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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.[3] The distance to this system, as determined from its annual parallax shift of 4.53 mas,[2] is 719 light-years. It is moving further from the Earth with a heliocentric radial velocity of +27 km/s.[4]
 A visual band light curve for the April 1989 superoutburst of SU Ursae Majoris, adapted from Udalski et al. (1990)[1] | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 08h 12m 28.26946s[2] |
| Declination | +62° 36′ 22.4280″[2] |
| Apparent magnitude (V) | 10.8–14.96[3] |
| Characteristics | |
| Variable type | SU UMa[3] |
| Astrometry | |
| Radial velocity (Rv) | +27.0[4] km/s |
| Proper motion (μ) | RA: +6.582[2] mas/yr Dec.: −24.538[2] mas/yr |
| Parallax (π) | 4.5347 ± 0.0286 mas[2] |
| Distance | 719 ± 5 ly (221 ± 1 pc) |
| Absolute magnitude (MV) | +5.1[5] |
| Orbit[6] | |
| Period (P) | 0.076351±0.000043 d |
| Inclination (i) | 42[5]° |
| Periastron epoch (T) | 2,446,143.6672±0.0015 HJD |
| Semi-amplitude (K1) (primary) | 59±7 km s–1 km/s |
| Details | |
| White dwarf | |
| Mass | 0.8[7] M☉ |
| Temperature | 28,000[7] K |
| Red dwarf | |
| Mass | 0.105[8] M☉ |
| Radius | 0.167[8] R☉ |
| Other designations | |
| Database references | |
| SIMBAD | data |
The variable nature of this star was discovered at the Moscow Observatory by Lidiya Tseraskaya (L. Ceraski) in 1908.[10] It was classified as a U Geminorum-type variable, or dwarf nova.[11] Observation since 1926 showed that this variable undergoes two different types of eruptions: a short maxima lasting around two days that ranged in brightness 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'.[12] Similar dwarf novae of this class have since been discovered, and SU UMa is now the prototype for this sub-category of variable stars.[13]
This is a single-lined spectroscopic binary with an orbital period of 1.83 hours.[6] 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.[14] For SU UMa, the accretion rate from the companion is 9.8×10−13 M☉·yr−1.[15] 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.[14]
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