The First Starlight

Peering through the unusual lens, they observed a faint cloud of stars lying 13.4 billion light-years from Earth. The cloud, like a very small galaxy, contained about a million stars. For comparison, typical galaxies in the Universe today contain hundreds of billions of stars. Ellis and his colleagues believe the diminutive cloud is a building block of the full-sized galaxies that now populate our Universe.

If the Universe is 14 billion years old, as some models hold, then the star cloud existed less than one billion years after the Big Bang - just when theorists think the first stars probably formed. Indeed, the stars in the cloud appear to be very young. Spectral evidence suggests that they are roughly 2 to 5 million years old, though Ellis cautions that this evidence is still being debated among researchers. The Sun, for comparison is about 4.8 billion years old.

Credit: the ESA, NASA, Richard Ellis (Caltech, USA) and Jean-Paul Kneib (Observatoire Midi-Pyrenees, France). A blow-up of the boxed area in the Hubble image above. Circled are the faint, twin images of the star cloud.

"We're seeing this cluster while it's switching on," Ellis says. The stars in the cloud were among the first, perhaps, to light up the heavens.

Without a boost from the gravitational lens, which brightened the star cloud's light by approximately 30 times, neither the Keck 10-meter telescope nor the Hubble Space Telescope would have detected these distant, young stars, notes Ellis.

Such magnifying lenses are one of the odd realities of Einstein's theory of general relativity: Einstein showed that mass creates a local curvature in the geometry of space-time that can bend the path of light. The strong curvature caused by the massive group of galaxies Ellis used for this research was able to bend and focus light rays from the star cloud far behind it, just like the lens in a magnifying glass.

"By looking at particular regions of the sky where the light from the very early Universe is highly magnified by a gravitational lens, we get a helpful boost from nature in searching for these feeble signals," Ellis adds.