Pedagoguery

Our solar system is isolated, with other stars light years away. Only 11 stars exist within 10 light years of us, and most of them are dim red dwarfs. It was thought that it was always so, and that the sun was born an isolated star. However, most young stars we see exist as part of clusters, ranging in size from around 100 to tens of thousands of stars, all born out of the same gigantic molecular cloud. Why should our sun be any different?

The first evidence that our sun was born as part of a cluster emerged in 2003, when a chemical analysis of an ancient meteorite was done. This meteorite were thought to be some of the most pristine leftovers of our solar system's formation. However, the scientists discovered nickel 60 in a compound that normally contains iron. Nickel 60 is the byproduct of the radioactive decay of iron 60, which itself has a half life of about 2.6 million years. Therefore, the nickel had to have come from iron 60 that was incorporated into the compound. Where did the iron 60 come from? Given its half life, it had to have come from a supernova explosion that was nearby, when the sun was only about 1.8 million years old, and the supernova had to have been within 5 light years. The chances of a massive star just happening to wander by and explode are miniscule. The far more likely explanation is that it was a massive star born as part of the same cluster as the sun.

When you look at existing young clusters, you see a range of stellar masses, with lots of smaller starts and a few giants. It seems likely that the same thing happened with the sun's formation. A likely scenario is this. A large molecular cloud starts to collapse. Within the cloud there develops a few massive stars. As the stars ignite, they pour out tremendous amounts of ultraviolet radiation, ionizing the cloud around them. The ionization proceeds out from the star in a spherical front, proceeded by a shock wave. As the shock wave hits nearby clumps of gas that are relatively denser than the surrounding medium, it causes them to collapse, forming a smaller star. Our sun was one such. Eventually, the ionization front reaches the newborn star, and starts to boil off the gas, perhaps leaving a “finger” of gas connecting it to the surrounding cloud. We can see similar such structures today, such as the Eagle Nebula in the famous “Pillars of Creation” picture by the Hubble Space Telescope.

Within about 10,000 years of the ionization front reaching the solar system, the loose gas in the protoplanetary disk has been driven off. Over the next 10,000 years, the disk itself is eroded, and completely eliminated beyond about 50 Astronomical Units (AU) away from the sun. This also likely inhibited the growth of Uranus and Neptune.

After about another 2 million years, the massive star goes supernova. This rains debris onto our solar system, including many heavy elements and radioisotopes. The decay of these elements causes the bodies containing them to heat up, which is why some asteroids show evidence of melting. These are also incorporated into planets, and power early geologic activity in the terrestrial planets. This also enriches the whole solar system, including the sun, with heavy elements. The sun appears to have a greater abundance of heavy elements than its location would otherwise indicate, and this is likely the reason why.

Sometime within the next 100 million years, another star in the cluster passes within a few thousand AU of the sun, stirring up the comets in the Oort Cloud and giving many of them inclined orbits. This would also have dramatic effects on the solar system itself, since many comets would be send into the inner solar system. This may well be one of the causes of a period called the Late Heavy Bombardment, which is where most of the moon's craters come from.

As the heavy stars in the cluster age and explode, the gravitational force holding the cluster together weakens. That, combined with close encounters between stars and the rotation of the galactic disk itself, disperses the cluster. Over the 4.6 billion years since the solar system's formation, the original inhabitants of the cluster are probably strung in an arc from a third to a half of the circumference of the galactic disk at our radius.

It is possible to identify the long lost siblings of our sun through detailed chemical analysis of other stars. Members of the original natal cluster of the sun would have a distinctive chemical signature, very similar to our sun's. They would also likely have a predictable location in the galaxy. By looking in the likely places, we can find our sun's siblings, and through that, learn something more about the early history of our solar system.

Next time, more speculations on the multiverse.