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First stars ten times smaller
Posted: 16 November 2011

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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.

At the centre of the box is a forming star. Red shows hot gas and blue cool gas. When the gas becomes so hot that it escapes the star's gravity, the star stops growing, limiting the mass of the first stars to several tens of solar masses, instead of hundreds of times more massive as originally thought. Image: NASA/JPL-Caltech/Kyoto Univ.

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.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!


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