An artist's concept of the James Webb Space Telescope. Credit: ESA

Somewhere in the distant, old Universe, a population of stars hide undetected. They were the first to form after the birth of the Universe and are supposed to be far bigger in mass than any star visible today.

Astronomers know they must have been out there: only in this way could they solve the riddle of the origin and composition of stars in today's Universe. A couple of ESA missions will help astronomers search for this elusive population.

When the Universe formed, there was just hydrogen and helium. Chemical elements such as oxygen, carbon, iron and so on were forged later, in the nuclear furnaces at the hearts of stars and then cast into space at the end of the star's life. Astronomers call everything that is heavier than helium a 'metal'. All stars we can observe today contain metals. The youngest contain the most metals and astronomers call them population I stars. The oldest contain only some metals and astronomers call these population II stars.

Where do these metals come from? Astronomers have theorised that a first generation of stars, which they call population III, must have existed in the early Universe. This first generation of stars must have formed using only hydrogen and helium, the only elements available in the early cosmic history. After living for 'just' a million years, they extinguished themselves, showering the metals they had created into space. The heavy elements lay dormant until they were collected into the next generation of stars and the first galaxies, sometime later.

The theory of population III stars suggests they are long dead in the local Universe. How can their existence then be confirmed? In the most distant realms of space, where what we observe is either very old or even extinguished, some signs of their existence might still be glimpsed. One mission that will help considerably in the search is the James Webb Space Telescope (JWST), ESA's collaboration with NASA to replace the Hubble Space Telescope with a six-metre-class telescope. There are many questions for it to answer.

"We don't really know what the first generation of stars are like and we don't know where exactly they formed," says Peter Jakobsen, ESA's Study Scientist for the JWST. "One of the biggest questions is whether the first stars formed in clumps or as isolated individuals. If they clumped, we'll be able to see them much more easily and further away than if they didn't." Even if JWST does not see the first stars directly, it will give astronomers an invaluable clue about how far away they are, allowing them to refine their theories. New research suggests that even if the population III stars are extremely far away, JWST would see them exploding as supernovae, at the ends of their individual lives.

In addition, some astronomers suspect that some gamma-ray bursts (GRBs) are created by the death of these earliest stars. Ironically, we may therefore already be seeing the farewell detonation of some population III stars. ESA's new gamma-ray observatory, Integral, is perfectly placed to shed light on these violent events. It will indirectly help provide clues about population III stars. "I suspect that in the next ten years, we'll know the answers to at least some of our questions about what went on in the early Universe," says Jakobsen. This includes learning more about the existence and role of the earliest stars.

JWST
The James Webb Space Telescope (JWST) is a collaboration between ESA and NASA. It is the successor to the Hubble Space Telescope and, with a six-metre mirror, it will be almost three times the size of HST. Engineers have designed the JWST to work best at infrared wavelengths. This will allow it to study the very distant Universe, looking for the first stars and galaxies that ever emerged. Current plans call for its launch in 2010.

Integral
The International Gamma Ray Astrophysics Laboratory (Integral) is the first space observatory that can simultaneously observe celestial objects in gamma rays, X-rays, and visible light. Integral was launched on a Russian Proton rocket in October 2002 into a highly elliptical orbit around Earth. Its principal targets are powerful phenomena known as supernova explosions, regions of the Universe thought to contain black holes and violent explosions known as gamma-ray bursts. In particular, when a gamma-ray burst goes off in Integral's field of view, an automatic alert is sent to the world's ground-based observatories within 30 seconds. This allows for rapid follow-up observations that are needed to analyse these mysterious phenomena.