First Stars May Not Have Been Monsters

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Astronomers are seeking to explore a little-studied time in our universe’s history known as “the Dark Ages.” The universe was dark in that no galaxies shined brightly as we see today. Yet somehow, out of this dark, mostly-hydrogen gas, the very first stars did form somehow, lighting up the cosmos for the very first time.

What these first stars were like is still a debate amongst astronomers since there is a lack of direct observations.

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Astronomers can simulate the formation of stars under the conditions that were present in the very early universe. However, the scenario is complex on large and small scales and involves many physical and chemical processes, all which need to be tracked carefully.

Often, some assumptions and approximations must be made in order for the simulation to fit into a realistic computational space and time.

Many previous models of the first stars were semi-analytic, meaning that some processes were approximated through the use of equations, rather than just letting the simulation progress in a purely numerical way. These have long predicted that, in the absence of heavier elements such as carbon, nitrogen, and oxygen, that the first stars would be truly massive, reaching hundreds of times the mass of the sun before finishing their growth.

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Such behemoths would live out very short lives and explode catastrophically in a type of supernova that is not seen in the universe today; a “pair-instability” supernova.

This collapse and subsequent explosion is so violent that it does not leave a black hole remnant behind, as other large supernovae do. However, the pattern of elements created by such a supernova should be detectable in the gas left over, now trapped in old “metal-poor” stars wandering the galaxy. (Remember, in this case, astronomers refer to “metals” as any element that is not hydrogen and helium. Sorry, chemists.)

The problem with this hypothesis up until now is that the pattern of elements in old, metal-poor stars does not match this prediction. Instead, it looks as if the gas was created in “regular” core-collapse supernovae that we still see today that herald the death of a star that is tens of times the mass of our sun. This explosion leaves behind a neutron star or black hole as a remnant.

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This new simulation of a star forming in the early universe explains why this might be the case. The group of astronomers, led by Takashi Hosokawa of the Jet Propulsion Laboratory (JPL) at NASA, used radiation hydrodynamical simulations to plot out the birth and life of this model star. This means they used principles of fluid flow mechanics and took into account the effects that the light of the star itself has on the process.

They found that the protostar formed a disk of material around it, as was expected, from which material flowed onto the star-to-be. As the star got denser and hotter, its radiation heated and ionized some of the gas around it, until the pressure was too great and gas began to flow out from the polar regions of the star. This outflow expanded until it cut off the gas falling on to the disk around the star, thus cutting off the star’s food supply.

ANALYSIS: Massive Star Birth Revealed

Finally, the disk itself was blasted away by the star’s bright light, and 100,000 years after the star began to form, it had reached 35 times the mass of the sun and had begun nuclear fusion in its core.

35 solar masses is still pretty huge, though not the hundreds of solar masses that were expected. This type of star may have been more typical of the first stars, leading to core-collapse supernovae within a few million years. Though the existence of the super-behemoth stars has not been ruled out, the early chemical evolution of the universe was probably dominated more like this kind of star.

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Today, with the higher abundance of heavier elements, stars are much more likely to be puny things, the size of our sun or less massive. However, the longer lifetimes of these stars is far more conductive to supporting the billions of years of evolution that life required on Earth, so this more peaceful existence is probably a good thing. That doesn’t mean we can’t dream and wonder at the largest stars in our universe, and at least enjoy the show.

Images: Top – The complicated environment around a forming star in a simulation, including the star itself, an accretion disk, and massive outflows of gas. Center - The evolution of the massive model star, as taken from the movie made from the simulations. Credits: NASA / JPL-Caltech / Kyoto University

This research was published in Science Express, and more information can be found at the first author’s website.