Many stars are born in clusters of hundreds or thousands crowded within a diameter of a few light-years. These clusters usually disperse over a period of billions of years. New evidence suggests that our sun also originated in a cluster, and thus has many sibling stars that have scattered, leaving our sun relatively isolated.
In 2003 scientists analyzed two meteorites dating from the formation of the solar system. They detected nickel 60, the product of radioactive decay of iron 60, in chemical compounds where iron would normally be found. This indicates that the compounds originally formed from iron, which then metamorphosed into nickel. The iron had to form, reach the solar system, and be incorporated into the meteorites fairly quickly, before it turned into nickel.
Therefore the iron must have originated nearby, probably from a supernova explosion within five light-years of the sun when the sun was about 1.8 million years old. If the sun had been as secluded as it is today, a supernova so close by would have been extremely unlikely. However, it would have been far more probable if the exploding star and the newborn sun were packed close together as part of a cluster.
This hypothesis could explain the puzzling levels of heavy elements in the sun. Generally, the farther a star's birthplace is from the galactic center, the poorer the star is in heavy elements. However, a nearby supernova that seeded the meteorites with iron 60 could also have enriched the sun with these elements.
The odd, skewed orbits of many comets also suggest that the sun once belonged to a cluster. The internal dynamics of the solar system cannot account for these orbits, since the comets are beyond the gravitational influence of the major planets. These comets were most likely stirred up by a star passing nearby, which would have been improbable unless the sun had many neighbors.
As further evidence, astronomers are looking for stars similar to the sun in an arc along which the hypothetical cluster would have dispersed. These stars should have a composition much like the sun's, since they would have been enriched by the same supernova that enriched the early solar system.
