07 March 2013

Eclipsing Binary Star System Accurately Determine Distance To The LMC

Astronomers at the European Southern Observatory have accurately determined the distance of one of the closest galaxies, The Large Magellanic Cloud, to the Milky Way to be 163,000 light-years.

A binary star system, as the name suggests is composed of two stars. These stars orbit around a common center of mass. The brightest of the two stars is called the primary star while the other is the secondary or companion star.

One of the closest galaxies to the Milky Way is the Large Magellanic Cloud (LMC). Astronomers at the European Southern Observatory searched through its 35 million stars to find a binary star system that would help to accurately determine the distance of the LMC to the Milky Way.

The stars found by ESO are known as Cepheid Variables. These are bright but unstable stars that pulsate and vary in brightness (see embedded video below). The star that pulsates more quickly are fainter than the those that pulsates more slowly. As the stars revolve, they eclipse each other and the observed brightness changes as this happens.

The exact light variations depend on the relative sizes of the stars, their temperatures and colors and the details of the orbit while the colors are measured by the comparing the brightness of the stars at various near-infrared wavelengths (the farther an object is, the closer it is to the infra-red wavelength).

By studying the relationship between the duration and its luminosity (period-luminosity relationship) of the Cepheids found at the Large Magellanic Cloud, astronomers can accurately determine its distance to the Milky Way.

The Large Magellanic Cloud is an irregular galaxy that is believed to be once a spiral galaxy but was pulled out of shape because of the gravitational effect of the Milky Way.

Eclipsing Binary Star System

Accurately Determining Distance of The Milky Way to The Large Magellanic Cloud

After nearly a decade of careful observations an international team of astronomers has measured the distance to our neighbouring galaxy, the Large Magellanic Cloud, more accurately than ever before. This new measurement also improves our knowledge of the rate of expansion of the Universe — the Hubble Constant — and is a crucial step towards understanding the nature of the mysterious dark energy that is causing the expansion to accelerate. The team used telescopes at ESO’s La Silla Observatory in Chile as well as others around the globe. These results appear in the 7 March 2013 issue of the journal Nature.

Astronomers survey the scale of the Universe by first measuring the distances to close-by objects and then using them as standard candles to pin down distances further and further out into the cosmos. But this chain is only as accurate as its weakest link. Up to now finding an accurate distance to the Large Magellanic Cloud (LMC), one of the nearest galaxies to the Milky Way, has proved elusive. As stars in this galaxy are used to fix the distance scale for more remote galaxies, 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.

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

Video: Eclipsing Binary Star System in the Large Magellanic Cloud

The improvement in the measurement of the distance to the Large Magellanic Cloud also gives better distances for many Cepheid variable stars. These bright pulsating stars are used as standard candles to measure distances out to more remote galaxies and to determine the expansion rate of the Universe — the Hubble Constant. This in turn is the basis for surveying the Universe out to the most distant galaxies that can be seen with current telescopes. So the more accurate distance to the Large Magellanic Cloud immediately reduces the inaccuracy in current measurements of cosmological distances.

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 colours of the stars remarkably accurate distances can be found.

This method has been used before, but with hot stars. However, certain assumptions have to be made in this case and such distances are not as accurate as is desirable. But now, for the first time, eight extremely rare eclipsing binaries where both stars are cooler red giant stars have been identified. These stars have been studied very carefully and yield much more accurate distance values — accurate to about 2%.

“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,” adds Grzegorz Pietrzyński (Universidad de Concepción, Chile and Warsaw University Observatory, Poland), lead author of the new paper in Nature.

“We are working to improve our method still further and hope to have a 1% LMC distance in a very few years from now. This has far-reaching consequences not only for cosmology, but for many fields of astrophysics,” concludes Dariusz Graczyk, the second author on the new Nature paper.


European Southern Observatory
Universidad de Concepción
Warsaw University Observatory
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