IN THE SKIES

Paris, May 20: Astronomers have discovered a new type of exploding star that spews huge quantities of calcium and defies the two known categories of supernovae, according to a pair of studies.

Only a handful of these novel star bursts have been spotted over the last few years, but they could explain the abundance of calcium observed in galaxies like our own Milky Way, the researchers said.

They could even account for the calcium present in our bones, and in all life on Earth.

Up to now, supernovae -- the most energy-intense and brilliant events visible in the universe -- have been grouped into two classes.

Type Ia are thought to arise when the gravity of a burnt-out rump star called a white dwarf draws off enough material from a similar star nearby -- a process known as accretion -- to become unstable.

At a critical tipping point, the star's ultra-dense core of carbon and oxygen ignites into a shattering thermonuclear blast.

Light curves generated by Type Ia supernovae are so regular and predictable that they are used as benchmarks to measure the speed with which the universe expands.

The other known path to stellar destruction involves the gravitational and catastrophic collapse of a hot, massive star's inner core.

When these stars -- dubbed Type II, or Type Ib or Ic -- become supernovae, they give birth to neutron stars or black holes.

But as to tools for peering into the heavens become more powerful, astronomers have come across supernovae that do not fit neatly into either category.

"We're discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes," said Alex Filippenko, a professor at the University of California at Berkeley and co-author of one of the studies.

Both studies, published yesterday in the British journal Nature, focus on one such anomalous stellar burst, known as SN 2005E, which became visible in the halo of nearby galaxy NGC 1032 in January 2005.

Like a Type Ia supernova, SN 2005E appeared to arise from white dwarfs. But the calcium and titanium thrust out by the blast some 110 million years ago was evidence of a nuclear reaction involving helium, rather than the Type Ia's signature carbon and oxygen.