Star Facts: Merak

Star Facts: Merak
Credit: The Big Dipper by Bob King

Merak (Beta Ursae Majoris) is the 5th brightest star in the Big Dipper asterism associated with the constellation Ursa Major, and the 80th most luminous star in the entire night sky. The star also serves as one of the pointer stars, along with Dubhe, at the front edge of the Dipper’s bowl, with a line drawn through both stars always pointing towards Polaris, the North Star.

Quick Facts

• Constellation: Ursa Major
• Coordinates: RA 11h 01m 50.47654s|Dec. +56° 22′ 56.7339″
• Distance: 79.7 light years
• Star Type: A1IVps
• Mass: 2.7 sol
• Radius: 3.021 sol
• Apparent Magnitude: +2.37
• Luminosity: 63.015 sol
• Surface Temperature: 9,377K
• Rotational Velocity: 47 km/sec
• Age: 500 ± 100 million years
• Other Designations: Merak, Mirak, ß Ursae Majoris, ß UMa, Beta UMa, 48 Ursae Majoris, BD+57°1302, FK5 416, GC 15145, HD 95418, HIP 53910, HR 4295, PPM 32912, SAO 27876


Ursa Major is the most recognizable constellation in the northern night sky, and since it is north circumpolar, the stars in the asterism are all visible throughout the whole year. Refer to the image above for the position of Merak relative to the other stars in the Big Dipper.

Physical Properties

Based on its spectrum, Merak is a fairly typical A-class blue main sequence star that is fusing hydrogen in its core. However, infrared observations have revealed the presence of a circumstellar disc of dust that surrounds the star, much like the dust discs that can be observed around the stars Vega and Fomalhaut. Based on the 120K mean temperature of the disc, it is believed to be centred on a radius of about 47 astronomical units from the star.

While the dust disc is estimated to be only about 0.27% as massive as Earth, in terms of size it would extend almost to the orbit of Saturn if it were placed over the Sun. Currently, it is not known if any planets exist in the disc, or if there are planets in the process of forming from material inside it.

Along with most of the Big Dipper’s stars, Merak is an inner member of a loosely bound open cluster known as the Ursa Major Moving Group, which is a relatively large group of stars (that may or may not include Sirius) that share a common proper motion across the sky. In this context, the Ursa Major Moving group consists of stars that share the same region of space, and not just the same region of the sky as seen from our perspective.


The star’s traditional name, Merak, is derived from the Arabic term “al-maraqq” which translates into English as “[the] loins of the Bear”, as a reference to the star’s position in the greater Bear figure.

In China, Merak is known as Bei Dou èr, which means “[the] Second Star of Northern Dipper”, due to its position in an asterism known as “Northern Dipper”, which consists of the stars Alpha Ursae Majoris, Gamma Ursae Majoris, Delta Ursae Majoris, Epsilon Ursae Majoris, Zeta Ursae Majoris, and Eta Ursae Majoris.

The star Merak also featured in the original Star Trek series in an episode entitled “The Cloud Minders”, in which life on a planet orbiting Merak, called Merak II, was almost destroyed by a botanical plague. Total disaster was averted when toward the end of the episode the plague was cured by the liberal application of a mineral called zenite.

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