<p>This graph is not the light curve of a cepheid. It is used to find the average brightness of the star, and from that, estimate its distance.</p>

<p>The rate at which cepheids pulse is related to their <em>average</em> luminosity (and so their average absolute magnitude). This graph relates the period to their average absolute magnitude.</p>

<p>To use the graph, observe a cepheid for long enough to establish a period (say 30 days, for example). Then read the graph (an absolute magnitude of just under -5.5 in the example)</p>

<p>You now know how intrinsically bright the star is on average. By looking at its average apparent brightness, you can calculate an estimate of its distance. See <a href="https://astronomy.stackexchange.com/questions/17826/how-does-the-cepheid-variables-help-in-finding-distance?rq=1">How does the cepheid variables help in finding distance</a></p>

CC BY-SA 3.0
2017-02-01T22:44:07.953Z
2017-02-01T22:44:07.953Z
2017-04-13T12:59:06.910Z
James K
<p>Basically, I've been seeing these graphs that say: Cepheid Period-Luminosity Relation and the Period axis is supposed to be "logarithmic";</p>

<p>How does this graph work? Since the luminosity of a star doesn't just increase, as we know, but it increases and decreases periodically, why does the magnitude keep raising?
How do I find the pulsating period of such a star through a graph like this?
<a href="https://i.sstatic.net/4mMpN.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/4mMpN.gif" alt="enter image description here"></a></p>

<distances><luminosity><cepheids><absolute-magnitude><variable-star>
How to find a Cepheid's pulsation period using its graph?
Potato Comet
# How to find a Cepheid's pulsation period using its graph?

(Asked by: username_0 on 2017-02-01)

Basically, I've been seeing these graphs that say: Cepheid Period-Luminosity Relation and the Period axis is supposed to be "logarithmic";

How does this graph work? Since the luminosity of a star doesn't just increase, as we know, but it increases and decreases periodically, why does the magnitude keep raising?
How do I find the pulsating period of such a star through a graph like this?
[![enter image description here](https://i.sstatic.net/4mMpN.gif)](https://i.sstatic.net/4mMpN.gif)

**Tags:** distances, luminosity, cepheids, absolute-magnitude, variable-star


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## 1 answers

### Answer by username_1 on 2017-02-01. Score: 3

This graph is not the light curve of a cepheid. It is used to find the average brightness of the star, and from that, estimate its distance.

The rate at which cepheids pulse is related to their *average* luminosity (and so their average absolute magnitude). This graph relates the period to their average absolute magnitude.

To use the graph, observe a cepheid for long enough to establish a period (say 30 days, for example). Then read the graph (an absolute magnitude of just under -5.5 in the example)

You now know how intrinsically bright the star is on average. By looking at its average apparent brightness, you can calculate an estimate of its distance. See [How does the cepheid variables help in finding distance](https://astronomy.stackexchange.com/questions/17826/how-does-the-cepheid-variables-help-in-finding-distance?rq=1)


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