Astronomers' puzzle: A forbidden star

An ancient star has been found lurking in the “forbidden zone” of star formation, which has astronomers puzzled as to how it could have formed.

Astronomers used instruments on ESO’s Very Large Telescope (VLT) to observe the star, which is called SDSS J102915+172927.

It has slightly less mass than our Sun, and could possibly be around 13 billion years old.

Stars are composed mainly of hydrogen and helium, with a small amount of heavier elements.

These heavier elements are referred to as “metals” by astronomers, but surprisingly this mysterious star is almost devoid of them. In fact, it has 20,000 times less metals than the Sun.

A team of European astronomers has used ESO's Very Large Telescope (VLT) to track down a star in the Milky Way that many thought was impossible.

They discovered that this star is composed almost entirely of hydrogen and helium, with only remarkably small amounts of other chemical elements in it.

This intriguing composition places it in the "forbidden zone" of a widely accepted theory of star formation, meaning that it should never have come into existence in the first place. The results will appear in the 1 September 2011 issue of the journal Nature.

A faint star in the constellation of Leo (The Lion), called SDSS J102915+172927 [1], has been found to have the lowest amount of elements heavier than helium (what astronomers call "metals") of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old.

"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," said Elisabetta Caffau (Zentrum fur Astronomie der Universitat Heidelberg, Germany, and Observatoire de Paris, France), lead author of the paper.

"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."

The team analyzed the properties of the star using the X-shooter and UVES instruments on the VLT.

This allowed them to measure how abundant the various chemical elements were in the star. They found that the proportion of metals in SDSS J102915+172927 is more than 20,000 times smaller than that of the Sun.

"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," said Piercarlo Bonifacio (Observatoire de Paris, France), who supervised the project.

"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."

Cosmologists believe that the lightest chemical elements - hydrogen and helium - were created shortly after the Big Bang, together with some lithium, while almost all other elements were formed later in stars.

Supernova explosions spread the stellar material into the interstellar medium, making it richer in metals. New stars form from this enriched medium so they have higher amounts of metals in their composition than the older stars. Therefore, the proportion of metals in a star tells us how old it is.

"The star we have studied is extremely metal-poor, meaning it is very primitive. It could be one of the oldest stars ever found," adds Lorenzo Monaco (ESO, Chile), also involved in the study.