Why the Seven Sisters so often show only six stars
Across the world, people pictured the Pleiades as seven sisters, and their stories often explain why one went missing. Some scientists suspect the tales reach back to around 100,000 BC, when the star Pleione sat farther from its neighbour Atlas and could be picked out by eye as a separate point.
The cluster lies about 444 light-years away in Taurus, closer than any other Messier object, and holds more than 1,000 stars, of which six or seven are easy to see without help. Its hot blue stars formed within the last 100 million years. The wisps of nebula around them were once taken for leftover birth material, but they now look like an unrelated dust cloud the cluster happens to be crossing. Simulations suggest the group began as a compact nursery resembling the Orion Nebula, and in roughly 250 million years galactic tides should pull it apart.
Few sky objects shaped calendars so widely. Around 2330 BC the cluster marked the vernal point, where the Sun crosses the celestial equator in spring. Babylonian catalogues called it MUL.MUL, star star, and placed it first among stars along the Moon's path. In ancient India it was the first lunar station, Kṛttikā, and early Arabs opened their seasonal calendar with it as al-Thurayyā. The oldest known depiction is probably the Nebra sky disc from Bronze Age Germany, dated to about 1600 BC. The Greek name may come from the verb to sail, because the cluster's dawn appearance opened the Mediterranean sailing season; the mother figure Pleione was likely invented later to explain it.
In Japan the eighth-century Kojiki calls the group Mutsuraboshi, six stars. Its modern Japanese name, Subaru, from a verb meaning to gather together, went to an 8.2-metre telescope on Mauna Kea and to a carmaker formed from five merged firms, whose logo shows six stars.
Galileo became the first to aim a telescope at the cluster, sketching 36 stars in his Sidereus Nuncius of March 1610. In 1767 John Michell reckoned the odds of so many bright stars lining up by chance at only 1 in 500,000, concluding they must be physically linked. Messier added it as M45 in 1771, oddly for a list of faint comet-like smudges, perhaps to outnumber his rival Lacaille's 42 objects.
Source: Pleiades
