These seven names are today the names of the seven stars visible to the naked eye in the star cluster called the Pleiades. (Many people can make out only six stars of the seven, and extra keen sight is needed to be able to spot the seventh, Sterope. See her name in this respect.)
There are two rival derivations for the name. The first takes it from peleia, 'pigeon', 'dove', so that the seven are a flock of pigeons. Zeus, it seems, set the seven sisters in the sky as seven pigeons to save them from the lust of Orion. (This story overlooks the fact that Orion went too, and today the constellation that bears his name can still be seen 'chasing' the Pleiades across the sky!)
A variant of this, with no reference to pigeons, is that the sisters were so distressed at the death of the Hyades, who according to one story were also the daughters of Atlas and Plei'one, that they killed themselves. Zeus therefore transformed them into stars.
The second theory derives their name from pleo, 'to sail'. This is because the seven stars 'rise' in the spring which marked the start of the sailing season for the ancient Greeks.
Other explanations for the name say that it comes from the phrase to pleion, and so means 'the full ones', since the stars are visible at a time when the earth is 'full' of crops, or that the origin is in to polein, 'the turners' (poleo, 'to go about' or pelo, 'to be in motion', as English 'pole'), since they 'turn' in the sky or in time.
There is also the completely unimaginative explanation, of course, that the seven get their name from their mother! As an astronomical name, Sterope is today known in the form Asterope for the faintest star in the cluster.
