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Eta Lupi

Proper NameCerberus
Bayer DesignationEta Lupi
Flamsteed NumberNone
HR (BSC)5948
Right Ascension16h 0m 7s
Declination-38° 23' 48"
Distance442 light years
136 parsecs
MagnitudeApparent: +3.43
Absolute: -2.23
Spectral ClassB2.5IV blue subgiant
Optimum VisibilityJune

One of a swarm of mostly blue stars that make up the constellation of Lupus the Wolf, lying along the clouded fringes of the Milky Way in the southern sky. Cerberus or Eta Lupi lies near the eastern borders of its constellation, where the Milky Way passes eastward from Lupus into the neighbouring constellation of Scorpius. The star takes its name from the three-headed guardian of Hades in Greek myth (a name it shares with a region on Mars, an asteroid and - with slightly different spelling - one of the moons of Pluto).

Cerberus shines with blue light at a visual magnitude of +3.43, though its absolute magnitude is a much brighter -2.23 (which means that, if it lay ten parsecs from the Sun, the star would be as bright as Jupiter in the night sky). Its true distance, based on parallax measurements, is some 442 light years. Cerberus belongs to the Scorpius-Centaurus Association, a group of more than four hundred stars whose motion through the Galaxy shows that they shared a common origin in the distant past, but which are now widely distributed across the sky.

Blue Cerberus in eastern Lupus shines at third magnitude. The star to the southeast (lower left) is a similar blue subgiant, fourth-magnitude HR 5967, in fact somewhat closer to the Solar System, but rather less luminous than Cerberus. Imagery provided by Aladin sky atlas

The luminous blue star (commonly classified as a subgiant, though in fact possibly a main sequence or dwarf star) is the primary component of a triple star system, and both of its companion stars follow distant orbits around this primary. Its inner companion is a white main sequence star some 2,000 AU distant (that is, approximately seventy times the distance of Neptune from the Sun). The outer companion is even more distant, an F-type dwarf star estimated to lie some nine times farther from the primary star, and to follow an orbit that takes approximately half a million years to complete.


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