Cepheid stars are remarkable beacons. Each pulses with a regular frequency related to its intrinsic brightness. An astronomer can therefore determine the distance to a Cepheid by comparing its observed and intrinsic luminosities. Cepheids, in turn, calibrate other classes of luminosity-signaling standard candles that can be viewed from farther away. Iterating the process yields a cosmic distance ladder that enables distance estimation across the cosmos. Astronomers had long thought the behavior of Cepheids was unchanging, but Pauline Barmby of the University of Western Ontario and colleagues now report that of 29 Cepheids that the team studied, about one-fourth are losing mass. As they do so, they shroud themselves with dust that could introduce errors in distance estimations. The figure, from an earlier, related study, shows an IR image obtained with the Spitzer Space Telescope of one such star, Delta Cephei. The red region is a shock created as the star plows through interstellar gas and dust. The detailed form of the shock reveals that it was shaped by a stellar wind emanating from Delta Cephei that’s a million times stronger than that from our Sun. So, is a lower rung on the cosmic distance ladder rotten, making the ladder itself unreliable? To the contrary, argued team member Massimo Marengo at last week’s American Astronomical Society meeting in Seattle. Even with its uncertainties, the Cepheid rung is strong and observations based on the Cepheids remain trustworthy. Furthermore, now that astronomers are gaining a deeper understanding of those stars, they will be able to better determine their brightness and construct an even sturdier ladder. (M. Marengo et al., Astrophys. J. 725, 2392, 2010; P. Barmby et al., Astron. J. 141, 42, 2011.)—Steven K. Blau
