Dubhe, also known as Alpha Ursae Majoris, is the second brightest star in the constellation of Ursa Major. It is also a spectroscopic binary star system and the 33rd brightest star in the night sky. Dubhe, along with Merak, are known as the Pointer Stars since they are used to find Polaris, the North Star.
Key Facts & Summary
- Dubhe is one of the seven stars that form the famous asterism, the Big Dipper.
- It is situated in the bowl of the asterism. The Big Dipper asterism is also known as “Plough” in the UK.
- The primary star of the spectroscopic binary system, Dubhe A, is an orange giant of spectral type K0II.
- The secondary star, Dubhe B, is a white hydrogen-fusing dwarf star with the stellar classification of F0V.
- The Dubhe star system has an apparent magnitude of 1.79 and it is located at around 123 light-years / 37.7 parsecs away from the Sun. Its absolute magnitude is around -1.10, and it is suspected that one of the stars are variable.
- Dubhe A has around 4.25 solar masses, and its radius hasn’t been yet determined.
- Dubhe A is around 316 times brighter than our sun but it is cooler, with surface temperatures at around 4.660 K.
- Dubhe A has a rotational velocity of around 2.6 km / 1.6 mi per second and has a surface gravity of around 2.46 cgs.
- The secondary star, Dubhe B is also more massive than our sun, having around 1.6 solar masses.
- Although Dubhe is part of the constellation of Ursa Major, it is not part of the Ursa Major Moving Group of stars.
- Dubhe shares its 33rd place as the brightest star in the night sky with Mirfak.
- Both stars are fainer than Alioth, Alnitak, and Alnair, but they outshine Regor, Wezen, and Kaus Australis.
- Many more stars are suspected to be part of the Dubhe stellar system.
Though Dubhe isn’t the brightest star in the constellation of Ursa Major, but rather it is around 2% fainter than the brightest star Alioth, it still bears the Alpha designation which usually is given to the brightest star.
This, however, is not necessarily an error, since when stars were designated a couple of hundreds of years ago, they weren’t exactly given these designations based on their luminosity. Both the traditional names Dubhe and Ak. Dubhe are derived from the Arabic for “bear.” Dubhe’s name actually comes from an Arabic phrase that translates to “the back of the Great Bear.”
As a comparison, some native American cultures saw the bowl of the Big Dipper as a celestial bear being chased by hunters which were represented by the Bowl’s handle.
Since Dubhe isn’t part of the Ursa Major Moving Group of stars, its origin remains a mystery. The star’s age is also currently unknown. The star though, most likely formed from a molecular cloud of gas and dust, or a nebula. Gravity pulled the swirling gas and dust together and formed the second-brightest star in the constellation of Ursa Major, that we today call Dubhe.
Distance, Size, and Mass
Dubhe is around 123 light-years / 37.7 parsecs away from the Sun. Both stars which form the binary system are believed to be bigger than our sun.
The primary star, Dubhe A, for example, has a mass of around 4.25 solar masses, and its radius hasn’t been yet determined but its mass may be a clear indication that it is certainly bigger than our sun.
When it comes to the second star, Dubhe B, even much less information is available. Currently, it is speculated that Dubhe B has around 1.6 solar masses, and thus it may be bigger than our sun as well.
The primary star of the spectroscopic binary system, Dubhe A, is an orange giant of spectral type K0II. It has an apparent magnitude of 1.79 and an absolute magnitude of -1.10. It is suspected to be a variable star since its brightness has been reported to change by a thousandth of a magnitude.
Dubhe A is around 316 times brighter than our sun but it way cooler, with surface temperatures at around 4.660 K. For comparison, our sun has an average surface temperature of around 5.778 K. Dubhe is also the coolest star and the most distant of the Big Dipper stars. However, it is the second most luminous of these stars, with only Alkaid being brighter.
Dubhe A has a rotational velocity of around 2.6 km / 1.6 mi per second and has a surface gravity of around 2.46 cgs. The secondary star, Dubhe B, is a white hydrogen-fusing dwarf star with the stellar classification of F0V.
Dubhe is a spectroscopic binary star system. Dubhe B completes one orbit around Dubhe A every 44.4 years, at an average separation of 23 AU.
Another class F-star with a spectroscopic companion at around 8 arcminutes away is believed to be part of the system. This 7th magnitude system is often referred to as Alpha Ursae Majoris C, though it is considerably farther away from Dubhe A. The star is catalogued separately as HD 95638 – their system is classified as an F8 and have an orbital period of around 6 days.
Dubhe is located in the constellation of Ursa Major, it is the westernmost star of the Big Dipper asterism. Ursa Major is the largest northern constellation and the third largest out of the 88 constellations.
The stars of the Big Dipper asterism are circumpolar to most northern locations and thus they can be seen throughout the year. One of the best months to observe them is during April. Both Dubhe and Merak are known as the Pointer Stars since they are often used to find the north star, Polaris.
In the opposite direction of Polaris, Dubhe and Merak can be used to find Regulus, the alpha star of the Leo constellation.
Much about Dubhe remains a mistery. Its companion, its radius, age, origin, and much more. The star seems stable and it appears that it will remain so. Since it is one of the pointer stars, and so much about it remains unknown, it will surely be studied in the future rigorously.
Did you know?
- Dubhe’s other traditional name “Ak” translates to “eye”, but it hasn’t been used as much as the other variant.
- For the Chinese, Dubhe was known as the First Star of Northern Dipper or the Star of Celestial Pivot.
- Hindu mythology portrays Dubhe as Kratu, one of the Saptarishi – seven sages – Kratu was a Vedic deity and the son of Lord Brahma.
- Dubhe along with Alioth and Alkaid, are the three stars of Ursa Major selected for celestial navigation. Only 58 stars are in this group, and only the brightest and the most easily recognizable are selected.
- The state of Utah officialy declared Dubhe as their official state star in 1996. This happened when the state was celebrating it’s Centennial year – sine Dubhe’s light reaches us at around 100 years.
- The stars in the Big Dipper represented a funeral procession for the Arabs. The ones that formed the Dippler’s handle – Alkaid, Mizar, Alioth, represented the mourners, while Dippler’s bowl – Megrez, Phecda, Dubhe, and Merak – represented the coffin. They’re slow-motion around Polaris and the north celestial pole was associated with the slow movement of a funeral procession.