Variability class

Eruptive and cataclysmic variables

This family covers stars that brighten or fade through accretion and eruption rather than pulsation or geometry: young stars still feeding from their disks, flaring dwarfs, and cataclysmic binaries where a white dwarf accretes gas from a companion. Most of it is fundamentally non-periodic.

118,684 starsYSO · CV · GCAS

dwarf nova · schematic

Timet 0 d

what is shining

Schematic: long quiescence with sharp, irregular outbursts. Usually not periodic.

What it is

Two populations make up most of this class. Cataclysmic variables (CVs) are close binaries in which a white dwarf accretes from a companion; instabilities in the accretion disk produce dwarf-nova outbursts, and thermonuclear runaways produce novae. Eruptive young stellar objects — T Tauri stars, FUors, EXors, and dust-dipping UXors — vary as they accrete from and are shadowed by their circumstellar disks. Flare stars and Be-shell stars round out the family.

The physics

The common thread is accretion and instability rather than a clean oscillator. A dwarf nova erupts when its disk switches between cool and hot states; a nova when accreted hydrogen ignites in a thermonuclear runaway on the white dwarf; a young star flickers as accretion streams and disk shadows come and go. The variability is stochastic — episodic, often dramatic, and rarely repeatable on a fixed cadence.

VSX types

Subtypes

Dwarf nova

U Gem-type CV: recurrent disk-instability outbursts brightening by several magnitudes over days.

T Tauri (young)

CTTS

Accreting pre-main-sequence star; irregular bursts and dust-driven fadings.

Flare star

UV Cet-type dwarf with abrupt, minutes-long magnetic flares superposed on a quiet star.

γ Cas (Be shell)

GCAS

Rapidly rotating Be star with irregular brightenings as it ejects equatorial material.

Method

Finding the period

For most of this class there is simply no period to find, and a confident GLS peak on red-noise accretion variability is a trap, not a discovery. These stars are better found by their variability indices (Stetson J) and the classifier's anomaly score. Some CVs show an orbital period or superhumps; they are the exception.

Pitfalls

Do not trust a periodogram peak here: stochastic, red-noise variability readily fakes a GLS signal.
Non-detections carry real information — a CV in deep quiescence sits below the limit between outbursts.
A single outburst is an event, not a period; repeated outbursts are quasi-recurrent at best.
Young stars mix accretion bursts and dust fadings, so the same star can both brighten and dim irregularly.

Archive stars