Star cluster ruled out as Sun’s birthplace

Astronomers have long wanted to pinpoint the location within our Galaxy where the Solar System was formed. Stellar cluster M67 has been suggested as the birthplace, but new computer detective work shows that this is not the case.

The star cluster M67. Credit: University of Massachusetts/CalTech/NASA

Most stars are formed within groups or clusters although isolated star formation can still occur, and it appears that our Sun likes to follow the crowd because evidence shows that it most likely formed among stellar companions. The Kuiper Belt contains many bodies on unusual orbits, such as the highly elliptical orbit of the minor planet Sedna.

“The best explanation for this is a flyby star that passed close to the Solar System,” says Barbara Pichardo from the Instituto de Astronomía, Universidad Nacional Autónoma de México. “This event could have happened only when the Sun was born, in its birth cluster.”

More evidence that the Sun was born in a cluster comes from meteorites found on Earth. Some meteorites were created billions of years ago around the time that the planets were being formed in the protoplanetary disc. They have thus “captured” the elements within the disc at the time of its formation just as a camera captures a moment in time. The meteorites contain short-lived radioisotopes, which were most likely produced by a massive star going supernova. For the meteorites to be enriched by such material, the young Solar System much have resided quite near to the massive star and as massive stars are rare, this must have happened in a cluster.

So if the Sun formed within a cluster, is the cluster still there today? And if so, which one is it? “It has been for a while a tempting idea that the Sun was born in M67, and it is clear why people thought that, those stars are the most similar stars to the Sun (even more than any star in the Solar Neighbourhood),” Pichardo tells Skymania News. Along with being chemically similar to stars in M67, the Sun is also of a similar age to the cluster.

However Pichardo and her colleagues have just published a paper where they used detailed computer simulations to show that the Sun was not born in M67. If it had been born in the cluster, then it would have had to be ejected in order to become the isolated star we know today. An interaction with a giant molecular cloud could cause this ejection, but the simulations show that the probability of this event is extremely low.

Another possible ejection method is an encounter between the Sun and a binary star system. The difference between the speed of the Sun and that of M67 is quite high, so for the Sun to be ejected at this speed it would have needed a very close encounter with a binary system. This would have put the Sun too close to the other stars and that would have completely destroyed the protoplanetary disc.

“We could have used a larger distance for the Sun from the binary, but in that case the velocity of the expulsion would have not been high enough to explain the difference in velocity between the Sun and M67,” adds Pichardo. “We needed 30 kilometres per second, which is A LOT, and only a violent expulsion could have explained it.” This high velocity also excludes the possibility that the Sun and M67 formed from the same molecular cloud, as the speeds within molecular clouds are much lower than this.

So if M67 has been ruled out, then where was the Sun born? “The natal cloud that gave birth to the Sun (and probably other stars -siblings of ours) probably disrupted a long time ago,” says Christine Allen, co-author on the paper. “But it is interesting to continue to look for such ‘siblings’.”

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