Alpha Lupi
Brightest star in Lupus, a nearby supernova candidate.
Alpha Lupi, also known by the Bayer designation α Lupi and the name Uridim, is a binary star system and the brightest star in the southern constellation Lupus. With an apparent visual magnitude of 2.3 on the Bortle Dark-Sky Scale, it remains easily visible to the naked eye even in areas with heavy light pollution. Parallax data from the Hipparcos mission places it roughly 460 light-years from the Solar System, and it is considered one of the nearest supernova candidates.
The primary star is a giant of spectral type B1.5 III, with about ten times the mass of the Sun and a luminosity 18,000 times greater. Its outer atmosphere reaches an effective temperature of 24,550 K, giving it a blue-white glow characteristic of B-type stars. In 1956, it was identified as a Beta Cephei variable, meaning its brightness fluctuates periodically due to atmospheric pulsations. The variability period is 0.29585 days—just over 7 hours and 6 minutes—and the magnitude changes by about 0.05 magnitudes, or roughly 5% of its total brightness.
The secondary star was reported in 2023 following an interferometric search. It was detected at a separation that corresponds to a projected physical distance, and it is 5.14 magnitudes fainter than the primary in the K-band. Its mass has been measured, and its orbital period is expected to be close to a decade—short enough that orbital motion should be detectable. However, radial velocity observations have found no evidence of variations caused by the companion, likely because its semi-amplitude falls below detection limits. A 14th-magnitude star located 26 arcseconds from Alpha Lupi is also listed as a companion in double star catalogues.
Alpha Lupi is a proper-motion member of the Upper Centaurus–Lupus subgroup within the Scorpius–Centaurus OB association, the nearest co-moving association of massive stars to the Sun. This gravitationally unbound stellar association is estimated to be 16–20 million years old and is the source of a bubble of hot gas, known as the Local Bubble, that contains the Sun.
In Chinese astronomy, the asterism Kekouan (Imperial Guards) includes Alpha Lupi along with several other stars. The Chinese name for Alpha Lupi itself is derived from this asterism. R. H.
- Bayer designation
- α Lupi
- Apparent visual magnitude
- 2.3
- Distance
- ~460 ly
- Spectral type
- B1.5 III
- Mass
- ~10 solar masses
- Luminosity
- 18,000 solar luminosities
- Effective temperature
- 24,550 K
Lore & Background
The primary star is a giant star with a stellar classification of B1.5 III. It has about ten times the mass of the Sun yet is radiating 18,000 times the Sun's luminosity. The outer atmosphere has an effective temperature of 24,550 K, which gives it the blue-white glow of a B-type star. In 1956 it was identified as a Beta Cephei variable by Bernard Pagel and colleagues, which means it undergoes periodic changes in luminosity because of pulsations in the atmosphere. The variability period is 0.29585 days, or just over 7 hours, 6 minutes. The magnitude varies by about 0.05 magnitudes, or about 5% of its brightness.
The secondary was reported in 2023 after an interferometric search. It was detected at a separation of , corresponding to a projected separation of . It is 5.14 magnitudes fainter than the primary in the K-band and has a mass of . The orbital period is expected to be close to a decade, short enough for orbital motion to be detectable. Radial velocity observations have found no evidence for variations induced by the companion, although its semi-amplitude is likely below the detection limits. A 14th magnitude star situated 26" from Alpha Lupi is listed as a companion in double star catalogues.
Alpha Lupi is a proper motion member of the Upper Centaurus–Lupus sub-group in the Scorpius–Centaurus OB association, the nearest such co-moving association of massive stars to the Sun. This is a gravitationally unbound stellar association with an estimated age of 16–20 million years. The association is also the source of a bubble of hot gas that contains the Sun, known as the Local Bubble.
Reader's Guide
Alpha Lupi is notable as the brightest star in Lupus and one of the nearest supernova candidates, making it a target for studies of stellar evolution and explosive endpoints. Its classification as a Beta Cephei variable, identified in 1956, places it among pulsating stars whose brightness changes over a period of just over 7 hours. The discovery of a binary companion in 2023 via interferometry adds to its significance, as the orbital period is expected to be close to a decade, allowing future observations to trace orbital motion. The star's membership in the Upper Centaurus–Lupus sub-group of the Scorpius–Centaurus OB association ties it to the nearest co-moving group of massive stars, which is also responsible for the Local Bubble, a hot gas cavity containing the Solar System. Historically, the star has been known under various names, including the Chinese name Yang Mun or Men (南門), meaning 'the South Gate', and the Babylonian name 'Kakkab Su-gub Gud-Elim' (Star Left Hand of the Horned Bull). In 2024, the IAU Working Group on Star Names approved the name Uridim for this star, derived from the Sumerian name for the constellation Lupus.
Did You Know?
- Alpha Lupi is one of the nearest supernova candidates.
- It is a Beta Cephei variable with a period of 0.29585 days (about 7 hours, 6 minutes).
- The star's name Uridim was approved by the IAU Working Group on Star Names on 12 September 2024.
More in Variable Stars, Part 6 1-24
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