Star Facts: Phecda

Star Facts: Phecda
Image Credit: Jeff at JSJ Photography

Phecda (Gamma Ursae Majoris), the 6th brightest star in Ursa Major and 85th brightest star in the night sky, marks out the lower left corner of the Big Dipper’s bowl. Along with most of the Dipper’s stars, Phecda is an inner member of the Ursa Major Moving Group, which is a relatively large group of stars (which may include Sirius) that share a common proper motion across the sky.

Quick Facts

• Constellation: Ursa Major
• Coordinates: RA 11h 53m 49.84732s |Dec. +53° 41′ 41.1350″
• Distance: 83.2 light years
• Star Type: A0 Ve
• Mass: 2.94 sol
• Radius: 3.04 sol
• Apparent Magnitude: +2.438
• Luminosity: 65.255 sol
• Surface Temperature: 9,355K
• Rotational Velocity: 178 km/sec
• Age: ± 300 million years
• Other Designations: Phad, Phecda, Phekda, Phegda, Phekha, Phacd, Fekda, Gamma Ursae Majoris, 64 Ursae Majoris, BD+54 1475, FK5 447, GC 16268, HD 103287

Visibility

Ursa Major and the 7 stars of its Big Dipper asterism are north circumpolar, making them visible the whole year round. Refer to the image above for the position of Phecda relative to the other stars in the Big Dipper.

Physical Properties

Phecda has an Ae classification, with the “e” being an indication that the star is surrounded by a huge gas disc. It is also one of only 100 or so known Ae-type stars, all of which are rapid rotators. Phecda, for instance, spins with an equatorial rotational velocity of 178 km/sec compared to around 2 km/sec for the Sun.

In terms of its spectrum, Phecda is a white, so-called “colorless” star, which means that it has the same appearance when viewed optically, as it has in photographs. This effect is ascribed to the fact that Phecda is at the high end of the temperature range for A-class stars, which partially explains why Phecda behaves much like the much hotter B-class stars, which as a class, cools downwards from between 9,500K and 30,000K into the A-classification. As a rule, A-class stars do not exceed 9,500K.

Although Phecda is listed in several sources as an astrometric binary star, with the companion thought to be a K-class star that is about 80% as massive as the Sun, many investigators have failed to find the companion, and it is now believed that the original data, including an orbital period of about 2.5 years, is likely spurious, and the result of fluctuations in the emissions of the envelope of gas that surrounds the star.

History

The star’s traditional name, Phecda, is derived from the Arabic phrase, “fakhth al-dubb”, which means “thigh of the Bear”, presumably as a reference to the stars’ location in the figure of the much larger bear, as opposed to its position in the Big Dipper asterism.

In Chinese astronomy, Phecda and the stars Alpha Ursae Majoris, Beta Ursae Majoris, Delta Ursae Majoris, Epsilon Ursae Majoris, Zeta Ursae Majoris, and Eta Ursae Majoris all form part of an asterism known as Bei Dou, which translates into “Northern Dipper”. Due to its position in the asterism, Phecda is known in China as Bei Dou san, meaning “[the] Third Star of Northern Dipper”.

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