Sen— Astronomers have been puzzled to discover a star within an ancient and spectacular cluster that seems to have found the secret of eternal youth.

They used a giant telescope at the European Southern Observatory's mountaintop observatory at La Silla in Chile to image the famous globular cluster that is labelled Messier 4, or M4.

M4 was first noted by the French comet hunter Charles Messier in a catalogue that he compiled of fuzzy objects in the sky that might be mistaken for the objects he was seeking. It is one of more than 150 similar clusters orbiting our own galaxy.

M4 is easily spotted in the constellation of Scorpius by amateur astronomers with binoculars or even the smallest of telescopes, though low in the sky for many northern hemisphere observers. It can be found close to the bright star Antares.

Lying at the relatively close distance of about 7,200 light-years away, it would be even more magnificent if it were not partly obscured by gas and dust in our own Milky Way.

The MPG/ESO 2.2-metre telescope's Wide Field Imager was easily able to resolve the tightly knit ball of light to reveal the tens of thousands of individual old stars. The cluster is noteworthy in being home to many white dwarfs — the cores of ancient, dying stars whose outer layers have drifted away into space.

Hubble's image of the heart of M4. Credit: NASA/ESA

Viewed against the backdrop of the Milky Way, it makes for a stunning photo. Astronomers at another major ESO observatory site in Chile, Paranal, have used the Very Large Telescope (VLT) to study many of the individual stars within the cluster in detail. In particular, spectroscopes have been able to split the light from individual stars into a rainbow spectrum to reveal what they are made of.

Because stars in globular clusters are ancient, they are generally far less rich in the heavier chemical elements than younger stars like our own Sun. This is the case with M4 too, but one of the stars checked out by the astronomers offered up a surprise because it was found to have much more of the rare light element lithium than expected.

They want to know how. Normally this element is gradually destroyed over the billions of years of a star's life, but this one star amongst thousands seems to have kept hold of it. Either the star has found a way to retain its original lithium, or it has discovered a process to enrich itself with freshly made lithium.

The 2.2-metre MPG/ESO telescope at La Silla. Credit: ESO

By chance, the Hubble Space Telescope operated by NASA and ESA also released an image of the heart of M4 this week as part of the Hubble Picture of the Week series.

The lithium star is not the only strange object in M4. In July 2003, Hubble helped discover a peculiar planet that orbits a binary system of a white dwarf and a pulsar. Labelled PSR B1620-26 b, the planet is 2.5 times as massive as Jupiter and is thought to be around 13 billion years old — almost three times as old as the Solar System.