Circumstellar habitable zone
Circumstellar habitable zone.Horologium is a constellation of six faintly visible stars in the southern celestial hemisphere. It was first described by the French astronomer Nicolas-Louis de Lacaille in 1756 and visualized by him as a clock with a pendulum and a second hand. The boundaries of Horologium (literally 'an instrument for telling the hour') were specified in 1922 by the International Astronomical Union, and it has since been one of their designated constellations. All parts of the constellation are visible to observers south of 23°N. The constellation's brightest star – and the only one brighter than an apparent magnitude of 4 – is Alpha Horologii (at 3.85), an ageing orange giant star that has swollen to around 11 times the diameter of the Sun. The long-period variable-brightness star, R Horologii (4.7 to 14.3), has one of the largest variations in brightness known for stars in the night sky visible to the unaided eye. Four star systems in the constellation are known to have exoplanets; one – Gliese 1061 – contains an exoplanet in its habitable zone.