Close binary systems with orbital periods from 0.05 to 230 days. One of the components of these systems is a hot dwarf star that suddenly, during a time interval from one to several dozen or several hundred days, increases its brightness by 7-19 mag in V, then returns gradually to its former brightness over several months, years, or decades. Cool components may be giants, subgiants, or dwarfs of K-M type (from GCVS).
- there is roughly 30 novae per year in the Milky Way and about 20 in other galaxies
- absolute magnitude <math>M_V = -8 mag</math>
- spectra near maximum light resemble A-F absorption spectra of luminous stars at first
- spectra at minimum light resemble spectra of Wolf-Rayet stars.
- amplitude between 7 and 19 mag in V
- quick rise of few days
- slow decline, between few days and 100 days (fast: type A, slow: type B, very slow: type C)
- 15 days after maximum its Mv = -5.5mag, independent on speed or class
- small changes at minimum light may be present
Exemplary observations of a very fast nova with 0.3m Meade telescope: [Munari and Dallaporta].
- are produced by a white dwarf star and a red giant circling about each other in a close orbit
- recurrent every 10-100 years
- smaller amplitude: 4-9 mag
- M_V at minimum is about 0-1 mag
- example: RS Oph - 3 outbursts between 1957 and 1991 (AAVSO), exhibits some small variation in the baseline in minimum
- decline within few days
Short transient searches in the other galaxies revealed possible new sub-class of novae: Kasliwal et al. 2010. They usually stay above the Gaia detection limit for not more than 10 days.
Interesting paper on classification and properties of novae: [Strope, Schaefer & Henden 2010]