SU Ursae Majoris stars are interacting binaries that show ordinary dwarf-nova outbursts and longer superoutbursts. Gas transferred from a companion forms an accretion disk around a white dwarf. The leading thermal-tidal picture links a hot disk with an eccentric, precessing outer disk. The white dwarf does not need to explode.
Separate the observation from the model
The observational clues are an extended bright event and characteristic brightness modulations called superhumps. The interpretation asks how thermal instability, disk expansion and tidal forces produce them. A long outburst is not, by itself, a direct measurement of disk eccentricity or resonance.
The AAVSO introduction to SU UMa provides observational context and explains the thermal-tidal framework. Individual systems need detailed light curves rather than classification from a single bright measurement.
What the 3:1 resonance means
At the relevant resonance radius, gas circles the white dwarf approximately three times during one binary orbit. Repeated forcing by the companion can excite an eccentric disk. Whether the disk can reach that radius depends on mass ratio and tidal truncation; a universal sharp mass-ratio threshold would oversimplify the hydrodynamics.
In the thermal-tidal model, increased tidal interaction helps prolong the hot state. The cause of the extended event should be described as a model tested against observations, not as a complete explanation of every superoutburst. Irradiation, changing mass supply and pressure effects complicate detailed behavior.
Why superhumps have a different clock
An eccentric disk can precess slowly in the same sense as orbital motion. For a positive superhump, a useful approximation is 1/Psh = 1/Porb − 1/Pprec. The modulation repeats when the companion and the changing disk orientation realign.
For an illustrative orbital period of 0.080 days and superhump period of 0.082 days, that relation gives a precession period of 3.28 days. The superhump is therefore neither the orbital period itself nor a pulsation of the white dwarf. These are teaching numbers, not measurements of SU UMa.
What an observing campaign should resolve
- The event’s rise, plateau and decline, with enough coverage to establish its duration.
- Superhump maxima over successive nights, preserving changes in period and amplitude.
- An independent orbital period, rather than assuming the photometric modulation equals it.
- Possible aliases and gaps that allow competing period solutions.
Superhumps evolve during an event. Combining every night into one constant period can erase physically useful changes. For the measurement method, see measuring superhumps.
Exceptions are scientifically useful
The observations of NY Serpentis showed standstills between superoutbursts, challenging an overly rigid separation of dwarf-nova subtypes. Such cases test which disk conditions matter; they do not make the classification meaningless.
Compare ordinary disk outbursts and Z Cam standstills to follow the different states of an accreting disk.
Revised October 26, 2026. The precession calculation is illustrative.