Cepheid variables are simply indispensable. It was Harvard’s Henrietta Leavitt who, in 1912, discovered a relationship between the cycle of variable brightness in these stars and their luminosity. With a classic Cepheid, the longer the period of the star, the greater its intrinsic brightness. That sets up the method: Determine the period of the variable, check its apparent magnitude with the absolute magnitude corresponding to that period, and you can measure the distance. The relevant term is ‘standard candle.’

But put telescopes into space and you can refine these measurements, as studies of Cepheid variables with the Hubble Space Telescope have now shown. That’s helpful because we’d like to know the Hubble constant — the universe’s rate of expansion — as accurately as possible, and Cepheids are one of our best tools. To fine-tune the Cepheid method, a team from the University of Texas at Austin has directly measured the distance to ten Cepheid variables, using Hubble to trace their apparent motion in the sky, called parallax.

Parallax has a distinguished history. Looking at a star from opposite sides of Earth’s orbit around the Sun, astronomers made early distance measurements by seeing how far the star was displaced — the star seems to make a small circle on the sky. That apparent motion is helpful for calculating the distance to nearby stars, but far trickier when measuring more distant ones. The Texas’ team’s Milky Way targets were tough — the circle they drew was the equivalent of a quarter seen at 1500 miles — demanding the use of Hubble’s Fine Guidance Sensors.

But once achieved, the parallax findings give us a precise distance that can be weighed against the intrinsic brightness measurements for the Cepheid, and that helps us tune the period-luminosity relationship. Having made the calibration, astronomers can more accurately deduce the distance to Cepheids in distant galaxies. Ultimately, using such data should improve the accuracy of the Hubble constant, giving us precise measurements to any galaxy whose redshift can be measured.

There are other standard candles besides Cepheid variables, and I want to look at some of these in a future post. For now, though, the paper is Benedict et al., “Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations,” Astronomical Journal 1816 (April 2007), pp. 1810–1827. Abstract available.