Upsilon Orionis
Blue-white main sequence star and suspected Beta Cephei variable.
Upsilon Orionis (υ Ori, υ Orionis) is a blue-white main sequence star in the constellation Orion, traditionally named Thabit or Tabit (Arabic for 'the endurer'). It is a suspected Beta Cephei variable with an apparent magnitude of 4.62, located over 1,300 light-years from the Solar System. The star is notable as a spectral standard for the O9.7V type and as one of the most massive members of the Orion OB1c association.
- Apparent magnitude
- 4.62
- Distance
- approximately 1,325 light-years (parallax) or 1,260 light-years (spectroscopic)
- Spectral type
- B0 main sequence (standard), also classified as O9V, O9.5V, and O9.7V
- Radius
- 5.5 solar radii
- Luminosity
- 32,000 solar luminosities
- Effective temperature
- 32,900 K
- Mass
- 17.5 solar masses
Lore & Background
Upsilon Orionis lies south of Iota Orionis and, together with 29 Orionis, marks the top of Orion's right boot in Johann Bayer's Uranometria (1603). John Flamsteed assigned it the number 36. Its proper name appears to derive from the Arabic Al Thabit, meaning 'the endurer'; American amateur naturalist Richard Hinckley Allen noted in 1899 that the name appeared on Elijah Hinsdale Burritt's star atlas Geography of the Heavens, though its ultimate origin was unknown. Since 1943, the star has been consistently defined as a B0 main sequence reference for the MK spectral classification system, though other studies have classified it as O9V or O9.5V. The Galactic O-Star Spectroscopic Survey designated it the standard star for the O9.7V spectral type in 2011.
Reader's Guide
Upsilon Orionis holds significance as a long-standing spectral standard for early-type stars, used since 1943 to calibrate the MK classification system. Its classification has varied between B0, O9V, O9.5V, and O9.7V, reflecting the challenges in precisely typing hot, massive stars. In 1981, nonradial pulsations with a period of about 12 hours were observed, leading to its classification as a slowly pulsating B star. Later analysis of Hipparcos data suggested it is most likely a Beta Cephei variable, a class of blue-white main sequence stars 10–20 times the Sun's mass that pulsate with periods of 0.1–0.3 days; their magnitude changes are more pronounced in ultraviolet light. The American Association of Variable Star Observers lists it as a Beta Cephei variable. With a mass of 17.5 solar masses and a luminosity 32,000 times that of the Sun, it is one of the most massive stars in the Orion OB1c association, which lies in Orion's Sword. Its distance is estimated at about 1,325 light-years from parallax and 1,260 light-years from spectroscopic measurements.
Did You Know?
- Upsilon Orionis shares the traditional name Thabit (or Tabit) with Pi3 Orionis.
- It has been classified as B0, O9V, O9.5V, and O9.7V by different studies.
- The star was observed in 1981 to have nonradial pulsations over a period of about 12 hours.
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