The distance to the LMC

Measuring the distance to celestial objects is probably the most difficult problem in astronomy: unless you know the intrinsic brightness or size of an object, you can’t tell its distance simply by measuring its apparent brightness or size. A large, bright, distant galaxy looks the same as a small, dim, nearby galaxy. Conversely, if we know the distance to an object then we can begin to understand many of its most important physical characteristics, such as its size and luminosity.

As I explain at length in Measuring the Universe, astronomers estimate how far away something is by making use of the so-called cosmological distance ladder. The idea is that by understanding the behaviour of objects that are nearby we can estimate distances to objects that are further away; by understanding behaviour on that larger distance scale we can reach out and estimate distances to objects that are even further away; and so on. We start by determining the radius of the Earth; that gives us a clue to the scale of the solar system; that in turn gives us a clue, via parallax, to the distance to the nearby stars; by understanding the nature of stars we can estimate the size of the Galaxy; and so on and so on, until we can determine the size of the Universe by determining the Hubble constant H0.

The problem with this approach is that an error in one of the early rungs of the distance ladder will propagate through to the later rungs: when astronomers in the last century corrected their misunderstanding of the brightness of a type of variable star called a Cepheid, their estimate of the size of the Universe doubled.

Establishing the distance to the Large Magellanic Cloud (LMC), one of the closest galaxies to our own, provides the basis for one of the early rungs on the cosmological distance ladder.  If we have an accurate determination of the LMC distance then we can calibrate various distance-measuring techniques that in turn let us measure objects at cosmological distances. In Measuring the Universe I gave the results of three distance estimates to the LMC. Cepheid measurements produced a distance of 50 kpc; a technique based on measuring the tip of the red giant branch yielded a distance of 48 kpc; and RR Lyrae stars gave a distance of 45 kpc. Each technique had its associated errors, of course, but there was a 10% difference between the Cepheid and the RR Lyrae methods. It would be good to measure the LMC distance with more accuracy.

Well, this week’s Nature contains a paper entitled “An eclipsing-binary distance to the Large Magellanic Cloud accurate to two per cent“. Grzegorz Pietrzyński and his colleagues observed eight binary systems over a period of almost ten years. These eight systems were “eclipsing” binaries: in other words, because of the orientation of their orbits with respect to Earth, our telescopes observe them to pass in front of each other. As one star eclipses another, the total brightness of the pair diminishes; and it diminishes by different amounts depending on which star is doing the eclipsing. By making careful observations of these changes in brightness, and combining them with measurements of the orbital speeds of the stars, the Pietrzyński team were able to estimate the distance to the LMC: it’s 49.91 ± 0.19 kpc away. (There is a systematic error of 1.11 kpc.)

Graphic of brightness vs time for an eclipsing binary

How the apparent brightness of an eclipsing binary system changes with time (Credit: NASA)

The eclipsing-binary measurement agrees well with the latest LMC distances using other techniques. Astronomers are firming up one of the key rungs of the distance ladder.