Elthor

A triple star system in the constellation of Taurus, Elthor lies almost directly southward from the Pleiades, and southwestward from the Hyades that form the 'face' of Taurus. It was traditionally said to represent the Bull's chest, and its name Elthor (along with several variant spellings) simply derives from the Arabic for 'the Bull'. The system is estimated to lie some 484 light years from the Solar System.

At the centre of the system is a very close binary pair, Lambda Tauri A and B, which orbit one another at a distance of just 0.1 AU (so close, indeed, that they are thought to exchange material with one another). This central pair of stars form an eclipsing binary: they are oriented so that they pass across one another as seen from Earth, causing the magnitude of the system to vary considerably. Elthor's maximum visual magnitude is +3.31, but during an eclipse (which occurs every 1.1 days) it can fall to +3.97.

The two stars A and B are both larger and more massive than the Sun. One is a hot and highly luminous blue main sequence star, generating nearly six thousand times as much light of the Sun, while its companion is a white subgiant. The subgiant is itself highly luminous, but much less so than its brilliant neighbour.

Orbiting this pair at a distance of about 0.4 AU (a distance comparable with Mercury's orbit around the Sun) is a third member of the system, Lambda Tauri C. This is a dwarf star, tiny by comparison with the other two members of the system, and thought to be rather less massive than the Sun. Nonetheless, its proximity to the two massive inner stars is sufficient that its meagre gravity can have detectable effects on the two main stars' orbit around one another.