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Distance to Neighbor Galaxy Measured More Accurately

the Large Magellanic Cloud

In this image of the Large Magellanic Cloud, the positions of the eight faint and rare cool eclipsing binary stars used in this study are marked with crosses (these objects are too faint to appear directly in this picture). By studying how their light changes, and other properties of these binary systems, astronomers can measure the distances to them very accurately. Credit: ESO/R. Gendler. LMC image © Robert Gendler

After nearly a decade of careful observations, an international team of astronomers, including two from the IfA, has measured the distance to our neighboring galaxy, the Large Magellanic Cloud, more accurately than ever before.

Astronomers ascertain the scale of the Universe by first measuring the distances to close-by objects and then using them as standard candles—objects of known brightness—to pin down distances farther and farther out in the Universe.

This chain is only as accurate as its weakest link. Up to now, finding an accurate distance to the Large Magellanic Cloud (LMC), a satellite galaxy to our own Milky Way, has proved elusive. Stars in that galaxy are used to fix the distance scale for more remote galaxies, so it is crucially important.

But careful observations of a rare class of double star have now allowed a team of astronomers to deduce a much more precise value for the LMC distance: 163,000 light-years.

“This is a true milestone in modern astronomy. Because we know the distance to our nearest neighbor galaxy so precisely, we can now determine the rate at which the Universe is expanding—the Hubble constant—with much better accuracy. This will allow us to investigate the physical nature of the enigmatic dark energy, the cause of the accelerated expansion of the Universe,” said IfA astronomer Rolf-Peter Kudritzki, a team member.

Rolf Kudritzki and Fabio Bresolin

IfA team members Fabio Bresolin and Rolf Kudritzki.

IfA astronomer Fabio Bresolin, also a team member, explained, “For extragalactic astronomers, the distance to the Large Magellanic Cloud represents a fundamental yardstick with which the whole Universe can be measured. Obtaining an accurate value for it has been a major challenge for generations of scientists. Our team has overcome the difficulties using an exquisitely accurate method, and is already working to cut the small remaining uncertainty by half in the next few years.

The team worked out the distance to the LMC 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, what their masses are, 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.

“Now we have solved this problem by demonstrably having a result accurate to 2 percent,” stated Wolfgang Gieren (Universidad de Concepción, Chile), one of the leaders of the team.

The team used telescopes at the European Southern Observatory’s Paranal and La Silla Observatories in Chile as well as others around the globe. The LMC is a southern sky object that is not observable from the Northern Hemisphere, so telescopes on Mauna Kea could not be used for this project.

These results appeared in the March 7 issue of the journal Nature.