Most stars are born in clusters rather than singly, and there’s plenty of evidence that the Sun was too.
For one thing, the material of the infant solar system (as preserved in the earliest meteorites) was enriched by fresh supernova debris from at least one very young, massive star (having 15 to 25 solar masses) that exploded less than 5 light-years away, no more than 2 million years after the Sun's formation. Today no such massive star exists within 300 light-years of the Sun. Clearly, the early solar system had stars close around it.
But that was 4.57 billion years ago. Where are the Sun’s cluster-mates now?
Some of them, it turns out, should remain surprisingly nearby. An analysis by Simon F. Portegies Zwart (University of Amsterdam) finds that the Sun’s birth cluster started off with about 500 to 3,000 solar masses and a diameter smaller than about 20 light-years — typical for open clusters. Evidence for the cluster's mass and size, Zwart writes, is preserved in the anomalous chemical abundances and structure of the solar system's Kuiper Belt — the realm of small, icy objects out beyond Neptune. Some of the Kuiper Belt's objects are dynamically "hot"; they were stirred up and scattered by the gravity of at least one nearby cluster star making a close pass in early days.
Like other open clusters, the Sun's birth cluster disintegrated with time. Most of its stars have long since drifted away and are mixed irretrievably into the swarms of the Milky Way — strung out during the approximately 27 orbits that they and the Sun have made around the galaxy since their origin long ago.
However, about 10 to 60 of the Sun's nestmates (a few percent) should still remain closer than 300 light-years from us and are still traveling in parallel with us, according to Portegies Zwart. The European Space Agency's upcoming Gaia astrometry satellite should be able to sort them out by their space motions. Their exact chemical abundances might then give them away for sure.
“Finding even a few,” writes Portegies Zwart, “will strongly constrain the parameters of the parental star cluster and the location in the Galaxy where we were born.”
Don't expect them to look like anything special, though. The Sun is very ordinary among stars, and so were the circumstances of its origin.
A draft preprint of the research paper appears on the Astro-ph preprint server (March 2, 2009).