Phecda

Phecda, Gamma Ursae Majoris (γ UMa), sometimes also known as Phad, is one of the bright stars forming the Big Dipper‘s bowl and the Great Bear’s hindquarters. With an apparent magnitude of 2.438, it is the sixth brightest star in the constellation Ursa Major. It lies at a distance of 83.2 light years from Earth. Northern observers can easily identify Phecda as the lower-left or southernmost star of the Big Dipper’s bowl.

Star type

Phecda is a white (class A) hydrogen fusing dwarf. It has a mass of 2.94 solar masses and a radius 3.04 times that of the Sun. With an effective temperature of 9,355 K, it shines with 65 solar luminosities. It is classified an an Ae star, a star embedded in an envelope of gas and dust that is adding emission lines to its spectrum. Herbig Ae and Be stars typically show lines of hydrogen and calcium in their spectra and they sometimes have circumstellar disks.

Phecda is a very fast spinner, with a projected rotational velocity of 178 km/s. The star’s estimated age is about 300 years.

Many sources list Phecda as an astrometric binary star system with a class K main sequence companion with 0.79 solar masses and an orbital period of 20.5 years. However, a number of recent studies have failed to detect the companion, so the effect is possibly deceiving.

Phecda (Gamma Ursae Majoris), image: Wikisky

Facts

Phecda’s spectrum has been used as one of the anchor points for the Morgan-Keenan (MK) spectral classification system since 1943. This was the year of the first publication of the list of standard stars in The Atlas of Stellar Spectra by W.W. Morgan, P.C. Keenan and E. Kellman.

Phecda is a member of the Ursa Major Moving Group (Collinder 285), a loose stellar association of stars with an estimated age of 300 million years that share a common origin and motion through space. Other notable members of the group include all the Big Dipper stars except Alkaid and Dubhe, as well as the likely members Menkalinan (Beta Aurigae), Alphecca (Alpha Coronae Borealis), Skat (Delta Aquarii), Adhafera (Zeta Leonis), and Beta Serpentis. The group was discovered by the English astronomer Richard A. Proctor in 1869. Proctor was the first to notice that most of the stars of the Big Dipper have proper motions that are taking them toward a common point located in the constellation Sagittarius. With most of its stars sharing the same proper motion, the Big Dipper would not dramatically change over time, but as the stars at the ends of the asterism – Alkaid and Dubhe – do not belong to the group, the Dipper as we know it will eventually dissolve.

Phecda is physically closer to the Mizar–Alcor system than it is to Merak. It is only 8.55 light years from Mizar and Alcor, while its distance to Merak is 11 light years.

Big Dipper stars, image: Wikisky

Ancient Arabic astronomers saw the stars of the Big Dipper’s bowl as a coffin or bier, with the stars of the Dipper’s handle representing mourners. The constellation symbolized a funeral and its slow circular motion around the north celestial pole was associated with the slow movement of a funeral procession.

Name

The name Phecda (pronunciation: /ˈfɛkdə/) or Phad, as the star has also been commonly known, refers to the star’s position in the Great Bear constellation. It is derived from the Arabic phrase fakhth al-dubb, meaning “the thigh of the bear.” The name was officially approved for Gamma Ursae Majoris Aa by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on July 20, 2016.

The Chinese know Phecda as the Third Star of Northern Dipper (北斗三), referring to its position in an asterism that corresponds to the Big Dipper. The star is also known as Tiān Jī (天璣), or the Star of Celestial Shining Pearl.

The Hindu name for Gamma Ursae Majoris is Pulastya, after one of the Saptarishis (Seven Great Sages) in Hindu mythology. Pulastya was one of the Prajapati, Vedic deities of Hinduism.

Location

Phecda is easy to find because it is part of a bright and recognizable northern asterism, the Big Dipper. The asterism is formed by the seven brightest stars of Ursa Major constellation: Alkaid (η UMa), Mizar (ζ UMa), Alioth (ε UMa), Megrez (δ UMa), Phecda (γ UMa), Dubhe (α UMa) and Merak (β UMa).

Phecda can be used to find several bright deep sky objects. The barred spiral galaxy Messier 109 lies just southeast of the star. It is the brightest galaxy in the M109 Group, which also includes the barred spiral galaxy NGC 3953 and more than 50 other galaxies. The M109 Group is a member of the Ursa Major Cluster, a galaxy cluster particularly rich in spiral galaxies. NGC 3718 (also designated Arp 214), located near Phecda, is one of the cluster’s brighter members.

Phecda and Messier 109, image: Wikisky

The bright planetary nebula Messier 97, also known as the Owl Nebula, and the barred spiral galaxy Messier 108 can be found using Phecda and Merak, the other star at the bottom of the Big Dipper’s bowl. The Owl Nebula is located about 2.5 degrees from Merak in the direction of Phecda and Messier 108 appears in the same field of view, about 48 arcminutes northwest of the nebula.

The spiral galaxy Messier 106 can be found by extending an imaginary line from Dubhe through Phecda. The galaxy lies in the neighbouring constellation Canes Venatici, the Hunting Dogs.

Constellation

Phecda is located in the northern constellation Ursa Major, the Great Bear. Ursa Major is one of the 48 Greek constellations, listed by the Greco-Roman astronomer Ptolemy in the 2nd century CE, but it has likely been known since prehistoric times. The bright asterism formed by the constellation’s seven main stars has been known by many different names, including the Big Dipper, the Plough, the Great Wagon and Charles’ Wain.

Ursa Major constellation map by IAU and Sky&Telescope magazine

Ursa Major is the third largest constellation in the sky, after Hydra and Virgo, and it contains a great number of interesting deep sky objects. In addition to those already mentioned, they include Messier 81 (Bode’s Galaxy) and Messier 82 (the Cigar Galaxy), Messier 101 (the Pinwheel Galaxy), the barred spiral galaxy NGC 3079, and the grand design spiral galaxy NGC 3310.

Ursa Major is circumpolar to most northern observers and can be seen throughout the year. The best time of year to observe its stars and deep sky objects is during the month of April, when it is particularly prominent in the evening sky.

The 10 brightest stars in Ursa Major are Alioth (Epsilon UMa, mag. 1.77), Dubhe (Alpha UMa, mag. 1.79), Alkaid (Eta UMa, mag. 1.86), Mizar (Zeta UMa, mag. 2.04), Merak (Beta UMa, mag. 2.37), Phecda (Gamma UMa, 2.438), Psi Ursae Majoris (mag. 3.01), Tania Australis (Mu UMa, mag. 3.06), Talitha (Iota UMa, mag. 3.14), and Theta Ursae Majoris (mag. 3.166).

Phecda – Gamma Ursae Majoris