Observations of Pulsating Stars

We've run into several types of variable stars:

Eclipsing Binaries

T Tauris

AGBs

Cepheids are another type of variable star named after the archetype, Delta Cephei, which varyies by a magnitude or so over a well defined period of 5.367 days.

Henrietta Leavitt (1868-1921) compared photographs of various parts of the sky, taken on separate occasions, to identify variable stars. She discovered many Cepheids, most in a nearby small companion galaxy known as the Small Magellenic Cloud. She noted that the brighter ones took longer to change brightness:

Since all the Cepheids in the SMC are (just about) at the same distance, the fact that the brighter ones have longer periods means that period and luminosity are related. If we could find the distance to a Cepheid somehow, we could derive a relationship between period and absolute magnitude which we could use as a distance indicator.

This is hard. Cepheids are far away, so parallax won't work (currently). But main sequence fitting (and more complicated methods) can be used, with some inherent uncertainty. We now have distances to many Cepheids, and can derive the Cepheid period-luminosity relationship:

From Udalski 1999; <W_I> = I - 1.55(V-I)

P-L relation	P-L-C relation
M_V = -2.76logP - 1.18 (scatter = 0.109 mag)	M_I = -3.246logP + 1.409(V-I) - 2.34 (scatter = 0.074 mag)

So if we watch a Cepheid vary and derive its period, we can then immediately calculate its absolute magnitude. Then, having observed its apparent magnitude, we can calculate the distance.

Advantages:

Cepheids are bright (1000x the Sun) -- we can see them even in other (relatively nearby) galaxies
Cepheids are driven (as we shall see) by stellar pulsation, which is fairly well understood.

Disadvantages:

They are relatively rare
Their period also depends on their metallicity (actually a P-L-Z relationship).

How do we know they are pulsating stars? Look at their change in brightness, temperature, and radial velocity:

So Cepheids are brightest when they are hottest, and when they are smallest. Does this make sense?

What kind of stars are these? Look at their positions on the H-R Diagram, and plot models of stellar evolution as well:

Cepheids are evolved high mass stars!

There are other types of pulsating stars, which define an instability strip on the H-R diagram:

W Virginis stars (metal-deficient, under luminous Cepheids)
RR Lyrae stars (evolved low mass stars on the horizontal branch)
Delta Scuti stars (evolved F stars a bit more massive than the Sun)
ZZ Ceti stars (pulsating white dwarfs)

Observations of Pulsating Stars

Pulsation Physics Explained