Universe contains more calcium than assumed

The universe contains one and a half times more calcium than previously assumed. Astronomers of the SRON Netherlands Institute for Space Research draw this conclusion after observations with ESA's X-ray observatory XMM-Newton. The research offers them new insights in the formation history of the elemental building blocks of the cosmos in which supernovae play a crucial role.

The iron in our blood, the oxygen we breathe, the calcium in our bones, the silicon in the sand box, nearly all the atoms we are made of are released during the violent final moments of a dying star. These so-called supernova explosions eject newly made chemical elements into space where they may be used as building blocks for new stars, planets, or even life. However, astronomers still struggle with questions like how the elements are formed and how they are distributed across the universe.

The Abell 1689 cluster of galaxies as seen by the Hubble Space Telescope (NASA/ESA)

According to SRON space researcher Jelle de Plaa, many answers can be found in distant clusters of galaxies. "Clusters are in many ways the big cities of the universe", he says. "They consist of hundreds of galaxies that each contain billions of stars. The galaxies are embedded in a gigantic cloud of hot gas that fills the cluster like a smog. Due to their enormous size and numbers, clusters contain a large fraction of the total amount of matter in the universe. During the past billions of years supernova explosions have enriched the surrounding hot gas with heavier elements, like oxygen, silicon and iron."

De Plaa determined the abundances of oxygen, neon, silicon, sulphur, argon, calcium, iron and nickel in 22 clusters of galaxies using XMM-Newton for which SRON built one of the scientific instruments. In total he saw the 'pollution' produced by about 100 billion supernovae. When he compared the measured amounts of elements in the clusters with theoretical supernova models, the measured calcium abundance appeared to be one and a half times higher than theoreticians assumed.

Clusters of galaxies Abell 1689 (left) and Abell 3558 (right) as seen by XMM-Newton (ESA)

Dance of death

De Plaa and his colleagues also found that many supernovae in clusters are the result of a dance of death between two stars that revolve around each other. A very compact white dwarf withdraws matter from its unfortunate companion star. The matter forms a layer on the surface of the white dwarf. When the dwarf reaches a certain mass, its core cannot support the weight of the matter any more and explodes as a supernova. "Roughly half of the number of supernovae that ever exploded in clusters appear to have exploded this way", says De Plaa. "This is much more than the fraction of this kind of supernovae in our own galaxy, which we estimate to be 15%."

The results will be valuable for the scientists who make supernova models. De Plaa: "Until now, supernova experts had to make educated guesses about how a supernova exactly explodes. Because we measure the remains of 100 billion supernovae at once, we find more accurate averages than before. This will help the supernova community to learn how white dwarfs die."

The findings of De Plaa and his colleagues will be published in the scientific journal Astronomy & Astrophysics. This research is a result of a cooperation between SRON Netherlands Institute for Space Research and the University of Utrecht, the Netherlands.