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.[5] Although the spectrum shows variations, particular in the H-alphaabsorption lines, this is considered typical for this type of luminous hot supergiant.[10] 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.[11] Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.[10]
Physical characteristics
Estimates of Alnilam's properties vary. 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☉.[8] Analysis of the spectra and age of the members of the Orion OB1 association yields a mass 34.6 times that of the Sun (40.8 M☉ on the main sequence) and an age of 5.7 million years.[13] A more recent detailed analysis of Alnilam across multiple wavelength bands produced very high luminosity, radius, and mass estimates, assuming the distance of 606 parsecs suggested by the Hipparcos new reduction.[2] Adopting the larger parallax from the original Hipparcos reduction gives a distance of 412 parsecs[14] 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.[10] Using precalculated models, a 2020 study found smaller values for luminosity (420,000 L☉), radius (30.61 R☉), and mass (40 M☉).[9] Another spectroscopicdistance modulus of 7.79 imply a distance of 361 parsecs.[7]
The traditional name Alnilam derives from the Arabic النظام al-niẓām 'arrangement/string (of pearls)'. Related spellings are Alnihan and Alnitam:[17] all three variants are evidently mistakes in transliteration or copy errors, the first perhaps due to confusion with النيلم al-nilam 'the sapphire'.[18] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[19] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[20] included a table of the first two batches of names approved by the WGSN; which included Alnilam for this star. It is now so entered in the IAU Catalog of Star Names.[21]
The three belt stars were collectively known by many names in many cultures. Arabic terms include Al Nijād ('the Belt'), Al Nasak ('the Line'), Al Alkāt ('the Golden Grains or Nuts') and, in modern Arabic, Al Mīzān al H•akk ('the Accurate Scale Beam'). In Chinese mythology, they were also known as the Weighing Beam.[17]
^Calculated, using a distance modulus of 7.79 in the following equation: . This yields a value of 361 parsecs.
References
^Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN978-1-931559-44-7.
^ abcDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
^ abRuban, E. V.; Alekseeva, G. A.; Arkharov, A. A.; Hagen-Thorn, E. I.; Galkin, V. D.; Nikanorova, I. N.; Novikov, V. V.; Pakhomov, V. P.; Puzakova, T. Yu. (September 2006). "Spectrophotometric observations of variable stars". Astronomy Letters. 32 (9): 604–607. Bibcode:2006AstL...32..604R. doi:10.1134/S1063773706090052. S2CID121747360.
^ abOplištilová, A.; Mayer, P.; Harmanec, P.; Brož, M.; Pigulski, A.; Božić, H.; Zasche, P.; Šlechta, M.; Pablo, H.; Kołaczek-Szymański, P. A.; Moffat, A. F. J.; Lovekin, C. C.; Wade, G. A.; Zwintz, K.; Popowicz, A.; Weiss, W. W. (2023). "Spectrum of the secondary component and new orbital elements of the massive triple star δ Ori A". Astronomy and Astrophysics. 672: A31. arXiv:2301.10290. Bibcode:2023A&A...672A..31O. doi:10.1051/0004-6361/202245272. S2CID256226821.
^Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; Hoeg, E.; Bastian, U.; Bernacca, P. L.; Crézé, M.; Donati, F.; Grenon, M.; Grewing, M.; Van Leeuwen, F.; Van Der Marel, H.; Mignard, F.; Murray, C. A.; Le Poole, R. S.; Schrijver, H.; Turon, C.; Arenou, F.; Froeschlé, M.; Petersen, C. S. (1997). "The HIPPARCOS Catalogue". Astronomy and Astrophysics. 323: L49. Bibcode:1997A&A...323L..49P.