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Astronomers wind back clock on supernova blast

Paul Sutherland, Feature writer
Apr 3, 2015, 23:27 UTC

Sen—Detective work by a team of astronomers has led to fresh insights into how one of the Universe’s biggest explosions, a supernova, can occur.

The researchers used a Japanese space telescope to study the remains of a supernova that blew itself to pieces thousands of years ago. Then they traced the expanding dust and gas backwards to discover what had caused it.

The result came as something of a surprise, because the object which exploded is revealed to have been a single, extremely dense, white dwarf star. Many supernovae of this kind—a Type Ia—are thought to be produced when two white dwarfs spin closer together and collide.

The supernova studied was a remnant labelled 3C 397, lying around 33,000 light-years away in the constellation of Aquila, the Eagle. It is thought the remnant is now being seen as it was between 1,000 and 2,000 years after the blast occurred.

The Japanese-led Suzaku X-ray satellite was turned onto 3C 397 to analyse the cloud of debris and check how massive the star that produced it was, by checking crucial elements with the spacecraft’s X-ray Imaging Spectrometer. 

They looked at how much nickel and manganese there was in the debris, and compared it to measurements of iron and chromium which are found in all Type Ia explosions. The relative quantities pointed to just one white dwarf being involved.

Further help came from NASA’s Spitzer Space Telescope which measured how much gas and dust had been gathered from surrounding interstellar space as the cloud expanded.

Team member Carles Badenes, of the Department of Physics and Astronomy at the University of Pittsburgh, Pennsylvania, said in a statement: “White dwarfs remain stable as long as they never tip the scales too closely to 1.4 solar masses.

“White dwarfs near this limit are on the verge of a catastrophic explosion. All it takes is a little more mass.”

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An artist's impression of the Suzaku X-ray satellite in orbit. Image credit: JAXA

Astronomers believe that most stars like our Sun will become white dwarfs at the end of their lives. Such a star will be as massive as our Sun but only about the size of Earth, making it extremely dense.

Ben Shappee is a professional astronomer, based at Carnegie Observatories in Pasadena, California, whose main research interest is also supernovae, though he was not involved in the study of 3C 397. 

He told Sen: “We already know that a Type Ia supernova involves a white dwarf star. Our challenge is to discover what happens to make it explode.  

“They go supernova when something causes a runaway explosion similar to a thermonuclear blast. And we want to know what’s triggering that blast because a lone white dwarf that’s just sitting there won’t explode on its own.

“Basically there have been two models to suggest an answer. It could be a Sun-like star or a red giant that the white dwarf strips of material over time until it becomes too massive to support itself and explodes. 

“The other main possibility is that the companion is a second white dwarf and those two stars become very close and merge and then explode.”

The results from Suzaku’s observations of 3C 397 suggest that in this case, a single white dwarf did accumulate matter from a normal companion star, rather than a white dwarf.

Lead researcher Hiroya Yamaguchi, of NASA’s Goddard Space Flight Center, in Maryland, said in a statement: “Mounting evidence indicates both of these mechanisms produce what we call type Ia supernovae.

“To understand how these stars explode, we need to study the debris in detail with sensitive instruments like those on Suzaku.”

Type Ia supernovae are of special interest to cosmologists because they can be used as distance markers or “standard candles” to help determine how far away galaxies are.

There is another main class of supernova, termed a Type II, which is caused when a red supergiant star becomes unstable and its core collapses in on itself, causing the explosion.