AN Ursae Majoris is a

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

system in the northern circumpolar constellation of Ursa Major. It is a variable star, with ''AN Ursae Majoris'' being the variable star designation, and ranges in brightness from 14.90 down to 20.2. Even at its peak brightness though, the system is much too faint to be visible to the naked eye. Based on parallax measurements, the system is located roughly 1,050

light year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s away from the Sun. This is a single-lined

spectroscopic binary A binary star is a system of two star, stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separa ...

system with a period of in a close, circular orbit. The pair form an eclipsing binary system that decreases from magnitude 14.9 down to 20.2, once per orbit. This object, along with

AM Herculis AM Herculis is a binary variable star located in the constellation Hercules. This star, along with the star ''AN Ursae Majoris'', is the prototype for a category of cataclysmic variable stars called '' polars'', or ''AM Her'' type ...

, define a class of cataclysmic variables known as '' polars''. The pair consist of a low mass white dwarf with a strong

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

, interacting with a low–mass

main sequence In astronomy, 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 Her ...

star that has filled its Roche lobe. Matter is being energetically accreted from the main sequence star onto one or both magnetic

poles Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, who share a common history, culture, the Polish language and are identified with the country of Poland in Ce ...

of the white dwarf star, producing emission lines in the spectrum. The magnetic field of the white dwarf has an estimated strength of .

References

{{Stars of Ursa Major White dwarfs Polars (cataclysmic variable stars) Eclipsing binaries Ursa Major (constellation) Ursae Majoris, AN