Perhaps the solar system was born in a bubble that surrounded a giant star

Slices of a simulation showing the evolution of a Wolf-Rayet star (moving clockwise from top left) (Image courtesy V. Dwarkadas & D. Rosenberg)
Slices of a simulation showing the evolution of a Wolf-Rayet star (moving clockwise from top left) (Image courtesy V. Dwarkadas & D. Rosenberg)

An article published in “The Astrophysical Journal” describes a research that offers a new explanation for the solar system’s birth. A team of scientists led by Vikram Dwarkadas of the University of Chicago gathered a series of clues that suggest the possibility that within the shell of a giant star of the type called Wolf-Rayet there were the conditions that triggered the formation of the Sun along with its planets and all other minor objects.

There are no certainties regarding the events that led to the birth of the solar system, even if the estimates indicate that it took place about 4.6 billion years ago. The observations made in recent years of stellar systems in various phases of their formation indicate that the solar system formed from a molecular cloud that coalesced into the Sun and in the other celestial bodies. Probably at the same time other stars formed and in May 2014 a team of researchers announced that they had probably discovered a Sun’s sibling.

The main problem is to determine what could cause a coalescing of the gas and dust inside that cloud to generate the solar system. So far, the most widely considered hypothesis was that of a supernova that caused a shock wave that compress the cloud enough to trigger that process.

Some clues to those primordial events can be found in objects that remained virtually unchanged since that time. In particular, some meteorites that date back to the dawn of the solar system contain various elements in different isotopes and their analysis can provide important information about that era.

The team led by Vikram Dwarkadas put together data collected in various studies indicating that in that type of meteorites there’s an abundance of aluminum-26 but also a limited amount of iron-60. Supernovae produce both elements so if the birth of the solar system had been triggered by one of them, one would expect both of them to be contained in abundance in those meteorites. Those elements were supposed to be ejected during the explosion and projected into the cloud from which the solar system was born.

The situation would be different in a Wolf-Rayet star. That’s a phase in the life of very massive stars, over 20 solar masses at their birth, where they start ejecting materials through very intense stellar winds. That mass loss is estimated at about a billion times that emitted by the Sun during the same period. This phenomenon causes the ejection of the outer shell of the star, composed of hydrogen. According to Nicolas Dauphas, one of the authors of the article, it’s a good place for the birth of stars.

In a Wolf-Rayet star the aluminum-26 is transported outwards together with the dust grains that form around the star. They can hit the inner side of the shell that wraps around the core of the star, leaving the aluminum trapped in that area. If part of that shell collapsed inward because of its gravity, it would have formed the solar system.

In this hypothesis, the Wolf-Rayet star that could somehow be the Sun’s mother died long ago, perhaps in a supernova or in a collapse that directly created a black hole. A supernova would have generated iron-60 but it’s possible that it pull through the bubble that surrounded the core or its distribution was uneven. A direct collapse into a black hole would have produced little iron-60.

Basically, these scenarios explain the abundance of aluminum-26 and the scarcity of iron-60. This makes this hypothesis interesting and Nicolas Dauphas stated that between 1% and 16% of the stars similar to the Sun could have been born in that kind of nursery. That’s one more reason to study this possibility.

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