Alnilam lights up NGC 1990.
Photograph by Glen Youman
Epoch J2000 Equinox J2000
|Right ascension||05h 36m 12.8s|
|Declination||−01° 12′ 06.9″|
|Apparent magnitude (V)||1.69|
|Spectral type||B0 Ia|
|U−B color index||−1.03|
|B−V color index||−0.18|
|Variable type||α Cygni|
|Radial velocity (Rv)||25.9 km/s|
|Proper motion (μ)||RA: 1.49 mas/yr
Dec.: −1.06 mas/yr
|Parallax (π)||1.65 ± 0.45 mas|
|Distance||approx. 2,000 ly
(approx. 600 pc)
|Absolute magnitude (MV)||−6.95|
|Surface gravity (log g)||2.9 cgs|
|Rotational velocity (v sin i)||91 km/s|
It is the 30th brightest star in the sky (the 4th brightest in Orion) and is a blue-white supergiant. Together with Mintaka and Alnitak, the three stars make up the belt of Orion, known by many names across many ancient cultures. Alnilam is the middle star. It is slightly variable, from magnitude 1.64 to 1.74. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. 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. Alnilam's relatively simple spectrum has made it useful for studying the interstellar medium. Within the next million years, this star may turn into a red supergiant and explode as a supernova. It is surrounded by a molecular cloud, NGC 1990, which it illuminates to make a reflection nebula. Its stellar winds may reach up to 2000 km/s, causing it to lose mass about 20 million times more rapidly than the Sun.
Estimates of its 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☉), and 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 mass 34.6 times that of the Sun (40.8 M☉ on the main sequence) and an age of 5.7 million years.
Other names and history
The name Alnilam derives from the Arabic النظام an-niżām, related to the word نظم nażm "string of pearls". Related spellings are Alnihan and Alnitam: all three variants are evidently mistakes in transliteration or copy errors.
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. The belt was also the Three Stars mansion (simplified Chinese: 参宿; traditional Chinese: 參宿; pinyin: Shēn Xiù), one of the Twenty-eight mansions of the Chinese constellations. It is one of the western mansions of the White Tiger.
In pre-Christian Scandinavia, the belt was known as Frigg's Distaff (Friggerock) or Freyja's distaff. Similarly Jacob's Staff and Peter's Staff were European biblical derived terms, as were the Three Magi, or the Three Kings. Väinämöinen's Scythe (Kalevala) and Kalevan Sword are terms from Finnish mythology.
The Seri people of northwestern Mexico call the three belt stars Hapj (a name denoting a hunter) which consists of three stars: Hap (mule deer), Haamoja (pronghorn), and Mojet (bighorn sheep). Hap is in the middle and has been shot by the hunter; its blood has dripped onto Tiburón Island.
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