Cepheid variables!

It's been a while since I wrote anything about astrophysics, so it's high time I came back to a really cool topic: Cepheid variable stars. These provide a so-called standard candle that allows astronomers to determine extraordinary long distances to far-away galaxies.

At its root, a Cepheid variable star is one that breathes in and out either in its first mode of oscillation (just expanding and contracting spherically) or its second (becoming slightly tubular, then bouncing back to a pancake shape, and so on). As it contracts, its surface temperature increases, so its luminosity increases. In most cases, these sorts of oscillations would damp out pretty quickly, but in Cepheid variables, there's a shell of partially ionized helium inside the star. As the star compacts, the internal temperature increases, and this results in extra ionization in the helium layer. As a result, the opacity of that thin shell increases, so the radial pressure from inside increases, pushing the helium layer (and thus the rest of the star) back outwards. At this point the helium ionization decreases thanks to a decrease in temperature, and the whole cycle begins again. To make a long story short, the partially ionized helium layer allows the star to expand and contract with very little damping pretty much indefinitely. Most importantly, there is a set relationship between the period of oscillation and the star's luminosity, which means that by observing how long a brightening and dimming period takes, we know exactly how bright the star really is. Based on this and how bright we see it, we can determine its distance from us!

One drawback of these candles is that they tend to be large (apparently 4-20 solar masses), and therefore short-lived, stars. That means it's hard to find Cepheids in particularly elderly galaxies. But hey, they're one of the coolest (hottest?) standard candles in the astrophysics arsenal, so I'm not complaining!