SU Ursae Majoris

SU Ursae Majoris, or SU UMa, is a close binary star in the northern circumpolar constellation of Ursa Major. It is a periodic cataclysmic variable that varies in magnitude from a peak of 10.8 down to a base of 14.96.[2] The distance to this system, as determined from its annual parallax shift of 4.53 mas,[1] is 719 light years. It is moving further from the Earth with a heliocentric radial velocity of +27 km/s.[3]

The variable nature of this star was discovered at the Moscow Observatory by Lidiya Tseraskaya (L. Ceraski) in 1908.[9] It was classified as a U Geminorum-type variable, or dwarf nova.[10] Observation since 1926 showed that this variable undergoes two different types of eruptions: a short maxima lasting around two days that ranged in brightess between 11.6–12.9 magnitude, and a longer maxima extending for 13 days that ranged between 10.4–11.8 magnitude. The later event came to be referred to as 'supermaxima'.[11] Similar dwarf novae of this class have since been discovered, and SU UMa is now the prototype for this sub-category of variable stars.[12]

This is a single-lined spectroscopic binary with an orbital period of 1.83 hours.[5] It consists of a white dwarf star that is acquiring matter from its close companion via an accretion disk. This disk is unstable and undergoes periodic outbursts which increase the luminosity of the system.[13] For SU UMa, the accretion rate from the companion is 9.8×10⁻¹³ M_☉·yr⁻¹.[14] X-ray emission has been detected in the vicinity of the white dwarf, which drops by a factor of four during outbursts. This emission is theorized to come from the boundary layer between the white dwarf and its accretion disk.[13]