Alnilam

Alnilam, Epsilon Orionis (ε Ori), is a bright blue supergiant located at a distance of 1,975.8 light years from Earth in the direction of Orion constellation. With an average apparent magnitude of 1.69, it is the 29th brightest star in the sky and 4th brightest in Orion, after Rigel, Betelgeuse and Bellatrix. It is only slightly fainter than Elnath in Taurus constellation and Miaplacidus in Carina, and it just outshines Alnair in Grus, its Orion neighbour Alnitak, and Alioth, the brightest star in Ursa Major.

Alnilam is the middle star of Orion’s Belt, one of the best known asterisms in the night sky, also formed by the bright Alnitak and Mintaka.

Star type

Alnilam is a supergiant star of the spectral type B0 Ia, appearing bluish or blue-white in colour. It is about 40 times more massive than the Sun and has a radius 32.4 times solar. Its estimated luminosity is in the range between 275,000 and 832,000 times that of the Sun.

The star is classified as an Alpha Cygni-type variable. It changes in brightness from magnitude 1.64 to 1.74. Alpha Cygni stars are supergiants with the spectral classification B or A that exhibit changes in brightness as a result of non-radial pulsations, with some portions of their surface contracting while others simultaneously expand.

Alnilam and Orion’s Belt, image: Wikisky

Alnilam’s physical properties are still mostly uncertain, as different surveys have yielded different results over the years. In 2006, a team of scientists used atmospheric modelling and found that the star had a radius of 24 solar radii, a luminosity 275,000 times that of the Sun and a temperature of 27,000 K. Two years later, a different team analysed Alnilam’s spectrum and calculated a radius of 32.4 times that of the Sun, a luminosity of 537,000 solar luminosities and a temperature of 27,500 K.

A recent survey of the star across several wavelengths, assuming the distance of 2,000 light years, gave a luminosity of 832,000 L_☉, the highest to date for the star.

Alnilam’s stellar winds can reach speeds of up to 2,000 km/s and, as a result, the star loses mass at a rate 20 million times greater than the Sun. Even though it is only a few million years old, Alnilam is already fusing heavy elements in its core. It may evolve into a red supergiant considerably more luminous than Betelgeuse in the next million years, before it inevitably goes out as a supernova.

Orion’s Belt

Alnilam is the central star of Orion’s Belt, an asterism known since ancient times, also formed by the bright stars Alnitak and Mintaka. Alnilam is the brightest of the three despite being the most distant. It is also the only single star of the three. Alnitak and Mintaka are both multiple star systems.

Orion’s Belt, image: Davide De Martin (http://www.skyfactory.org); Credit: Digitized Sky Survey, ESA/ESO/NASA FITS Liberator

Even though Alnilam appears closer to the stars to its right and left, Alnitak and Mintaka are in fact closer together because they lie at similar distances to Earth (Alnitak at 1,260 light years and Mintaka at 1,200 light years).

The three stars have a common origin and share the same proper motion, which is why Orion’s Belt has retained an almost identical shape since prehistoric times.

Orion’s Belt stars have been known across many different cultures since ancient times. The English folk names for the three stars include Peter’s Staff, Jacob’s Rod (Staff), the Yard-stick, the Golden Yard-arm, the Three Kings (the Magi), the Three Marys and Our Lady’s Wand.

In South America and Portugal, the stars are known as “Las Tres Marías” (Spanish) and “As Três Marias” (Portuguese). In Puerto Rico and the Philippines, they are associated with the Biblical Magi and called “Los Tres Reyes Magos.”

Arabic names for the stars include Al Nijād (the Belt), Al Alkāt (the Golden Nuts) and the modern Arabic name, Al Mīzān al Ḥaqq (the Accurate Scale Beam), while the Chinese know them as the Weighing Beam. The Chinese also call them Three Stars (參宿), in reference to an asterism originally formed by the three Belt stars. Other bright stars of Orion – Betelgeuse, Rigel, Bellatrix and Saiph – were added to the asterism later. Alnilam’s Chinese name is the Second of Three Stars (參宿二).

Ancient Egyptians associated Orion and Sirius with Osiris and his wife, and believed that gods came from Orion’s Belt and Sirius to start the human race. The air shafts in the pyramids, believed to be there to send the pharaoh’s soul toward Orion, point toward the constellation. The alignment of the three pyramids of Giza is believed by some to represent the Belt of Orion, but the Orion correlation theory has been heavily contested.

Facts

Alnilam is one of the four Orion stars used in celestial navigation, along with Rigel, Betelgeuse and Bellatrix, the stars that outline Orion’s hourglass shape together with Saiph, the constellation’s sixth brightest star.  Navigational stars are selected both because they are very bright and because they are easy to identify. Many of them are either part of a prominent asterism or located near a recognizable pattern.

Alnilam was reported to be surrounded by a reflection nebula, designated NGC 1990, but it is unclear whether or not this object really exists because modern instruments have not been able to pinpoint its location and size. The nebula was first reported by the German-British astronomer William Herschel in 1786.

Name

The name Alnilam (pronunciation: /ˈælnɪlæm/) was officially approved by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on July 20, 2016. It is derived from the Arabic al-niẓām, meaning “string of pearls.”

Location

Alnilam and other bright stars of Orion are very easy to find because the constellation’s hourglass shape dominates the evening sky during the northern hemisphere winter. With the bright Betelgeuse and Bellatrix marking the Hunter‘s shoulders and Saiph and Rigel, his feet, the stars of Orion’s Belt, standing prominently in the middle, are easy to identify. Alnilam lies between Alnitak on the left and Mintaka on the right.

Orion stars, image: Wikisky

Orion’s Belt stars can be used to find other bright stars in the vicinity. Going east, they point in the direction of Sirius in Canis Major, the brightest star in the sky, and to the west they point toward Aldebaran, the brightest star in Taurus.

The best time of year to observe Orion’s Belt is in mid-December, when it reaches its highest point in the sky.

Orion’s Belt and the Orion Nebula (M42), image: Wikisky

Constellation

Alnilam is located in the constellation Orion, one of the most recognizable constellations in the sky. Orion is known for its exceptionally bright stars – mainly the first-magnitude supergiants Rigel and Betelgeuse – and several bright deep sky objects. These include the Orion Nebula (Messier 42) with the Trapezium Cluster, the neighbouring De Mairan’s Nebula (Messier 43), the emission nebulae Barnard’s Loop and the Flame Nebula (NGC 2024), the reflection nebula Messier 78, and the dark nebula Barnard 33, also known as the Horsehead Nebula.

Orion constellation map by IAU and Sky&Telescope magazine

Orion is a prominent feature of the night sky from November to May in the northern hemisphere and can also be seen in the early morning hours from late July to November. For southern observers, Orion is best seen during the summer months, appearing upside down.

The 10 brightest stars in Orion are Rigel (Beta Ori, mag. 0.05 – 0.18), Betelgeuse (Alpha Ori, mag. 0.0 – 1.3), Bellatrix (Gamma Ori, mag. 1.59 to 1.64), Alnilam (Epsilon Ori, mag. 1.64 – 1.74), Alnitak A (Zeta Ori A, mag. 2.00), Saiph (Kappa Ori, mag. 2.09), Mintaka AB (Delta Ori AB, mag. 2.23), Hatysa (Iota Ori, mag. 2.77), Tabit (Pi³ Ori, mag. 3.16), and Eta Orionis (mag. 3.31 – 3.6).

Alnilam – Epsilon Orionis