New computer simulations have changed the view that the Universe's first stars were giants. Although still huge, they were several tens of solar masses rather than the hundreds that standard theory predicted, say NASA Jet Propulsion Laboratory scientists.

After the big bang 13.7 billion years ago, the Universe consisted only of hydrogen and helium. The first stars formed a few hundred million years later from collapsing gas clouds and fused heavier elements in their cores, which were subsequently flung out into space once they exhausted their fuel supply and exploded as supernovae. The stellar debris then enriched the interstellar environment with a greater variety of chemical ingredients from which the next generation of stars were born.

Standard theory speculates that these first stars would need to be several hundred times the mass of our Sun, in order to compensate for their light elemental composition. But the new simulations produced stars with masses equivalent to a few tens of our own Suns, with the smallest just 43 solar masses.

"The maximum stellar mass we got in our simulations was about 85 solar masses," JPL's Takashi Hosokawa tells Astronomy Now. "It is difficult to make more massive stars with typical settings. Our work does not completely reject the possibility of the extremely massive first stars exceeding 100 solar mass, but suggests that such stars, if any, should have formed in very limited situations."

The team's simulations reveal that the growth of these stars is stunted earlier than expected, resulting in smaller final sizes. What prevents this growth could be the temperature of the matter in the vicinity of the forming stars heating up to much higher temperatures than previously believed – to some 50,000 Kelvin – as a result of radiation being emitted by the new born star. While cooler gas would normally sink onto the forming star and contribute to its mass, the hot gas expands and easily escapes from the star's gravitational clutches.

The result also has implications for how the first stars died. A star several hundred times the mass of our Sun is expected to leave a specific imprint of the heavier elements within the next generation of star, but this has never been seen. But if the first stars were less massive than previously thought, they would explode in a similar way to other stars seen in the present day Universe, thus supporting the null result in the search.