NASA telescopes help solve ancient supernova mystery

ScienceDaily (Oct. 24, 2011) — A mystery that began nearly 2,000 years ago, when Chinese astronomers witnessed what would turn out to be an exploding star in the sky, has been solved. New infrared observations from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer, or WISE, reveal how the first supernova ever recorded occurred and how its shattered remains ultimately spread out to great distances.

The findings show that the stellar explosion took place in a hollowed-out cavity, allowing material expelled by the star to travel much faster and farther than it would have otherwise.

"This supernova remnant got really big, really fast," said Brian J. Williams, an astronomer at North Carolina State University in Raleigh. Williams is lead author of a new study detailing the findings online in the Astrophysical Journal. "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."

A new image of the supernova, known as RCW 86, is online at http://go.nasa.gov/pnv6Oy .

In 185 A.D., Chinese astronomers noted a "guest star" that mysteriously appeared in the sky and stayed for about 8 months. By the 1960s, scientists had determined that the mysterious object was the first documented supernova. Later, they pinpointed RCW 86 as a supernova remnant located about 8,000 light-years away. But a puzzle persisted. The star's spherical remains are larger than expected. If they could be seen in the sky today in infrared light, they'd take up more space than our full moon.

The solution arrived through new infrared observations made with Spitzer and WISE, and previous data from NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton Observatory.

The findings reveal that the event is a "Type Ia" supernova, created by the relatively peaceful death of a star like our sun, which then shrank into a dense star called a white dwarf. The white dwarf is thought to have later blown up in a supernova after siphoning matter, or fuel, from a nearby star.

"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 observations also show for the first time that a white dwarf can create a cavity around it before blowing up in a Type Ia event. A cavity would explain why the remains of RCW 86 are so big. When the explosion occurred, the ejected material would have traveled unimpeded by gas and dust and spread out quickly.

Spitzer and WISE allowed the team to measure the temperature of the dust making up the RCW 86 remnant at about minus 325 degrees Fahrenheit, or minus 200 degrees Celsius. They then calculated how much gas must be present within the remnant to heat the dust to those temperatures. The results point to a low-density environment for much of the life of the remnant, essentially a cavity.

Scientists initially suspected that RCW 86 was the result of a core-collapse supernova, the most powerful type of stellar blast. They had seen hints of a cavity around the remnant, and, at that time, such cavities were only associated with core-collapse supernovae. In those events, massive stars blow material away from them before they blow up, carving out holes around them.

But other evidence argued against a core-collapse supernova. X-ray data from Chandra and XMM-Newton indicated that the object consisted of high amounts of iron, a telltale sign of a Type Ia blast. Together with the infrared observations, a picture of a Type Ia explosion into a cavity emerged.

"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. "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."

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA. For more information about Spitzer, visit http://spitzer.caltech.edu/ and http://www.nasa.gov/spitzer .

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Oxygen blew up ancient amoebas

Single-celled creatures' size spiked as oxygen levels roseWeb edition : Thursday, October 13th, 2011 Got oxygen?Fossil shells of Paleozoic amoebas, like the one shown here, grew to giant proportions as oxygen levels rose, suggesting that the gas fueled the creatures' evolution.John Groves

MINNEAPOLIS — Giant armor-clad amoebas that once swam Paleozoic seas may have owed their monstrous size to something in the water: oxygen. 

A new look at the fossil record suggests that a spike in oxygen levels supersized many species of these fusulinids, a now-extinct type of single-celled microbe called foraminifera. About 300 million years ago, when the atmosphere contained almost enough oxygen to spontaneously combust, some of these critters grew to be 10 centimeters long. They would have been visible to the naked eye.

“Their average volume increased by at least factor of 100, maybe up to a factor of 1,000,” says Jonathan Payne, a paleobiologist at Stanford University who presented his team’s research October 12 at the Geological Society of America annual meeting.

Searching for evidence of oxygen’s influence, Payne and his colleagues recruited undergraduates and high school students to compare the intricate, multilayered shells left behind by more than 1,800 foraminifera that lived between 250 million and 325 million years ago. At first, the biggest species grew increasingly larger as atmospheric oxygen rose. After oxygen peaked at levels 66 percent higher than today and began to fall, the size of the creatures also declined.

The shapes of new, giant species fit with the idea that oxygen controlled their growth. Instead of ballooning like beach balls, the amoebas elongated. They grew no wider than about 2 millimeters — limited by how far oxygen could penetrate into cells after being absorbed, according to Payne’s calculations.

Some scientists believe that oxygen also boosted the size of insects (SN: 10/21/06, p. 270) and animals during the Paleozoic era. Fossils have revealed species of millipedes longer than a meter, amphibians longer than 2 meters and dragonflies with wingspans of more than 70 centimeters.

But tracing the evolution of size in larger organisms — and linking it to oxygen — has proven difficult because fewer of these species evolved into giants, says Phil Novack-Gottshall, a paleobiologist at Benedictine University in Lisle, Ill.

“You might have one or two species of giant centipedes from this time,” he says. “The advantage of foraminifera is that you have many giant species to work with.”

Payne also sees signs of oxygen influencing the size of modern amoebas. Among species living in waters off the coast of Australia, those that evolved to live close to the surface are bigger than those living at a depth of 500 meters, where oxygen concentrations are halved. “The trends we see in these modern species are quite similar to patterns we see in the fossil species,” says Payne.

These microscopic modern species are pretty puny compared to their ancestors. But some day, if a pulse of oxygen revs up their evolution, they might be giants once again.

Found in: Earth and Paleontology

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