News in Science

Star that shouldn't exist found

Astronomers have found a tiny star at the edge of our galaxy, which may send them back to the drawing board to explain how stars are formed.

The star, called SDSS102915+172927, is reported today in the journal Nature.

It has about 80 per cent the mass of the Sun and is composed almost entirely of hydrogen and helium, something previously thought impossible.

An analysis of the star's properties by scientists led by Dr Elisabetta Caffau from the Zentrum fur Astronomie der Universitat Heidelberg in Germany indicates it has the lowest level of elements heavier than helium of any star ever studied, and more than 20,000 times less than the Sun.

All stars contain hydrogen and helium with trace amounts of lithium. These were the original elements created after the big bang which seeded the first generation of stars.

It's these first generation stars which produced all other elements from which subsequent stars were made. Astronomers refer to these other elements as 'metals'.

Turning theory on its head

"Accepted theory predicts stars with low mass and extremely low quantities of metals, shouldn't exist because the clouds of material from which they formed could never have condensed", says Caffau.

"But the work shows low mass stars can form at very low metallicity."

Scientists use the amount of metallic elements within a star, known as metallicity, to determine its age. The smaller the proportion of metals in its composition, the older it's thought to be.

Based on its metallicity, SDSS102915+172927 is about 13 billion years old, making it one of the oldest stars ever found.

It was discovered in the constellation of Leo the lion in the galactic halo, a region of the Milky Way populated by ancient stars.

Caffau and colleagues believe the star is probably not unique.

"We have identified several more candidate stars that might have metal levels similar or even lower," she says.

The search for more

Professor Mike Bessell, a stellar evolution astronomer from the Australian National University's Mount Stromlo Observatory, is conducting his own search for these stars.

He says the research is important in discovering whether low-mass, low-metallicity stars are unusual or the norm.

"We need to build up the numbers of these stars (identified) to work out how elements were created in these first and second generations and if it's different from what's happening now."

Bessell and colleagues are currently developing a telescope at Siding Spring Observatory called SkyMapper.

"It will scan the southern skies for at least five years, using modern CCD's giving quick read out, allowing us to take snapshots of the sky with higher photometry precision," he says.

Bessell hopes the new telescope will turn up at least 500 of these strange stars that shouldn't exist.