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

<p>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.</p>

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

CC BY-SA 4.0
2020-02-23T02:34:53.973Z
Maybe include a link to the General Catalog of Variable Stars (GCVS): [original site](http://www.sai.msu.su/gcvs/gcvs/) or the [VizieR](http://cdsarc.u-strasbg.fr/viz-bin/cat/B/gcvs) catalog/query form ?
astrosnapper
CC BY-SA 4.0
2020-02-21T16:59:56.917Z
2020-02-21T16:59:56.917Z
James K
<p>Certainly sunspots (or &quot;starspots&quot;) won't count, as they are statistically random and thus unpredictable, as well as causing a very small delta in total output.</p>
<h1>edit:</h1>
<p>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.<br />
The wiki page on general(periodic) variable stars identifies two  classes:</p>
<blockquote>
<p>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:</p>
<p>Intrinsic variables, whose luminosity actually changes; for example,
because the star periodically swells and shrinks.<br />
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.</p>
</blockquote>
<p>After that, one descends into pulsars, binary stars, Cepheids,  and lots more.</p>

CC BY-SA 4.0
2020-02-21T16:44:20.083Z
[Irregular variables](https://en.wikipedia.org/wiki/Irregular_variable) are non-periodic but are certainly considered to be variable stars.
HDE 226868
CC BY-SA 4.0
2020-02-21T16:56:55.967Z
@HDE226868 thank you for pushing me to fix this answer.
Carl Witthoft
CC BY-SA 4.0
2020-02-21T22:31:41.493Z
Lots of spotted stars are named variables. Try PZ Tel and AB Dor for instance.
ProfRob
CC BY-SA 4.0
2020-02-21T16:41:42.397Z
2020-02-21T16:56:17.717Z
2020-06-17T09:47:40.653Z
Carl Witthoft
<p>Variable stars are <a href="https://www.aavso.org/variables-what-are-they-why-observe-them" rel="nofollow noreferrer">stars whose apparent magnitude varies</a>. But <a href="https://www.aavso.org/vsx/help/VariableStarTypeDesignationsInVSX.pdf" rel="nofollow noreferrer">there are so many phenomena that can cause a star to be variable</a>, 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! </p>

<p>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?</p>

CC BY-SA 4.0
2020-02-21T11:23:31.510Z
a companion question might be "What variable star varies the least?"
uhoh
<star><variable-star><definition>
How variable does a star have to be, to be a variable star?
usernumber
# How variable does a star have to be, to be a variable star?

(Asked by: username_0 on 2020-02-21)

Variable stars are [stars whose apparent magnitude varies](https://www.aavso.org/variables-what-are-they-why-observe-them). But [there are so many phenomena that can cause a star to be variable](https://www.aavso.org/vsx/help/VariableStarTypeDesignationsInVSX.pdf), 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?

**Tags:** star, variable-star, definition
> **Comments:**
> **username_1**: a companion question might be "What variable star varies the least?"  

---

## 2 answers

### Answer by username_2 on 2020-02-21. Score: 3 | Accepted answer

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.

> **Comments:**
> **username_3**: Maybe include a link to the General Catalog of Variable Stars (GCVS): [original site](http://www.sai.msu.su/gcvs/gcvs/) or the [VizieR](http://cdsarc.u-strasbg.fr/viz-bin/cat/B/gcvs) catalog/query form ?  

---

### Answer by username_4 on 2020-02-21. Score: -1

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.

> **Comments:**
> **username_5**: [Irregular variables](https://en.wikipedia.org/wiki/Irregular_variable) are non-periodic but are certainly considered to be variable stars.  
> **username_4**: @HDE226868 thank you for pushing me to fix this answer.  
> **username_6**: Lots of spotted stars are named variables. Try PZ Tel and AB Dor for instance.  

---

