Cepheid Variables | Universe Today

Written by Jean Tate

Cepheid variables are named after the star Delta Cephei, which is in the constellation of Cepheus. As is sometimes the case in the history of astronomy, this was actually the second example of this kind of variable discovered; the first was Eta Aquilae, discovered a few months earlier, in September 1784.

If Cepheids were people, they'd be celebrities (and doubly deserving of the name 'stars'). They were what Hubble used to find the Hubble Law; they are the original 'standard candle' for estimating distances in extra-galactic astronomy.

Why?

Because they are bright (so they can be clearly seen at multi-megaparsec distances), easy to distinguish from other bright stars (their variability is unique), and it's easy to work out how bright they really (so the difference between how bright they appear tells us how far away they are).

Cepheids are hot, massive stars, five to twenty times as massive as the Sun. Every Cepheid pulsates, in a regular cycle part of the star (a bit below its surface) becomes opaque, gets hotter (because it absorbs more light from deeper in), expands, becomes transparent (because it cools down), and contracts. This produces a distinctive 'shark fin' shape in its light curve. The relationship between luminosity (intrinsic brightness, or absolute magnitude) and period for Cepheids is quite precise; measure the period, and you can work out its luminosity.

Actually, there are two types of stars which have Cepheid light curves, and only in 1942 did Baade distinguishing cleanly between them. Today's Cepheids are Population I Cepheids, found in the galactic plane. The other kind, Population II Cepheids, are called W Virginis variables (after the prototype), and are found in the galactic bulge or old disk population; they have considerably less 'metals' than the real Cepheids (astronomers call all elements beyond helium 'metals'). An interesting consequence of Baade's discovery was that the value of the Hubble constant dropped a lot!

AAVSO (American Association of Variable Star Observers) has a good write-up of Cepheids; and the ESO has an easy exercise on measuring galaxy distances using Cepheids.

Given their importance to astronomy, it should come as no surprise that there are many Universe Today stories on Cepheids; some examples are Astronomers Find New Way to Measure Cosmic Distances, Astronomers Use Light Echo to Measure the Distance to a Star, and Astronomers Closing in on Dark Energy with Refined Hubble Constant.

The Astronomy Cast Stellar Populations explains the differences between Population I and II stars.

Filed under: Astronomy

Tags: cepheid, cepheid variables, cepheids, Hubble constant, Variable Stars