Scientific Method / Science & Exploration

Helium-burning binary stars give us a new tape measure for the Universe

Gives the distance to the nearest galaxy with 1/3 the uncertainty.

The Large Magellanic Cloud, the largest satellite galaxy of the Milky Way. Accurate estimates of the distance to this galaxy help calibrate measurements of the expansion rate of the Universe.

The measurement of cosmic distances is like a scaffold, where levels are built upon the structure below. To measure the distances to far galaxies, astronomers must calibrate their measurements using closer objects that we know the distance to; those in turn must be calibrated by closer objects still. As a result, if we are to trust the measurements we've made of the structure and expansion of the Universe, we'd better have really good measurements to nearby galaxies.

For that reason, astronomers have devoted a lot of effort to measuring the distance to the Milky Way's brightest satellite galaxy, Large Magellanic Cloud. Now G. Pietrzyński and colleagues have determined that galaxy's distance with unprecedented accuracy. By identifying a set of rare binary stars, their properties allowed the astronomers to measure their distances from Earth to 2.2 percent accuracy. These results will help refine the measurements on which cosmology is founded: the expansion rate of the Universe.

Distances within the Solar System can be measured a number of ways, including direct methods like radar ranging. With a truly accurate estimate of the size of Earth's orbit, astronomers can use parallax—the apparent displacement of stars in the sky as Earth orbits the Sun—to find the distances to stars in the Milky Way. Some stars are variable, changing brightness in a predictable way; these provide a means of measuring how far it is to neighboring galaxies.

These and other measurements allow astronomers to calibrate distances to galaxies containing Type Ia supernovae. These explosions are visible billions of light-years away, and outshine everything else in their host galaxies. As acknowledged by the 2011 Nobel Prize in physics, Type Ia supernovae are currently our best way to track the expansion rate of the Universe.

The current study provides a possible alternative means to measure distances on an intermediate scale with more accuracy than we can get using variable stars. The Large Magellanic Cloud (LMC) is the second-closest galaxy to the Milky Way; you can think of it as the second layer of scaffolding, above measurements inside our galaxy. (Only the Sagittarius dwarf spheroidal galaxy is closer than the LMC, but it's much smaller and fainter, and therefore less useful for cosmological calibrations.)

The researchers used data from the Optical Gravitational Lensing Experiment (yes, it's nicknamed OGLE), which was designed to look for fluctuations in dark matter density by observing stars in the LMC. While OGLE hasn't succeeded in its primary goal of spotting clumps of dark matter, it has amassed a lot of data from 35 million stars, going back as far as 1992.

From those 35 million stars, the astronomers identified 12 eclipsing binary stars; of those, they analyzed data from eight pairs for a period of eight years. These pairs they chose are rare, consisting of stars in the helium-burning stage, which occurs after they have exhausted their core's hydrogen fuel. Aging stars of this type have well-known intrinsic brightness in relation to their color.

The researchers also selected the eight binaries for the length of their orbits: relatively long periods ranging between 60 and 772 days. Combined with the amount of light blocked during the eclipses, these long orbits enabled the astronomers to reconstruct the sizes of the stars through the same techniques that are commonly used for spotting exoplanets. That provided a detailed physical picture of the binary star systems, pinpointing their exact intrinsic brightness to approximately 2 percent accuracy. If an object's intrinsic brightness is known, it's a remarkably simple matter to estimate its distance from Earth based on how much of that light reaches us.

The LMC has a fairly simple structure—a flattened disk—and all the binaries were found close to the galaxy's center. As a result, the astronomers used the stars' distances to estimate how far the LMC's center is from Earth: 49.97 kiloparsecs, give or take 0.19 kiloparsecs. (One kiloparsec is a thousand parsecs, or roughly 3,300 light-years.) That accuracy is about 2.2 percent, a vast improvement over previous estimates based on variable stars, which did no better than 8 percent.

With these estimates in hand, it will be possible to calibrate the expansion rate of the Universe—a quantity known as the Hubble parameter—to about 3 percent accuracy. Future observations should be able to improve both the LMC distance measurement and the Hubble parameter even further.

Nature, 2013. DOI: 10.1038/nature11878  (About DOIs).

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