Dubhe (Alpha Ursae Majoris)

Dubhe, Alpha Ursae Majoris (α UMa), is the primary component in a multiple star system located at an approximate distance of 123 light years from Earth. Shining at magnitude 1.79, it is the second brightest star in the constellation Ursa Major (the Great Bear), after Alioth. Dubhe is the most distant of the seven bright stars that form the Big Dipper.

Dubhe and its Big Dipper neighbour Merak are known as the Pointer Stars. The two stars can be used to find Polaris, the North Star. They form the outer part of the Big Dipper’s bowl.

Star system

Dubhe is the primary component in a spectroscopic binary star system with a combined apparent magnitude of 1.79. The system is composed of Alpha Ursae Majoris A (Dubhe) and Alpha Ursae Majoris B.

Another spectroscopic binary system lies at a separation of 7.1 arcminutes. The fainter pair has the spectrum of a class F star of the spectral type F8 or F7V and shines at seventh magnitude. It has an orbital period of 6.035 days. The system is catalogued as HD 95638 and sometimes called Alpha Ursae Majoris C. It forms a quadruple star system with Alpha Ursae Majoris A and B.

The primary star, Alpha UMa A, is an orange giant of the spectral type K0III. The companion, Alpha UMa B, is a white hydrogen-fusing dwarf with the stellar classification A5V. Dubhe was previously classified as a yellow giant of the spectral type G9 III, but more recent observations have found it to be a K-type star.

The two stars orbit each other with a period of 44.45 ± 0.11 years. They are separated by only 0.590 ± 0.026 arcseconds and cannot be resolved visually. The physical separation between them is around 23 astronomical units (Earth – Sun distances).

Dubhe has a mass of 3.7 solar masses and a radius 26.85 times that of the Sun. With an effective temperature of 4,650 K, it shines with 339 solar luminosities. The star spins at 2.63 km/s, taking up to 1.35 years to complete a rotation. It has an estimated age of 280 ± 30 million years.

Dubhe is a suspected variable star. Its brightness has been reported to change by a thousandth of a magnitude. In 2000, a team of astronomers identified multimodal oscillations in the star using the WIRE satellite star camera. They found ten radial oscillation models with periods from 6.4 hours to 6.4 days.

The companion, Alpha UMa B, is also more massive than the Sun, but not as massive as Dubhe. It has an estimated mass 2.5 times that of the Sun. Unlike the evolved Dubhe, it is still a main sequence star.

Dubhe (Alpha Ursae Majoris), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Facts

Dubhe is the second brightest star in the constellation Ursa Major and, on average, the 34th brightest star in the sky. It is about as bright as Mirfak (Alpha Persei), the brightest star in the constellation Perseus. Dubhe is only imperceptibly fainter than Alioth (Epsilon Ursae Majoris) in Ursa Major, Alnair (Alpha Gruis) in Grus, and Alnitak in Orion. It slightly outshines Regor (Gamma Velorum in Vela, Wezen (Delta Canis Majoris) in Canis Major and Kaus Australis (Epsilon Sagittarii) in the Sagittarius constellation.

Dubhe forms the Big Dipper with Alkaid (Eta Ursae Majoris), Mizar (Zeta Ursa Majoris), Alioth (Epsilon Ursae Majoris), Megrez (Delta Ursae Majoris), Phecda (Gamma Ursae Majoris), and Merak (Beta Ursae Majoris). Six of the seven Big Dipper stars (all except Megrez) shine at second magnitude, making the Dipper one of the brightest and most familiar asterisms in the northern celestial hemisphere. The asterism is also known as the Plough, the Saucepan, and the Great Wagon.

Five Dipper stars – Mizar, Alioth, Merak, Phecda and Megrez – are all class A stars with an estimated age of around 300 million years. They belong to the Ursa Major moving group (Collinder 285), a group of stars that formed in the same molecular cloud at around the same time. These stars share common velocities and motion through space.

Big Dipper stars, image: Stellarium

Dubhe and Alkaid, the easternmost star of the Big Dipper, do not share a common origin with the other stars nor do they have the same proper motion. As a result, the Big Dipper asterism will not stay the same over time and it will eventually dissipate.

Dubhe is the coolest (4,650 K) and most distant of the Big Dipper stars. It lies at a distance of 123 light years, while most other stars are about 80 light years distant. Dubhe is the second most luminous of the seven stars. Only Alkaid, a hot, blue B-type star, has a greater energy output. Like Dubhe, Alkaid is more distant than the middle five stars of the Dipper. It lies around 104 light-years away.

Dubhe is one of three Ursa Major stars selected for use in the field of celestial navigation. The other two stars are Alioth and Alkaid. These stars have a special role in navigation because they are bright and easy to recognize. Many navigational stars form well-known asterisms with other bright stars, and this is the case with the three Ursa Major stars. Dubhe is part of the Big Dipper’s bowl and Alioth and Alkaid form the handle with Mizar.

Dubhe has the Bayer designation Alpha Ursae Majoris even though it is not the brightest star in the constellation Ursa Major. The German uranographer Johann Bayer, who assigned the Greek letters to bright stars in his star atlas Uranometria (1603), was guided by the order of magnitude and not by individual brightness.

In the case of Ursa Major, Bayer went from west to east. Dubhe got the Alpha designation because it is the westernmost second magnitude star in Ursa Major. Merak, the constellation’s fifth brightest star got the designation Beta, and Alioth, the brightest star in Ursa Major, was assigned the letter Epsilon because it is located more to the east, in the Great Bear’s tail. With a visual magnitude of 1.77, Alioth is only slightly brighter than Dubhe (mag. 1.79). The two stars appear equally bright to the unaided eye.

In the ancient Arabic-speaking world, the stars of the Big Dipper represented a funeral procession. The stars that form the Dipper’s handle – Alkaid, Mizar and Alioth – represented mourners, while the stars of the Dipper’s bowl – Megrez, Phecda, Dubhe and Merak – represented the coffin. Their slow motion around Polaris and the north celestial pole was associated with the slow movement of a funeral procession.

Dubhe became the official star of the State of Utah in 1996. It was selected as the official state star in Utah’s Centennial year because its light was thought to take about 100 years to reach Earth. The star is now estimated to lie 123 light years away, so its light takes a little longer.

Pointer Stars

Dubhe and Merak are known as the Pointer Stars (or simply the Pointers) because they are commonly used to find true north. The two stars point the way to Polaris, the nearest visible star to the north celestial pole. An imaginary line drawn from Merak through Dubhe and extended by about five times the distance between the two stars leads directly to the North Star.

Polaris is the brightest star along this imaginary line. It has an apparent magnitude of 1.98 and is slightly fainter than Alioth, Dubhe and Alkaid in the Big Dipper.

Polaris (Alpha Ursae Minoris) as seen by the Hubble Space Telescope, image: NASA/HST (CC BY 3.0)

Polaris is part of the Little Dipper, a relatively faint asterism that dominates the constellation Ursa Minor. The supergiant star marks the end of the Little Dipper’s handle and the tip of the Little Bear’s tail. Unlike the bright Big Dipper, the Little Dipper is not very easy to spot because its middle five stars are quite dim. For this reason, Dubhe and Merak are commonly used to identify the North Star.

In the other direction, Dubhe and Merak point towards the constellation Leo. The celestial Lion is one of the most recognizable constellations in the sky because it looks like the animal it represents. Regulus, its brightest star, can be found on the imaginary line extended from Megrez through Phecda, the inner stars of the Big Dipper’s bowl. Regulus marks the Lion’s heart and appears at the base of an asterism known as the Sickle. The Sickle looks like a backward question mark and outlines the Lion’s head and mane.

The Big Dipper, Arcturus, Spica and Regulus, image: Stellarium

Name

The name Dubhe (pronunciation /ˈdʌbiː/) comes from the Arabic word dubb, meaning “bear.” It was taken from the phrase żahr ad-dubb al-akbar, which means “the back of the Greater Bear.” The name indicates Dubhe’s position in Ursa Major. It was officially approved for Alpha Ursae Majoris A by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on June 30, 2016. The name formally applies only to the primary component in the system, Alpha Ursae Majoris A, but is often used for the whole star system.

The star’s other traditional name, Ak, means “the eye,” but it has not been used as commonly as Dubhe.

The Chinese know Dubhe as the First Star of Northern Dipper (北斗一). The name refers to the Chinese Northern Dipper asterism, which is equivalent to the Big Dipper. The star’s other Chinese name is Tiān Shū, the Star of Celestial Pivot (天樞).

In Hindu mythology, Dubhe is associated with Kratu, one of the Saptarishi (Seven Sages). Kratu was a Prajapati (Vedic deity) and the son of Lord Brahma.

Location

Dubhe is very easy to find because it is part of the Big Dipper. It marks the northwestern corner of the asterism. The star pattern appears in the northern part of the sky from most inhabited locations.

The location of Dubhe (Alpha Ursae Majoris), image: Stellarium

Dubhe can be used to find a pair of famous galaxies in Ursa Major. Messier 81, also known as Bode’s Galaxy, lies around 10 degrees northwest of the star. It can be found by extending an imaginary line from Phecda through Dubhe by about the same distance as that between the two stars. The Cigar Galaxy (Messier 82) lies in the same field of view, only 38 arcminutes from M81.

Going in the opposite direction, the spiral galaxy Messier 106 in the constellation Canes Venatici can be located by drawing a line from Dubhe though Phecda. A line drawn from Dubhe through Megrez leads directly to the famous Whirlpool Galaxy (Messier 51), also located in Canes Venatici.

Dubhe, Phecda, Messier 81, Messier 82, Messier 106 and Messier 51, image: Stellarium

Dubhe and other stars of the Big Dipper are circumpolar, i.e. they never set, for most northern observers and can be seen at any time of the year. In the northern hemisphere, the Big Dipper is particularly prominent in the months of spring, when it dominates the evening sky.

Constellation

Dubhe is located in the constellation Ursa Major, the Great Bear. Best known for the Big Dipper asterism, Ursa Major is the largest northern constellation and the third largest of all 88 constellations, after Hydra and Virgo. It occupies an area of 1,280 square degrees.

The Great Bear is one of the 48 Greek constellations that were catalogued by the Greco-Roman astronomer Claudius Ptolemy in his Almagest in the 2nd century CE. In Greek mythology, it is associated with the nymph Callisto, who was turned into a bear by the goddess Hera.

Notable stars in the constellation include the hot blue star Alkaid (Eta Ursae Majoris), the bright A-type stars Alioth (Epsilon Ursae Majoris), Mizar (Zeta Ursae Majoris), Merak (Beta Ursae Majoris), Alcor (80 Ursae Majoris), Phecda (Gamma Ursae Majoris) and Megrez (Delta Ursae Majoris), the orange giant Psi Ursae Majoris, and the yellow bright giant star Muscida (Omicron Ursae Majoris) at the Great Bear’s snout.

Ursa Major constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Ursa Major is also home to the red dwarf Lalande 21185, the fourth nearest star system to the Sun, the yellow dwarf Chalawan (47 Ursae Majoris), which has three orbiting extrasolar planets, the orange dwarf HIP 57274, also with three planets, and the contact binary star W Ursae Majoris.

Ursa Major contains a great number of interesting deep sky objects. In addition to M81 and M82, it is home to the planetary nebula Messier 97, also known as the Owl Nebula, the famous Pinwheel Galaxy (Messier 101), and the barred spiral galaxies Messier 108 and Messier 109.

The best time of the year to see the stars and deep sky objects in Ursa Major is during the month of April, when the constellation appears high in the sky in the early evening. The entire constellation is visible from locations north of the latitude 30° S, but it never rises high above the horizon for observers in the southern hemisphere.

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).

Dubhe – Alpha Ursae Majoris

Alpha Ursae Majoris C