By Paul Rincon
BBC News science reporter
Astronomers have detected a faint glow from the first stars to form in the Universe, Nature journal reports.
This earliest group of stars, called Population III, probably formed from primordial gas less than 200 million years after the Big Bang.
These objects cannot be seen by any present or planned telescopes.
Nasa scientists detected the stars from the imprint they have left on the general glow of infrared radiation dispersed throughout the cosmos.
This glow, which is composed of radiation from stars past and present, is known as the Cosmic Infrared Background (CIB).
The observations used in the latest study were made by the Infrared Array Camera (Irac) on the US space agency's Spitzer Space Telescope.
The results present the first evidence for cessation of the so-called cosmic Dark Ages.
The term, coined by the English Astronomer Royal, Sir Martin Rees, refers to the period in cosmic history when hydrogen and helium atoms had formed but had not yet had the opportunity to condense and ignite as stars.
Blazing into existence
The first stars after the Dark Ages were probably composed solely of hydrogen, helium and a little lithium.
After blazing into existence, their lives would have been intense and short, burning up their hydrogen in only a few million years.
Energy radiated by the Population III stars must have contributed to the CIB; the problem for researchers is that many more much younger stars have also contributed.
In order to isolate a signal from the earliest stars, Alexander Kashlinsky and his colleagues at Nasa's Goddard Space Flight Center in Maryland carefully removed the contributions from other stars and galaxies to the CIB.
"It took us a year to remove the signal sufficiently accurately in order to convince ourselves there was something out there that could not be explained by anything else we could think of," Dr Kashlinsky told the BBC News website.
The team discovered clustering in the distribution of infrared light over and above that expected from the combined effect of known galaxies.
In fact, the total contribution of foreground galaxies is small compared with the residual signal ascribed by the authors to the primordial stars.
In order to contribute this large signal, the primordial stars must have been extremely massive, in the region of hundreds of solar masses, Dr Kashlinsky explained.
"It seems these first stars were quite unlike those we see today. They were huge thermonuclear furnaces; few and far between, but they burned ferociously because they were so massive," Dr Kashlinsky explained.
The distribution of cosmic infrared light suggests these stars were clustered together, which might be partially explained if they were around only for a short time - perhaps a few hundred million years.
It is believed that these earliest stars manufactured the metals that would become important for later populations of stars.
However, other researchers wondered whether the analysis had missed, for example, foreground galaxies with low luminosities.
Richard Ellis, of the California Institute of Technology (Caltech), in Pasadena, said that "even a minor blunder in removing these foreground signals might lead to a spurious result".
He added: "A number of untested assumptions involved in allowing for unobserved galaxies could represent a weakness in the analysis."