How do Cepheid variables indicate distance?

The duration of the pulsations of these stars is always related to their intrinsic brightness. Astronomers can use this knowledge to calculate the distance.

RS Puppis is a Cepheid variable star whose brightness varies by a factor of five approximately every 40 days. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration; Acknowledgements: H. Bond (STScI and Penn State University)

How do Cepheid variables indicate distance?

Roger Brady
Tamale, California

Cepheid variable stars are a type of star that undergo regular pulsations. The duration of a Cepheid's pulsations is always related to its intrinsic or absolute brightness in a simple way: the longer its pulsations last, the larger and brighter the star. This means that if you can measure the period of a Cepheid variable star, or the time it takes to undergo a complete cycle of brightness changes (for example, from bright to dim to bright again), you can use a relationship called Leavitt's law to calculate the star's intrinsic brightness. The law is named after Henrietta Swan Leavitt, who first recognized this property of Cepheids while cataloging stars at Harvard University in the early 20th century.

What makes Cepheids so valuable as distance indicators is that obtaining the intrinsic brightness of a star is otherwise very difficult. Intrinsic brightness is an innate property of a star. It is much easier to measure the intrinsic brightness of a star. apparent brightness, which is simply how bright a star appears to us here on Earth.

If we know a star's intrinsic brightness as well as its apparent brightness, we can use those values ​​to easily calculate its distance. In the absence of other factors, all light sources appear dimmer with distance. A 30-watt light bulb, for example, always shines at a brightness of 30 watts, but it will appear much dimmer if you are far away.

Specifically, the brightness of an object dims by 1/d.²where d is the distance to the object. Therefore, since we can easily deduce the absolute luminosity of a Cepheid from its period, we have exactly the information we need to confidently calculate its distance.

Alison Klesman
Chief editor

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