The origin of the Sun: anomalies in chondritic meteorites indicate that it was not born alone

The Sun was born from the local collapse of a cloud of gas and dust. These components were progressively added by the effect of the gravitational field of that nebula. The formation of everything that came after, such as the structure of the planetary system that includes the Earth, took place at a high temperature, in fact, at too much temperature for there to be practically nothing solid in its beginnings.

The material in the solar environment was almost entirely in the gaseous phase, due to the temperatures above about 2000 K that occurred. However, when it acquired enough mass to be born as a star, it began to generate light at a dazzlingly powerful rate.

The light and the stellar wind progressively swept away the residual gas, thereby decreasing the gas density and this led to a drastic drop in temperature that gave rise to the condensation of the first solid particles from the gas.

The first surviving solids, the so-called calcium aluminum refractory inclusions (CAI), were able to survive their melting about 4,568 million years ago.

The first solid aggregates still form the so-called Interplanetary Dust Particles (IDPs) that come from primitive asteroids and comets. Precisely from them and due to thermal processing the chondrules arose. Those materials formed the protoplanetary disk.
Image by Josep M. Trigo (CSIC-IEEC) except the upper left frame with the IDP courtesy of Don Brownlee (NASA)

The meteorites that were born with the Sun

We call meteorites chondrites that formed part of small rocky bodies whose sizes varied between tens and a few hundred kilometers, formed around the Sun in that early stage after its birth. Thus, chondritic meteorites come from asteroids that are composed of solid primordial materials, whose isotopic dating reveals that they are about fifty million years older than Earth.

They were porous bodies that grew from collisions between the first existing solid particles in the protoplanetary disk. Due to its porous nature, part of the heat generated, both from the disintegration of its radioactive elements and from the collisions between them, was able to escape efficiently into space. The momentous consequence of this heat-loss process was that its materials never got hot enough, did not melt, and therefore retained their primordial chemical composition.

The bricks of rocky planets

We call these primordial rocks generically chondrites because they are formed mainly by typically submillimeter igneous spherules that we call chondrules. They might remind us of sedimentary rocks on Earth, except that these chondrules, plus concentrations of calcium and aluminum (CAI) and certain grains of sulfides and metals, represent the sediments of creation. Such solid particles were the components of authentic rivers of materials, arranged in a kind of toroidal regions around the young Sun. Later these particles collided with each other to give rise to chondritic asteroids, the building blocks of rocky planets.

All of this has been corroborated by the recent discoveries made by the ALMA radio telescope, which allows us to delve into the environment of young stars, in a stage similar to that passed by the Sun some 4,565 million years ago.

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The protoplanetary disk around HL Tau seen by ALMA exemplifies the distribution of solid materials in rings around that young star.
(ALMA Partnership et al. 2015, ApJ, 808, L3)

Extrasolar materials contained in chondrites

A really small percentage, typically less than one percent, of the chondrite-forming materials originated from other stars. They are called presolar grains, generally micrometric particles that condensed in the circumstellar environments of stars formed before the Sun itself.

On the other hand, as the solid materials condensed from the vapor phase, both the chondrules and other components of the chondrites contain daughter isotopes of others of a radioactive nature.

The existence of such isotopes in meteorites reveals that in the environment of the Sun there were stars whose emanated gases bathed the solar nebula. In fact, years ago we showed that the isotopic abundances of the chondrites could be consistent with the formation of the Sun in a stellar association of stars from the asymptotic branch of the giants, known as AGB stars.

In other words, to those chemical elements that condensed into the first solid components of the Solar System, another component could be incorporated which, being partially radioactive, originated from a certain type of stars that we know produce certain peculiar isotopes. We were able to show that if we mixed the radioactive isotopes theoretically expected from a star six times the mass of the Sun with another three hundred parts of gas with the solar composition, we would have just the isotopic anomalies measured in the chondrites.

However, science is advancing thanks to controversy, and a new study by Alan Boss of the Carnegie Institution suggests that it was a supernova explosion that incorporated many of these intermediate-lived radioactive isotopes, particularly iron-60. For years these authors have suggested that the shock wave produced by the explosion of a supernova could also be capable of encouraging the formation of the Sun and other stars from the primordial gas cloud.

For sure, our planetary system received presolar grains and gases from all these types of stars, and meteorites teach us something surprising: some were very close to our Sun.

The Sun was born in a molecular cloud, forming part of a stellar association

A valuable lesson from those true fossils of creation that are chondrites is that our star was born in a star-rich environment. Such stars contributed to the formation materials of the chondrites with tiny stellar grains, such as silicon carbide whose isotopic anomalies suggest that they come from AGB stars, and also with gases rich in radioactive isotopes generated from the nucleosynthesis of elements in the interior. of such stars. These short-lived isotopes were incorporated into the chondrite-forming materials before they disintegrated and are therefore evidence of the proximity of these stars to the Sun.

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Silicon carbide presolar grain. The small scale corresponds to a micrometer.
(Courtesy Sachiko Amari)

In this way, by studying the isotopic peculiarities contained in these tiny meteoritic components, we can know the types of stars that assisted in the birth of the Sun, stars that helped form the solid materials from which everything we know would be built.

That the Sun was not born alone should not surprise us. We are used to seeing huge nebulae in which stars are forming, such as the Carina Nebula. In them we contemplate the birth of stars while others, outstanding sisters, shine in a surprising way, illuminating the gaseous environment while they begin to create other worlds.

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The Carina Nebula captured by the NIRIM camera of the James Webb Telescope.
(Image: NASA, ESA, CSA, STScI)

In light of everything learned about the Cosmos, humans are nothing but conscious star matter.

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