Variability class · YSO · CV · GCAS

Eruptive / CV

Accretion, flares, and outbursts — variability with no clock.

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 strips matter from a companion. Most of it is fundamentally non-periodic.

21,733 active stars YSO · CV · GCAS

Representative light curve

g · schematic

A representative light curve, not a fold: long quiescence punctuated by sharp, irregular outbursts. There is usually nothing periodic to phase here.

What it is

Two overlapping populations rolled into one display 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 detonates 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.

The families within

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.

The varchive method

Finding the period

The same science code runs for every star. Here is how it behaves for this class — and where it can be fooled.

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. varchive surfaces these stars instead through their variability indices (Stetson J), the classifier's anomaly score, and the nightly refresh, which is what catches a fresh outburst or a new fading episode. Some CVs do hide an orbital period (or superhumps), but it is the exception, recovered case by case — never assumed.

What to watch for

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.

Keep exploring

Browse the Eruptive / CV population

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