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Variable stars are stars whose apparent magnitude varies. But there are so many phenomena that can cause a star to be variable, that I would expect all stars to be variable. A rotating star has a starspot? It's variable! A planet is transiting? It's variable! A cloud of dust passes in front? It's variable!

This makes me wonder what exactly distinguishes a variable from a non-variable star? Must the variations in brightness be larger than a given magnitude? Must the variations in brightness be periodic? Do dips (or peaks) in brightness be observed more than once?

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    $\begingroup$ a companion question might be "What variable star varies the least?" $\endgroup$
    – uhoh
    Commented Feb 21, 2020 at 11:23

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There is no lower limit, and as you say, all stars are somewhat variable.

However catalogues of variable stars exist, and they can record a wide range of levels of variability. For example, the general catalogue of variable stars lists stars like Alpha Triangulum, with a variability of 0.01 magnitudes.

Ultimately a variable star is a star which has had its variability measured and studied and recorded in a catalogue.

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  • $\begingroup$ Maybe include a link to the General Catalog of Variable Stars (GCVS): original site or the VizieR catalog/query form ? $\endgroup$
    – astrosnapper
    Commented Feb 23, 2020 at 2:34
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Certainly sunspots (or "starspots") won't count, as they are statistically random and thus unpredictable, as well as causing a very small delta in total output.

edit:

I stand corrected. There's a Wikipedia page (thanks, HDE) which describes several classes of aperiodic stars. These stars do continue to undergo significant variation for at least many days, apparently.
The wiki page on general(periodic) variable stars identifies two classes:

This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it. Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

After that, one descends into pulsars, binary stars, Cepheids, and lots more.

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  • $\begingroup$ Irregular variables are non-periodic but are certainly considered to be variable stars. $\endgroup$
    – HDE 226868 ♦
    Commented Feb 21, 2020 at 16:44
  • $\begingroup$ @HDE226868 thank you for pushing me to fix this answer. $\endgroup$
    – Carl Witthoft
    Commented Feb 21, 2020 at 16:56
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    $\begingroup$ Lots of spotted stars are named variables. Try PZ Tel and AB Dor for instance. $\endgroup$
    – ProfRob
    Commented Feb 21, 2020 at 22:31
  • $\begingroup$ The BY Draconis variables are an entire class of variable stars where the variability is caused by starspots rotating in and out of view. $\endgroup$
    – user24157
    Commented Feb 22, 2020 at 20:01

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