Dwarf nova

[1] The first one to be observed was U Geminorum in 1855; however, the mechanism was not known until 1974, when Brian Warner showed that the nova is due to the increase of the luminosity of the accretion disk.

[2] They are similar to classical novae in that the white dwarf is involved in periodic outbursts, but the mechanisms are different.

Current theory suggests that dwarf novae result from instability in the accretion disk, when gas in the disk reaches a critical temperature that causes a change in viscosity, resulting in a temporary increase in mass flow through the disc, which heats the whole disc and hence increases its luminosity.

[3] The luminosity of the outburst increases with the recurrence interval as well as the orbital period; recent research with the Hubble Space Telescope suggests that the latter relationship could make dwarf novae useful standard candles for measuring cosmic distances.

[3][4] There are three subtypes of U Geminorum star (UG):[5] In addition to the large outbursts, some dwarf novae show periodic brightening known as “superhumps”.