λ Orionis (Meissa) is the head of the constellation Orion.
Epoch J2000 Equinox J2000
|Right ascension||05h 35m 08.27761s|
|Declination||+09° 56′ 02.9611″|
|Apparent magnitude (V)||3.54 / 5.61|
|Spectral type||O8 III + B0.5 V|
|U−B color index||–1.01 / –0.77|
|B−V color index||–0.21 / +0.04|
|Radial velocity (Rv)||+33.5 km/s|
|Proper motion (μ)||RA: –0.34 mas/yr
Dec.: –2.94 mas/yr
|Parallax (π)||2.97 ± 0.55 mas|
|Distance||approx. 1,100 ly
(approx. 340 pc)
|Absolute magnitude (MV)||–4.25 / -1.94|
|Mass||27.9 ± 3.3 M☉|
|Surface gravity (log g)||4.0 cgs|
|Temperature||35,046 ± 454 K|
|Age||3.0 ± 0.8 Myr|
Meissa (Lambda Orionis, λ Ori) is a star in the constellation Orion. "Meissa" derives from the Arabic "Al-Maisan" which means "The Shining One". This term was used for Gamma Gemini (Alhena), but was somehow also mistakenly applied to Meissa and the name stuck. The original Arabic name for this star, "Al Hakah" (the source for another name for it, "Heka") refers to the Arabic lunar mansion that includes this star and the two of φ Ori (Al Haḳʽah, "a White Spot").
In Chinese, 觜宿 (Zī Sù), meaning Turtle Beak (asterism), refers to an asterism consisting of Meissa and both of φ Ori Consequently, Meissa itself is known as 觜宿一 (Zī Sù yī, English: the First Star of Turtle Beak.)
Meissa is a giant star with a stellar classification of O8 III and an apparent visible magnitude 3.54. It is an enormous star with about 28 times the mass of the Sun and 10 times the Sun's radius. The outer atmosphere has an effective temperature of around 35,000 K, giving it the characteristic blue glow of a hot O-type star. Meissa is a soft X-ray source with a luminosity of 1032 erg s−1 and peak emission in the energy range of 0.2–0.3 keV, which suggests the X-rays are probably being generated by the stellar wind.
This star is the dominant member of a 5-million-year-old star-forming region known as the λ-Orionis cluster, or Collinder 69. The intense ultraviolet energy being radiated by this star is creating the Sh2-264 H II region in the neighboring volume of space, which in turn is surrounded by an expanding ring of cool gas that has an age of about 2–6 million years. The expansion of this gaseous ring may be explained by a former binary companion of Meissa that became a Type II supernova. Such an event would also explain the star's peculiar velocity with respect to the center of the expanding ring, as the explosion and resulting mass loss could have kicked Meissa out of the system. A potential candidate for the supernova remnant is the neutron star Geminga. However, the last is unlikely given the distance between Geminga and the cluster.
Meissa is actually a double star with a companion at an angular separation of 4.41 arcseconds along a position angle of 43.12° (as of 1937). This fainter component is of magnitude 5.61 and it has a stellar classification of B0.5 V, making it a B-type main sequence star. There is an outlying component, Meissa C, which is an F-type main sequence star with a classification of F8 V. This star in turn may have a very low mass companion that is probably a brown dwarf.
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