There is reason to think that the first stars of the observable universe were very massive and exploded by giving supernova and black holes. These supernovae were supposed to be more powerful than we thought until now and they had to be accompanied by powerful jets of matter injecting heavy nuclei into the interstellar medium.
There are still many uncertainties about what happened when the first stars began to shine in the observable cosmos, ending the Dark Ages with the beginning of the cosmic Renaissance. It is certain that these first stars could not have formed exactly like those observed for less than 13 billion years. Nowadays, we can say the gravitational collapse of molecular clouds is made possible by the presence of dust and carbon and oxygen-containing molecules that act as radiators to dissipate heat released by this collapse, preventing the pressure caused by thermal energy to end the process leading to the formation of stars.
But when the universe came out of the Big Bang after a few minutes of primordial nucleosynthesis, no nucleus heavier than lithium, and therefore even more oxygen and carbon, had been generated. The star formation had to be different, probably involving molecular hydrogen and perhaps also dark matter, as astrophysicists exploring the concept of black stars think.
Still, we are led to think that these first stars were to be very massive, perhaps several times the mass of the Sun and they therefore burned their nuclear fuel very quickly as we teach the theory of stellar evolution. They would have exploded very violently in the form of supernova, leaving behind only black holes. The heavy elements synthesized in these stars would be found in the interstellar medium where they would have allowed the birth of the second generation of stars.
Supernovae at the origin of reionization?
A group of astronomers from MIT may have come to provide further details on the nature of the first stars as the researchers explain in an open-access article on arXiv and published today in Astrophysical Journal.
Everything started with new observations in 2016 with the Cosmic Origins Spectrograph instrument fitted to the Hubble telescope. The target was HE star 1327-2326, located about 5,000 light-years away in the Milky Way. It was discovered in 2005 in the female Hydra constellation, and its chemical analysis revealed in its atmosphere an abundance of metals more than 200,000 times lower than that of the Sun. Clearly, it was a very old star formed just after the very first stars.
To explain this anomaly, the researchers made about 10,000 numerical simulations of supernova explosions by varying the parameters describing their spawning stars and the explosions themselves. It turned out that only the most powerful supernovae – and whose explosions were asymmetrical with powerful jets of material rich in heavy nuclei and thus removed from the gravitational collapse that should have led them to be absorbed by a black hole newly formed – were able to account for the amount of zinc measured.
This result is interesting in more than one way because, in addition to providing us with a window on the physics of the first stars, it suggests that the supernovae of the early times were able to contribute significantly to the famous reionization of the observable universe.
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