williams, an astronomer at North Carolina State University in Raleigh, said in a statement. "It's two to three times bigger than we would expect for a supernova that was witnessed exploding nearly 2,000 years ago. Now, we've been able to finally pinpoint the cause."

Williams is the lead author of the new study, which is detailed online in the Astrophysical Journal.

Ancient supernova
The ancient supernova, called RCW 86, is about 8,000 light-years from Earth. But while its location was known, much of its details were shrouded in mystery.

One enigma is the fact that the star's spherical remains are larger than expected. If the star's exploded guts could be seen in infrared light in the sky today, they would take up more space than the full moon, researchers said.

By combining the new data from Spitzer and WISE with existing information from NASA's Chandra X-Ray Observatory and the European Space Agency's XMM-Newton Observatory, astronomers were able to grasp the missing pieces of the puzzle.

They found that RCW 86 is a so-called Type Ia supernova, triggered by the relatively peaceful death of a star similar to our sun. This star shrank into a dense star called a white dwarf before siphoning matter, or fuel, from a nearby companion star. The white dwarf is then thought to have exploded in a brilliant supernova explosion.

"A white dwarf is like a smoking cinder from a burnt-out fire," Williams said. "If you pour gasoline on it, it will explode."

The study showed for the first time that a white dwarf can create a cavity-like empty region of space around itself before exploding in a Type Ia supernova event. The presence of a cavity would explain why the remnants of RCW 86 are so big, researchers said.

When the explosion occurred, the cavity would have allowed the resulting ejected material to spew out unimpeded by gas and dust. This would also have allowed the star's remains to be cast out rapidly.

More cosmic clues
Using Spitzer and WISE, the researchers measured the temperature of the dust that makes up the RCW 86 remnant. They then calculated how much gas had to be present inside the supernova remnant to heat the dust to those temperatures.

They found that the supernova remnant existed in a low-density environment for much of its life, which points to the presence of a cavity.

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Earlier, scientists suspected that RCW 86 formed from a so-called core-collapse supernova, which occurs when a star's core reaches a tipping point mass and implodes. Core-collapse supernovas are the most powerful type of supernova.

While there were hints of a cavity around RCW 86,  at that time the phenomenon was only associated with core-collapse supernovas. In these cosmic blasts, massive stars blow material away from them before they explode, which carves out dust-free voids around them.

Yet, Williams and his colleagues were able to rule out the possibility of RCW 86 being a core-collapse supernova. X-ray data from Chandra and XMM-Newton indicated that the object consisted of high amounts of iron, which is traditionally a clear indicator of a Type Ia supernova.

Combining these observations with infrared data, the astronomers were able to show that RCW 86 was a Type Ia explosion in a cavity.

"Modern astronomers unveiled one secret of a two-millennia-old cosmic mystery only to reveal another," said Bill Danchi, Spitzer and WISE program scientist at NASA Headquarters in Washington, D.C. "Now, with multiple observatories extending our senses in space, we can fully appreciate the remarkable physics behind this star's death throes, yet still be as in awe of the cosmos as the ancient astronomers."

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Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
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  5. Accidental art

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  6. Supersonic test flight

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    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

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  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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