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Alnilam
Alnilam is the central star of Orion's Belt in the equatorial constellation of Orion. It has the Bayer designation ε Orionis, which is Latinised to Epsilon Orionis and abbreviated Epsilon Ori or ε Ori. This is a massive, blue supergiant star some 1,250 light-years distant. It is estimated to be 270,000 times as luminous as the Sun, and 28 times as massive.
It is the 29th-brightest star in the sky (the fourth brightest in Orion) and is a blue supergiant. Together with Mintaka and Alnitak, the three stars make up Orion's Belt, known by many names across many ancient cultures. Alnilam is the middle star.
Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified, for the spectral class B0Ia. Although the spectrum shows variations, particular in the H-alpha absorption lines, this is considered typical for this type of luminous hot supergiant. It is also one of the 58 stars used in celestial navigation. It is at its highest point in the sky around midnight on December 15.
It is slightly variable from magnitude 1.64 to 1.74, with no clear period, and it is classified as an α Cygni variable. Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.
Estimates of Alnilam's properties vary. Crowther and colleagues, using stellar wind and atmospheric modelling in 2006, came up with a luminosity 275,000 times that of the Sun (L☉), an effective temperature of 27,000 K and a radius 24 times that of the Sun (R☉). Searle and colleagues, using CMFGEN code to analyse the spectrum in 2008, calculated a luminosity of 537,000 L☉, an effective temperature of 27,500 ± 100 K and a radius of 32.4 ± 0.75 R☉. Analysis of the spectra and age of the members of the Orion OB1 association yields a current mass 34.6 times that of the Sun, initial mass of 40.8 M☉, and an age of 5.7 million years.
A more detailed analysis from 2015 across multiple wavelength bands produced very high luminosity, radius, and mass estimates, assuming the distance of 606 parsecs suggested by the Hipparcos new reduction. Adopting the larger parallax from the original Hipparcos reduction gives a distance of 412 parsecs and physical parameters more consistent with earlier publications. The luminosity of 832,000 L☉ and the mass of 64.5 M☉ at 606 parsecs is the highest ever derived for this star. Using precalculated models, a 2020 study found smaller values for luminosity (420,000 L☉), radius (30.61 R☉), and mass (40 M☉). Another spectroscopic distance modulus of 7.79 imply a distance of 361 parsecs.
interferometric observations in 2025 with the Very Large Telescope have uncovered that Alnilam is a fast rotator being viewed at low inclination. As of such, it has an oblate shape, with a pronounced equatorial circumference that is 51% larger than the polar circumference, which at a distance of 384±8 pc results in polar and equatorial radii of 22.3 and 33.6 R☉. Rotating close to its breakup velocity, its fast rotation suggest Alnilam formed after the merger of two stars in a binary system.
Alnilam's relatively simple spectrum has made it useful for studying the interstellar medium. It is surrounded by a molecular cloud, NGC 1990, which it illuminates to make a reflection nebula. Its stellar winds may reach up to 2,000 km/s, causing it to lose mass about 20 million times more rapidly than the Sun.
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Alnilam
Alnilam is the central star of Orion's Belt in the equatorial constellation of Orion. It has the Bayer designation ε Orionis, which is Latinised to Epsilon Orionis and abbreviated Epsilon Ori or ε Ori. This is a massive, blue supergiant star some 1,250 light-years distant. It is estimated to be 270,000 times as luminous as the Sun, and 28 times as massive.
It is the 29th-brightest star in the sky (the fourth brightest in Orion) and is a blue supergiant. Together with Mintaka and Alnitak, the three stars make up Orion's Belt, known by many names across many ancient cultures. Alnilam is the middle star.
Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified, for the spectral class B0Ia. Although the spectrum shows variations, particular in the H-alpha absorption lines, this is considered typical for this type of luminous hot supergiant. It is also one of the 58 stars used in celestial navigation. It is at its highest point in the sky around midnight on December 15.
It is slightly variable from magnitude 1.64 to 1.74, with no clear period, and it is classified as an α Cygni variable. Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.
Estimates of Alnilam's properties vary. Crowther and colleagues, using stellar wind and atmospheric modelling in 2006, came up with a luminosity 275,000 times that of the Sun (L☉), an effective temperature of 27,000 K and a radius 24 times that of the Sun (R☉). Searle and colleagues, using CMFGEN code to analyse the spectrum in 2008, calculated a luminosity of 537,000 L☉, an effective temperature of 27,500 ± 100 K and a radius of 32.4 ± 0.75 R☉. Analysis of the spectra and age of the members of the Orion OB1 association yields a current mass 34.6 times that of the Sun, initial mass of 40.8 M☉, and an age of 5.7 million years.
A more detailed analysis from 2015 across multiple wavelength bands produced very high luminosity, radius, and mass estimates, assuming the distance of 606 parsecs suggested by the Hipparcos new reduction. Adopting the larger parallax from the original Hipparcos reduction gives a distance of 412 parsecs and physical parameters more consistent with earlier publications. The luminosity of 832,000 L☉ and the mass of 64.5 M☉ at 606 parsecs is the highest ever derived for this star. Using precalculated models, a 2020 study found smaller values for luminosity (420,000 L☉), radius (30.61 R☉), and mass (40 M☉). Another spectroscopic distance modulus of 7.79 imply a distance of 361 parsecs.
interferometric observations in 2025 with the Very Large Telescope have uncovered that Alnilam is a fast rotator being viewed at low inclination. As of such, it has an oblate shape, with a pronounced equatorial circumference that is 51% larger than the polar circumference, which at a distance of 384±8 pc results in polar and equatorial radii of 22.3 and 33.6 R☉. Rotating close to its breakup velocity, its fast rotation suggest Alnilam formed after the merger of two stars in a binary system.
Alnilam's relatively simple spectrum has made it useful for studying the interstellar medium. It is surrounded by a molecular cloud, NGC 1990, which it illuminates to make a reflection nebula. Its stellar winds may reach up to 2,000 km/s, causing it to lose mass about 20 million times more rapidly than the Sun.