Mapping the cosmos with Cepheid stars

Cepheid variable stars are a vital rung on the “ladder” astronomers use to determine the distance to astronomical objects. These pulsating stars, which change in brightness as they also change in physical size over time, allowed Edwin Hubble to measure the distance to the Andromeda Galaxy and determine that our Milky Way was one of what we now know are trillions of “island universes” — galaxies — scattered throughout the larger, expanding universe.

These stars have been used as distance indicators since the early 1900s, thanks to the hard work of Henrietta Leavitt. And today, a young astronomer is using the Sloan Digital Sky Survey’s Apache Point Galactic Evolution Experiment (APOGEE) to make more precise measurements of Cepheid variables than ever before. Her project, featured in a press conference during last week’s 231st Meeting of the American Astronomical Society, showed that astronomers are still striving to understand the intricacies of these stars — and how they are closer than ever before to the final piece of the puzzle.

Why Cepheid variables are special
Kate Hartman, an undergraduate from Pomona College working with Rachael Beaton, the NASA Hubble and Carnegie-Princeton Postdoctoral Fellow at Princeton University, was tasked with using APOGEE data to determine the amount of elements present in each Cepheid star measured. Though it may sound simple, this type of measurement is vital to ensure astronomers’ calibration of the period-luminosity relationship for Cepheid variables — called the Leavitt Law — is correct. Problems with that calibration would affect distances measured using the law, which would increase uncertainties in other types of distance measurements, as well.

The Leavitt Law works like this: Cepheid variables are giant stars that pulsate — physically — over the course of hours or days. As Beaton explained in a press release, “over a pulsation cycle of a Cepheid variable, the star’s properties change. Its temperature, surface gravity, and atmospheric properties can vary greatly over a fairly short time.”  These stars demonstrate a clear period-luminosity relationship — a correlation between the period over which a star varies and its intrinsic brightness — that means once an astronomer has measured a Cepheid’s light curve (the variation in its light over time), they immediately know the brightness of the star. Knowing the intrinsic brightness of the star — rather than simply how bright the star appears — allows the astronomer to calculate its distance, because objects get dimmer with distance via a simple and universal law.

This is why only certain types of stars can be used as distance indicators — without the ability to know for certain how bright a star or other object actually is, astronomers cannot use it to measure distance.

APOGEE’s Cepheid catalog
So then the question arises: Do variables with slightly different chemical compositions or in different chemical environments have different period-luminosity relationships? Or is the single Leavitt Law capable of letting astronomers view any Cepheid variable, anywhere, and measuring its distance in a more straightforward way? And, additionally: Are surveys like APOGEE capable of producing reliable information, such as composition, about these stars, when a single image could catch the star at any random time during its pulsation period?

Hartman and Beaton wanted to find out. APOGEE was perfect for this job, because although it’s “optimized to study the cool, old giant-type stars found all across our galaxy,” said Beaton, “Cepheid variables … are similar in temperature, so they are well suited for APOGEE.”

Thus, APOGEE not only provides a large catalog of Cepheids, it also provides a tool to measure their properties in the same way that other (older) stars are measured. This type of consistency lets astronomers study both young and old parts of the galaxy to better trace its evolution.