So my understanding of how Cepheid is used as standard candle to determine distance of galaxies is like this: Astronomers observe a galaxy, notice an object within that galaxy whose brightness pulsates periodically, measure its period, translate it into luminosity by using Cepheid period-luminosity relationship, and get the distance from the modulus.

But how do they confirm that object really is a Cepheid and not another type of variable (say, RR Lyrae) that may have different (or even nonexistent) period-luminosity relationship? Is there any observable properties that is unambiguously unique to Cepheids?

  • $\begingroup$ Cepheids are yellow giant stars, so their spectrum and the regularity of their light curves are defining characteristics. There may be more; that’s why this is a comment and not an answer. $\endgroup$ Nov 5, 2022 at 23:41

1 Answer 1


Unambiguously unique (is there another kind of uniqueness?)? No. In fact in the 1950s it was discovered that Cepheid identification up to that point had often been inaccurate.

But Cepheids do exist in a particular area of the H-R diagram (the 'instability strip') - see here on Wikipedia: https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram#/media/File:HR-diag-instability-strip.svg - and so modern probabilistic/heuristic methods of identification are strongly supported.

As you may know stellar distances (and hence luminosities) are estimated using a variety of over-lapping 'yard stcks' which run from the highly-accurrate parallax to estimate the distances in the stellar neighbourhood to methods such as the brightness of supernovae at large distances and ultimately the red-shift.

Cepheids fall in the lower to 'middle' of this system and , in general, it can be assumed their identiification and classification is now well attested.


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