We know that the Milky Way is surrounded by an array of satellite galaxies, the largest of which is the rather aptly named Large Magellanic Cloud. But figuring out exactly how far away our cosmic neighbor is has proved fiendishly difficult.
This isn't simply an accounting problem — fixing a precise measurement of the distance between our galaxy and its satellites is a vital first step in figuring out the Hubble Constant, which describes the rate of the expansion of the universe. Astronomers determine the Hubble Constant through careful observation of far distant galaxies, but the only way to get an accurate measure of those galaxies is to know how the distance of more proximate objects and then extrapolate out from there.
As such, if we don't have a fairly precise figure for the distance to galaxies like the Large Magellanic Cloud, then any measuring errors are liable to be compounded many times over as we look further and further away. That's a big reason why astronomers at the European Southern Observatory have spent the last decade determine the distance to the Large Magellanic Cloud, arriving at a measurement of 163,000 light-years that's accurate to within 2%. That's a huge improvement over any previous estimate, and the ESO explains how they figured it out in a recent statement:
The astronomers worked out the distance to the Large Magellanic Cloud by observing rare close pairs of stars, known as eclipsing binaries. As these stars orbit each other they pass in front of each other. When this happens, as seen from Earth, the total brightness drops, both when one star passes in front of the other and, by a different amount, when it passes behind. By tracking these changes in brightness very carefully, and also measuring the stars' orbital speeds, it is possible to work out how big the stars are, their masses and other information about their orbits. When this is combined with careful measurements of the total brightness and colors of the stars remarkably accurate distances can be found.
For more, you can read the complete original paper, which has just been published in Nature, right here.
Artist's impression of eclipsing binary by ESO/L. Calçada.