Megrez (Delta Ursae Majoris) is the least luminous of all the seven Dipper stars, with an apparent visual magnitude of +3.3. This blue-white main sequence dwarf star is located around 58.4 light years from Earth, and is also an outlying member of the Ursa Major Moving Group, a collection of stars that share the same proper motion across the sky, and therefore likely have a common origin. Megrez has two faint 10th and 11th magnitude companions, both of which are separated from it by two arcminutes.
• Constellation: Ursa Major
• Coordinates: RA12h 15m 25.56063s |Dec. +57° 01′ 57.4156″
• Distance: 58.4 light years
• Star Type: A3 V
• Mass: 1.63 sol
• Radius: 1.4 sol
• Apparent Magnitude: +3.312
• Luminosity: 14 sol
• Surface Temperature: 9,480K ± 570K
• Rotational Velocity: 233 km/sec
• Age: 300 million years
• Other Designations: Delta UMa, 69 Ursae Majoris, BD+57 1363, CCDM J12155+5702A, FK5 456, GC 16736, HD 106591, HIP 59774, HR 4660, IDS 12105+5735 A, PPM 33469, SAO 28315, WDS J12154+5702A
Since the Big Dipper asterism in the constellation of Ursa Major is north circumpolar, most northern hemisphere observers can see Megrez all year round. Look for this star at the point where the Dippers’ bowl attaches to the handle, with Megrez forming the “attachment point”.
Like most other Dipper stars, Megrez is a run-of-the-mill A-class main sequence star, and there is nothing about Megrez that distinguishes it from most other stars of its spectral class that are in the prime of their lives, and converting hydrogen in their core into helium. These type of stars aren’t exactly small, either, and Megrez has twice the size and two-thirds more mass than the Sun, with 14 times its brightness.
One remarkable feature, however, is that it displays an excess of infrared radiation, which indicates that the star is surrounded by a dense debris disc. Unlike most other stars that have debris discs at large orbital radii, though, the disc around Megrez averages an orbital distance of only 16 astronomical units, which is unusually small. While the reason for the small separation is not entirely clear, most investigators believe that the matter in the disc is being pulled toward the star by the Poynting–Robertson effect, otherwise known as Poynting–Robertson drag.
In simple terms, this effect involves forces being exerted on the dust particle that orbits a star. Essentially, radiation pressure from the star causes a drag on the particle when the particle moves through it an angle, which causes the particle to lose some orbital energy, which in turn, causes the particle to spiral inward towards the star.
Note, however, that generally, only particles in the size range between 1 µm to 1 mm in diameter are affected by this force. Smaller particles are usually blown out of the dust disc by radiation pressure, while larger particles are most commonly reduced in size by collisions with other large particles before the drag effect can exert a significant influence on their orbital velocity, thus largely maintaining the debris discs’ structure and orbital distance from the star.
Although the traditional name “Megrez” derives from the Arabic phrase al-maghriz meaning “the base [of the bear’s tail]”, the star was also at one point known as “Kaffa”. The latter name first appeared in 1951 in the Atlas Coeli, a star atlas published by the Czech astronomer Antonín Becvár. However, despite diligent and exhaustive research by several astronomical historians, including Professor Paul Kunitzch, the origin of the name “Kaffa” remains a mystery.
In China, Megrez forms part of an asterism known as Bei Dou, “[the] Northern Dipper”, along with the stars Alpha Ursae Majoris, Beta Ursae Majoris, Gamma Ursae Majoris, Epsilon Ursae Majoris, Zeta Ursae Majoris and Eta Ursae Majoris. Due to its position in this asterism, Megrez is known as either Bei Dou sì, or Tian Quán, which translate into English as “[the] Fourth Star of Northern Dipper”, and “Star of Celestial Balance”, respectively.