How Many Supernovae Were There?

But this isn't the question they set out to answer. So what were they studying, and what are Type II supernovae, anyway? Here is the story of the Japanese discovery.

A Type II supernova is the last hurrah of a massive star. The ashes of nuclear fusion (nickel and iron) build up in the core, to the point it can no longer support itself against its own weight. The core collapses, creating a neutron star or a black hole. A shock wave triggered by the core collapse blows the rest of the star to pieces. In the process, many of the elements created by that star over millions of years (familiar elements such as oxygen and magnesium) are dispersed into interstellar space.

The Sun has far too little mass to explode as a supernova. Instead, it will end its life more gently, puffing off its outer layers and leaving behind a dense star called a white dwarf composed mainly of carbon and oxygen. In the case of the Sun, that will be the end of the story.

But some white dwarfs in binary star systems can steal matter from their companion stars and grown in mass. Under certain circumstances, the carbon and oxygen can be pressure-cooked to such an extreme that they will suddenly start a runaway chain reaction of nuclear fusion. The white dwarf converts a large fraction of its mass to iron and explodes completely - leaving nothing behind. Astronomers call such an event a Type Ia supernova.

These two types of supernovae, Type Ia and Type II, are our galaxy's main sources of elements such as oxygen and iron. But the two different types of supernovae produce various elements in different amounts. Type Ia supernovae are the main source of iron in the universe, whereas Type II supernovae contribute most of the oxygen.

A Suzaku X-ray image of AWM7, one of the clusters of galaxies studied by Dr. Sato's team.

The contributions of two types of supernovae for different elements.

Using Suzaku, Dr. Sato and his team studied clusters of galaxies, which not only contain the galaxies, but are filled with hot gas. The hot gas mainly comes from millions of supernovae. After measuring the X-ray spectra of this cluster gas, Dr. Sato and his colleagues could estimate the amounts of various elements it contains. By comparing these amounts with predictions from theoretical calculations, they could figure out what fraction of the gas came from Type Ia supernovae, and what fraction originated from Type II supernovae. They determined that there were probably three times more Type II supernovae than there were Type Ia supernovae.

This is not the first time such measurements have been made, but Dr. Sato's study is a marked improvement over the previous ones in several key respects. Also, as a byproduct of this study, his team calculated the number of supernovae in these clusters of galaxies, and by extension, the number of supernovae in our own Galaxy.

For the full story, see the Japanese team's press briefing at:

http://www.rs.kagu.tus.ac.jp/~ksato/gingadan-e.html

(This is an external link.)

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