From: Andy Soos, ENN
Published November 26, 2012 08:38 AM

Type 1A Supernovas

A supernova is a stellar explosion that is more energetic than a nova. I Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. A Type Ia supernova is a sub-category of supernovae that results from the violent explosion of a white dwarf star. Supernova can be easily picked out and are often used as a way to measure distances. A study using a unique new instrument on the world’s largest optical telescope has revealed the likely origins of especially bright supernovae that astronomers use as easy-to-spot mile markers to measure the expansion and acceleration of the universe.

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Kris Stanek, professor of astronomy at Ohio State and a co-author of the study, explained: "We really want to know more about these supernovae, given their importance in our understanding of how the universe is expanding," he said. "Many observations have been done over the years, and I think many astronomers are starting to accept one explanation — that two white dwarfs are probably responsible."

MODS measures the frequencies and intensities of light shining from a star. Stars shine at different frequencies depending on the chemical elements they are made of; a star like the sun, which is made mostly of hydrogen, shines at different frequencies than a star that is made mostly of helium.

Rick Pogge, professor of astronomy and lead designer of MODS, said: "MODS is one of the most sensitive optical spectrometers in operation today, and being used on what is currently the world's largest optical telescope. If we couldn't kill this debate with MODS and the LBT, something would be dreadfully wrong."

Type Ia supernovae make good mile markers for the universe because their extreme brightness — 5 billion times brighter than the sun — makes them easy to see, and their distinctive pattern of brightening and dimming in the weeks after they appear makes them easily identifiable.

Astronomers use that information to calculate the distance from Earth to the supernova, and in turn, calculate how fast the universe is expanding. Knowing more about the composition of the stars that create the supernovae could open up new ideas in the understanding of that expansion.

Here’s what nearly all astronomers agree on: Type Ia supernovae originate in binary systems, where one star or star-like object is orbiting another. The main object — the one that initiates the explosion — is a white dwarf, the massive remnants of a dead star. Over time, the white dwarf’s gravity peels off gas and dust from the companion and absorbs that material. Eventually, the white dwarf becomes unstable, and explodes in a supernova.

The Type Ia supernova is considered a sub-category in the Minkowski-Zwicky supernova classification scheme. There are several means by which a supernova of this type can form, but they share a common underlying mechanism. When a slowly-rotating, carbon-oxygen white dwarf accretes matter from a companion, it cannot exceed the Chandrasekhar limit of about 1.38 solar masses, beyond which it would no longer be able to support its weight through electron degeneracy pressure and begin to collapse.

The Ohio State astronomers found their answer in the light spectrum emanating from the supernova. If the companion were a star like ours, or even a giant star, a sizeable portion of the debris blown away from the supernova would contain atoms of the element hydrogen.

Supernova 2011fe provided a good chance for the researchers to test for the presence of hydrogen. Located in the Pinwheel Galaxy some 21 million light-years away, it was the closest near-Earth Type Ia supernova to occur in the last 20 years.

"If the companion were a star such as ours or even a red giant, we would expect to see a lot of hydrogen in the signal — maybe even half a solar mass’ worth, as the companion was blown away. But instead, we saw at most only one tenth of one percent of a solar mass’ worth of hydrogen. That suggests that the white dwarf’s companion had very little if any hydrogen in it, and is likely another white dwarf," Shappee said.

Pogge called the study "a beautiful demonstration of the kind of data we are able to get on a routine basis with the LBT and MODS. Our entire instrument team is very proud of how well MODS is working."

For further information see Mile Markers,

Pinwheel Galaxy and SN2011fe via Lawrence Berkeley National Laboratory

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