Tianguan

The 'horns' of Taurus the Bull are marked by two stars in the eastward parts of the constellation. At the northern horn-tip is Elnath, a blue giant about 130 light years from the Sun, while the southern horn is marked by Tianguan, another blue giant more than three times further away from the Sun than Elnath. Tianguan lies directly northward from Orion, and is the nearest bright star in the sky to Taurus' famous Crab Nebula (this is a line of sight effect: the Nebula is much more distant from the Sun than either of the two horn-stars).

Tianguan is a binary system: its primary component is a massive blue giant, but that giant star has something - its identity is uncertain - in a very close orbit around it. This companion has a mass only about a tenth that of the primary star, and orbits the blue giant at a distance only marginally greater than that of Earth from the Sun, causing the brightness of Tianguan to vary periodically. There is also evidence of circumstellar disc of matter in this system, emanating outwards from the blue star at its core.