Binary stars help pin down distance to neighbouring galaxy


The Large Magellanic Cloud — one of the 'satellite' galaxies that orbit the Milky Way — has repeatedly thwarted attempts to narrow down its exact distance from us, but a group of astronomers have announced that they now have the most accurate measurement of that distance ever.

"I am very excited because astronomers have been trying for a hundred years to accurately measure the distance to the Large Magellanic Cloud, and it has proved to be extremely difficult," said Wolfgang Gieren, an astronomer at the Universidad de Concepción, in Chile.

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Measuring distances in space isn't easy. There's no cosmic metre-stick to put out there to give us accurate measurements to the objects around us.

Astronomers find the distance to nearby stars using parallax — the apparent change in position of a star (to our viewpoint) from one 'side' of our orbit to the other — and geometry. They can then look at similar stars that are further away (where parallax doesn't help as much), compare the brightness of the light they receive from both, and determine distance by how much the light from the farther star has dimmed as it travels through space. Even this method becomes more difficult, though, the further the object is away, because measurements of the exact brightness of stars become harder to get. They can have a pretty good idea of the distance, but there is always a margin of error involved, and the farther away the star is, the larger that margin is likely to be.

For galaxies, the margin of error can be huge. However, there are some tricks that can be used to help. If you can find pairs of 'eclipsing binary' stars — two stars that orbit each other, and periodically pass in front of each other from our point of view (click here for a great animation) — you can use the relative dip in the brightness as they pass in front of each other to get a much better measurement of each star's brightness. With that value narrowed down, it's much easier to tell the distance to the stars.

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The astronomers used telescopes at the European Southern Observatory's (ESO) La Silla Observatory and the Las Campanas Observatory, both in Chile, and found eight different eclipsing binary systems in the Large Magellanic Cloud (LMC) that they could use in this way.

"ESO provided the perfect suite of telescopes and instruments for the observations needed for this project: HARPS for extremely accurate radial velocities of relatively faint stars, and SOFI for precise measurements of how bright the stars appeared in the infrared," said Grzegorz Pietrzyński, who is the lead author of the new paper in Nature.

It took over 10 years of observations of these stars, but once they had collected all the data, they were able to refine the distance to the LMC to 163,000 light-years, with only a 2 per cent margin of error.

"We are working to improve our method still further and hope to have a 1 percent LMC distance in a very few years from now," said astronomer Dariusz Graczyk, who was the second author on the paper. "This has far-reaching consequences not only for cosmology, but for many fields of astrophysics."

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