The bull's face in Taurus is a star family slowly drifting apart
The V of stars forming the face of Taurus looks like one group, but its brightest point, orange Aldebaran, is an impostor only 65 light-years away. The rest belong to the Hyades, the nearest open star cluster, about 153 light-years off. Born together roughly 625 million years ago, its members are gradually escaping into the galaxy.
Hundreds of stars in the Hyades share an age, a birthplace, a chemical makeup and a direction of travel. Its five brightest members have exhausted the hydrogen in their cores and swollen into giants; one of them, Epsilon Tauri, nicknamed the Bull's Eye, hosts a candidate giant planet, the first found in any open cluster. The stars are richer in heavy elements than the Sun, and their motion and age match the more distant Praesepe cluster, suggesting the two were born in the same region.
Because the Hyades is so close, astronomers can measure its distance directly, by watching member stars shift slightly as Earth circles the Sun, using the Hipparcos satellite and the Hubble telescope. A completely separate method, comparing the stars' colours and brightness with standard models, gives the same answer. That agreement makes the cluster a key rung on the ladder astronomers climb to measure distances to other galaxies.
Most star clusters disperse within 50 million years, a process called evaporation, so the Hyades is a survivor. It probably began with 800 to 1,600 times the Sun's mass. Heavy stars have sunk toward its centre, where the inner two parsecs hold only systems at least as massive as the Sun, while lighter stars get flung into the outer halo and then stripped away by the galaxy's tides. About a third of confirmed members already sit beyond the cluster's gravitational boundary, and many of its small red dwarfs seem to have gone.
Humans have watched these stars since prehistory. In Greek myth they were five daughters of Atlas, weeping for their dead brother Hyas and linked ever after with rain. Homer's Iliad places them on the shield Hephaistos forged for Achilles, and English folk knew them as the April Rainers. Charles Messier left them out of his 1781 catalogue, so they have no Messier number.
Source: Hyades (star cluster)
