Galaxy clusters are the largest objects in the universe, with optical-wavelength observations showing hundreds or even thousands of galaxies crammed into an area only a few million light-years across.

However, most of the atoms in these clusters � five times as many as appear in visible light - are in the form of optically invisible hot gas, the remnants of supernova explosions.

Astronomers divide supernovae into two basic types: core- collapse and Type Ia explosions. A core-collapse supernova originates when a star at the end of its life collapses into either a neutron star or black hole. The event produces large quantities of oxygen, neon and magnesium.

A type Ia supernova occurs when a white dwarf star consuming matter from a larger companion becomes too massive and completely disintegrates. The explosion produces lots of iron and nickel.

Using the XMM-Newton space telescope, the astronomers made deep observations in X-ray light of two galaxy clusters, 2A 0335+096 and Sersic 159-03, in November 2002 and August 2003, for a day and a half each time. Based on the data collected, the team could determine the abundances of nine chemical elements in the plasma, or hot electrically charged gas, occupying the clusters.

The elements include oxygen, iron, neon, magnesium, silicon, argon, calcium, nickel and - detected for the first time ever in a galaxy cluster - chromium.

"Comparing the abundances of the detected elements to the yields of supernovae calculated theoretically, we found that about 30 percent of the supernovae in these clusters were exploding white dwarfs (Type Ia) and the rest were collapsing stars at the end of their lives (core- collapse)," said team co-leader Norbert Werner of the SRON Netherlands Institute for Space Research in Utrecht.

"This number is in between the value found for our own Galaxy (where Type Ia supernovae represent about 13 percent of the supernovae population) and the current frequency of supernovae events as determined by the Lick Observatory Supernova Search project (according to which about 42 percent of all observed supernovae are Type Ia)," Werner added.

The team also found that the distribution of elements in 2A 0335+096 indicates an ongoing galactic merger, while the distribution of oxygen and iron across Sersic 159-03 suggests an epoch differential between the two types of explosions. Most of the gas production by core-collapse supernovae seems to have happened long before gas production by Type Ia supernovae, particularly in the core of the cluster.

One surprise involved the failure of current supernova models to predict correct calcium and nickel abundance � the team observed much more calcium than expected, for example. They said such discrepancies suggest there are details of supernova evolution that are not yet clearly understood.

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