Nunki

sigma sagittarii

Nunki, Sigma Sagittarii (σ Sgr), is a hot blue-white main sequence star located in the constellation Sagittarius. It is the second brightest star in the constellation, after Kaus Australis. With an apparent magnitude of 2.05, it is the 52nd brightest star in the sky, only slightly fainter than Algieba in Leo, Hamal in Aries and Diphda in Cetus, but outshining Menkent in Centaurus and Alpheratz and Mirach in Andromeda.  Nunki lies at a distance of 228 light years from Earth. In Sagittarius, it marks the vane of the celestial Archer’s arrow.

sigma sagittarii
Nunki (Sigma Sagittarii), image: Wikisky

Star type

Nunki is a hydrogen-fusing dwarf with a mass 7.8 times that of the Sun and a radius 4.5 times solar. It is 3,300 times more luminous than the Sun, with a surface temperature of 18,890 K. The star emits much of its light in the ultraviolet spectrum. Its estimated age is 31.4 million years. An optical companion, a 10th magnitude star, lies about 5.2 arcminutes away.

Nunki’s high mass and fast internal fusion rates indicate that it will not live very long. Even though it is still very young, the star will probably end its life within the next 50 million years. At the end of its life cycle, it will turn into a white dwarf with a mass comparable to that of the Sun.

Name

The name Nunki (pronunciation: /ˈnʌŋki/) has an Assyrian or Babylonian origin, but its meaning is unclear. The name was recovered by archaeologists and revealed by Richard Hinckley Allen in his Star Names: Their Lore and Meaning (1899). Sigma Sagittarii was identified with Nunki, the Star of the Proclamation of the Sea, as mentioned in the Euphratean Tablet of the Thirty Stars. The sea in question refers to the area of the sky occupied by several constellations associated with water: Delphinus (the Dolphin), Aquarius (the Water Bearer), Capricornus (the Sea Goat), Pisces (the Fish) and Piscis Austrinus (the Southern Fish). The Greek astronomer Aratus called this area the Water around 270 B.C.

Nunki was associated with the city of Eridu and the Akkadian Gu-shi-rab‑ba, the Yoke of the Sea, the patron asterism of the city. Ancient Sumerians believed that Eridu was the first city in the world. In Sumerian mythology, it was the home of Enki (known as Ea in Akkadian and Babylonian myths), the god of water, creation, knowledge, crafts and mischief. Gu-shi-rab‑ba was believed to be the pattern formed by Nunki with Ascella (Zeta Sagittarii) and Albaldah (Pi Sagittarii).

The name Nunki was officially approved for Sigma Sagittarii Aa on August 21, 2016 by the International Astronomical Union’s Working Group on Star Names (WGSN).

Sigma Sagittarii is also sometimes known as Sadira. 17th century Egyptian astronomer Al Achsasi al Mouakket listed the star as Thanih al Sadirah, “the second returning ostrich,” in his Calendarium, written around the year 1650. Sigma, Phi, Zeta, Chi and Tau Sagittarii were the Arabic Al Naʽām al Ṣādirah, or the Returning Ostriches, while Kaus Borealis was known as their keeper.

English astrologer Vivian E. Robson listed the star as Pelagus (“the sea” in Latin) in his book The Fixed Stars and Constellations in Astrology.

The Chinese know Sigma Sagittarii as the Fourth Star of the Dipper (斗宿四), referring to the Chinese Dipper asterism that it forms with Phi, Lambda, Mu, Tau and Zeta Sagittarii.

Facts

Nunki lies only 3.45 degrees south of the ecliptic, which means that it can be occulted by the Moon and by planets. Occultation by a planet takes place far less frequently; the last one occurred on November 17, 1981, when the star was occulted by Venus.  The occultation was visible from Europe around sunset.

Nunki is one of the 58 stars selected for navigation. The only other navigational star in Sagittarius is Kaus Australis, the constellation’s brightest star.

An X-ray survey of 12 nearby near-main sequence B stars conducted using the Roentgen Satellite (ROSAT) in the 1990s revealed that Nunki is a source of X-ray emissions. It has an X-ray luminosity of 1.2 x 1028 erg s-1.

Nunki is a very fast spinner, with a rotational velocity of 165 km/s. For comparison, the Sun rotates at the speed of 1.997 km/s (or about 7 km/h).

Location

Nunki is very easy to identify because it is one of the bright stars that form the prominent summer asterism known as the Teapot. It marks the top left corner of the asterism. Nunki is also a part of the Little Milk Dipper, a pattern of five stars that outline the Teapot’s handle, along with Ascella (Zeta Sagittarii), Kaus Borealis (Lambda Sagittarii), Phi and Tau Sagittarii. Other stars that form the Teapot are Alnasl (Gamma Sagittarii), Kaus Media (Delta Sagittariii) and Kaus Australis (Epsilon Sagittarii), the constellation’s brightest star.

teapot in sagittarius
The Teapot in Sagittarius, image: Wikisky

For northern observers, the Teapot is prominent above the southern horizon in the summer months.

There are several notable globular clusters in the vicinity of Nunki. The Sagittarius Cluster (Messier 22) has an apparent magnitude of 5.5 and is located about 10,600 light years from the solar system. It lies about 2.5 degrees northeast of Kaus Borealis, located at the top of the Teapot.

Messier 28 can be seen less than a degree northwest of Kaus Borealis. With a visual magnitude of 7.66, it is slightly dimmer than M22, and it lies at an approximate distance of 17,900 light years from Earth.

Messier 54 lies west and slightly south of Ascella. It was the first globular cluster discovered that belongs to a different galaxy. Its estimated distance is 87,400 light years and it is believed to belong to the Sagittarius Dwarf Elliptical Galaxy (SagDEG).

Constellation

Nunki is located in the constellation Sagittarius. Representing the Archer, the constellation is usually depicted as a centaur with a bow and arrow, and can be seen in the rich star field of the Milky Way in the southern sky.

sagittarius stars
Sagittarius constellation, image: Roberto Mura

Sagittarius is easy to identify because its brightest stars, including Nunki, form the Teapot asterism. It is a popular target for stargazers because it contains a number of interesting deep sky objects. These include the bright nebulae Messier 8 (the Lagoon Nebula), Messier 17 (the Omega Nebula) and Messier 20 (the Trifid Nebula), the open clusters Messier 18, Messier 21 and Messier 23, and the globular clusters Messier 22 and Messier 28.

The best time of year to observe the stars and deep sky objects in Sagittarius is during the month of August.

The 10 brightest stars in the constellation are Kaus Australis (Epsilon Sgr, mag. 1.85), Nunki (Sigma Sgr, mag. 2.05), Ascella (Zeta Sgr, mag. 2.59), Kaus Media (Delta Sgr, mag. 2.70), Kaus Borealis (Lambda Sgr, mag. 2.82), Albaldah (Pi Sgr, mag. 2.89), Alnasl (Gamma² Sgr, mag. 2.98), Eta Sagittarii (mag. 3.11), Phi Sagittarii (mag. 3.17), and Tau Sagittarii (mag. 3.326).

Nunki – Sigma Sagittarii

Spectral classB2.5 V
U-B colour index-0.761
B-V colour index-0.204
Apparent magnitude2.05
Absolute magnitude-2.17
Distance228 ± 5 light years (70 ± 1 parsecs)
Parallax14.32 ± 0.29 mas
Radial velocity-11.2 km/s
Proper motionRA: +15.14 mas/yr
Dec.: -53.43 mas/yr
Mass7.8 ± 0.2 M
Luminosity3,300 L
Radius4.5 R
Temperature18,890 K
Age31.4 ± 0.4 million years
Rotational velocity165 km/s
ConstellationSagittarius
Right ascension18h 55m 15.92650s
Declination-26° 17′ 48.2068”
DesignationsNunki, Sigma Sagittarii, σ Sagittarii, σ Sgr, 34 Sagittarii, Sadira, HD 175191, HR 7121, HIP 92855, GC 25941, SAO 187448, CCDM J18552-2618A, CPD-26 6590, PPM 269078, FK5 706, IDS 18491-2625 A, WDS J18553-2618Aa,Ab, TYC 6868-1829-1, ALS 15063, CD-26 13595