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# 9.8: Why the Solar System Collapsed

When scientists began to study meteorite compositions in the 1970s, an interesting mystery soon emerged. High-temperature minerals, like those containing aluminum and calcium, were among the first things to condense out of the solar nebula. Inclusions of these minerals are found in certain carbonaceous chondrite meteorites. When scientists studied these inclusions, they found peculiar abundances of certain isotopes. Most elements in nature occur in one main, stable isotopic form, as well as several rarer isotopic forms, some stable and some unstable (or radioactive). Researchers found certain isotopes in the meteorites that could only have been created very shortly before the Solar System itself.

Some carbonaceous chondrite meteorites. From left to right: Allende, Tagish Lake and Murchison. These meteorites can tell us about the earliest times in our solar system formation. Click here for original source URL

Astronomers studying meteorites found xenon-129, a form of xenon that arises from the decay of radioactive iodine-129. This decay process is very fast, geologically speaking. Once the iodine is created, the half-life of iodine-129 is only 17 million years. Xenon is chemically inert and unlikely to form mineral grains, so the chemical must have been trapped before it decayed, when it was still iodine. That leaves only a brief interval of 1 to 20 million years between the creation of the iodine and its incorporation into meteorite material. These inclusions were among the first minerals condensed in the solar nebula, and they are the main minerals to get a dose of the mysterious iodine. We must conclude that the iodine was injected essentially at the time the Solar System was born.

Where did the radioactive iodine in meteorites come from?  How was it trapped in the first planetary material? We believe the iodine and other isotopes were created by nuclear reactions inside nearby massive stars, which burn their nuclear fuel quickly and then explode in supernovae. The evidence indicates that such a star exploded near the pre-solar nebula, spewing short-lived radioactive isotopes and other debris into the cloud that would become the solar nebula.

In fact, the blast from the explosion probably instigated the compression of the pre-solar cloud, pushing the atoms closer together and thus helping to initiate the collapse that produced the Sun. We may not only have solved the mystery of iodine isotopes, but also discovered why the pre-solar cloud collapsed — the death of another star triggered the birth of our star. This cycle may be common among stars. Stars form in clusters, embedded in a region of dense interstellar gas. The largest stars in the cluster tend to be unstable and explode, thus compressing nearby gas and initiating further star formation.