Astronomers are mystified by a star they have discovered in the Milky Way – because theories say that it shouldn’t even exist. The star, which may be one of the oldest in the universe, is made up almost entirely of hydrogen and helium. 

The star’s position is marked in this photo of the region (ESO/Digitized Sky Survey 2)

Other chemical elements that ought to be there are missing, according to observations using the Very Large Telescope (VLT) at the European Southern Observatory in Chile.

That puts the star, dubbed SDSS J102915+172927 after its positional number in the Sloan Digital Sky Survey, in what accepted theory calls the “forbidden zone” meaning it should never have formed at all.

The star, which is in the constellation of Leo the Lion, is smaller in size and is thought to be much older than the Sun – 13 billion years compared to the Sun’s 4.5 billion years. Astronomers call all elements heavier than helium “metals”, even though they may not resemble what the rest of us mean by that term. The discovery team were surprised to find that the “impossible star” has incredibly small amounts of these metals – fewer than ever detected in a star before.

This suggests to them that the star is extremely old because younger stars contain more heavy elements, produced from the remains of supernova explosions spread throughout space.

Chief discoverer Elisabetta Caffau, of Heidelberg University, Germany and the Paris Observatory, France, said: “A widely accepted theory predicts that stars like this, 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

“It was surprising to find, for the first time, a star in this ‘forbidden zone’, and it means we may have to revisit some of the star formation models.”

Colleague Piercarlo Bonifacio, of Paris Observatory, said: “The star is faint, and so metal-poor that we could only detect the signature of one element heavier than helium — calcium — in our first observations.

“We had to ask for additional telescope time from ESO’s Director General to study the star’s light in even more detail, and with a long exposure time, to try to find other metals.” Two spectrographs on the VLT, called X-shooter and UVES, were used to analyse the star’s light and determine what chemicals it is made of from its spectrum.

Also surprising to the team was the lack of lithium in the star. Because of its age, it was expected to contain 50 times more lithium than it does because that metal existed along with hydrogen and helium in the early Universe.

Dr Bonifacio said: “It is a mystery how the lithium that formed just after the beginning of the Universe was destroyed in this star.”

The team, whose discovery is revealed in the journal Nature, say that their “impossible star” is probably not unique. They have found candidates which appear to have similar, or even lower, levels of metals and are preparing to observe them with the VLT to check them out.