Xi Ursae Majoris
Xi Ursae Majoris (ξ UMa, ξ Ursae Majoris) is a star system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...

 in the constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 Ursa Major
Ursa Major
Ursa Major , also known as the Great Bear, is a constellation visible throughout the year in most of the northern hemisphere. It can best be seen in April...

. On May 2, 1780, Sir William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

 discovered that this was a binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

 system, making it the first such system ever discovered. It was the first visual double star for which an orbit was calculated, when it was computed by Félix Savary
Félix Savary
Félix Savary, who was born on October 4, 1797 in Paris and died on July 15, 1841 in Estagel, was a French astronomer.He studied at the École Polytechnique, where he was later a professor of astronomy...

 in 1828
1828 in science
The year 1828 in science and technology involved some significant events, listed below.-Astronomy:* Félix Savary computes the first orbit of a visual double star when he calculates the orbit of the double star Xi Ursae Majoris.-Biochemistry:...


The two components are yellow G-type
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 main sequence dwarfs
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

. The brighter component, Xi Ursae Majoris A, has a mean apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of +4.41. It is classified as an RS Canum Venaticorum type
RS Canum Venaticorum variable
RS Canum Venaticorum variables are a type of variable star. They are close binary stars having active chromospheres which can cause large stellar spots. These spots are believed to cause variations in their observed luminosity...

 variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

 and its brightness varies by 0.01 magnitudes. The companion star, Xi Ursae Majoris B has an apparent magnitude of +4.87. The orbital period of the two stars is 59.84 years, and they are currently separated by 1.2 arcseconds, or at least 10 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....


Each component of this double star is itself a spectroscopic binary. B's binary companion, denoted Xi Ursae Majoris Bb, is unresolved, but the binary star is known to have an orbital period of 3.98 days. The masses of both A and B's companions (Ab and Bb) (deduced by the sum total mass of the system minus the likely masses of Aa and Ba determined by their class) indicate that they are probably MV stars (red dwarfs), Bb being on the cool end of the M spectrum, not much hotter than a brown dwarf.

It also has the proper name Alula Australis (and erroneously Alula Australe) meaning "the Southern (star) of Alula." The words Alula, El Acola and el-awla (with ν
Nu Ursae Majoris
Nu Ursae Majoris is a binary star in the constellation Ursa Major. It is approximately 400 light years from the Earth.The primary component, Nu Ursae Majoris A, is an orange K-type giant with an apparent magnitude of 3.5m...

) come from the Arabic phrase (al-Qafzah) al-Ūlā meaning "the first (leap)" (plus "Australis", "southern" in Latin).

It and Nu Ursae Majoris
Nu Ursae Majoris
Nu Ursae Majoris is a binary star in the constellation Ursa Major. It is approximately 400 light years from the Earth.The primary component, Nu Ursae Majoris A, is an orange K-type giant with an apparent magnitude of 3.5m...

 are Hea Tae (下台), the Lower Dignitary, in Chinese astronomy
Chinese astronomy
Astronomy in China has a very long history, with historians considering that "they [the Chinese] were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs."...


External links

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