SU Ursae Majoris star

SU Ursae Majoris stars are one of three subcategories of U Geminorum stars (dwarf novae), the others being SS Cygni stars and Z Camelopardalis stars. SU UMa stars are characterized by, in addition to normal U Gem-type outbursts (which consist of a rise from quiescence of 2 to 6 magnitudes and 1- to 3-day durations), super-outbursts. These occur once for every 3 to 10 normal outbursts, last for 10 to 18 days, and add one to two magnitudes (up to a factor of 5) in brightness over the normal maxima.


Super-outbursts are both brighter and longer in duration than normal outbursts. The rise to super-outburst is indistinguishable from the rise to a normal outburst and, while in super-outburst, a small periodic fluctuation of several tenths of a magnitude known as a super-hump is observed at maximum. Super-humps are unique in that the period of fluctuation is 2 to 3% longer than the orbital period of the system. Therefore, by observing the super-humps, the orbital period can be obtained.


In almost all cases, SU UMa stars have been found to have orbital periods of less than two hours. While the normal outbursts observed in SU UMa are thought to be the same in nature as U Gem-type outbursts, several theories exist to explain super-outbursts. The most favored one, known as the thermal-tidal instability model, supposes that both the narrow outbursts and the super-outbursts are governed by accretion disk instability. But whereas outbursts are explained in terms of thermal instability, super-outbursts are held to be due to an additional tidal instability. Specifically, physical processes cause the disk to expand until it reaches a critical radius at which a 3:1 resonance is achieved and tidal instabilities produce the super-outburst, bringing the disk back to its normal size. Super-hump activity is always associated with super-outbursts, and never with normal outbursts, so that super-humps and super-outbursts seem to be inherently related. Super-humps appear a day or so after the start of a super-outburst and decrease in amplitude as the super-outburst comes to an end. The super-hump can contribute up to 30% of the total light output. According to the thermal-tidal instability model, super-humps result from a precessing eccentric disk.


The interval from one super-outburst to the next is called the super-cycle. The super-cycle lengths of most known SU UMa stars are around a few hundred days, but a few systems have much shorter or much longer super-cycles. The short super-cycle systems have become known as ER Ursae Majoris stars, while the long super-cycle systems are called WZ Sagittae stars.