Key galaxy distance re-measured

By Jonathan Amos
Science correspondent, BBC News

image captionThe LMC is famed for containing some of the most vigorous star formation in the nearby Universe

Astronomers have updated one of their most important numbers.

It is the distance to the Large Magellanic Cloud, a close-by galaxy to our own Milky Way.

By studying a clutch of rare double stars in the LMC, the scientists were able to determine the separation to be 163,000 light-years.

The calculation is said to be very accurate. It is significant because the number is used to calibrate techniques that measure even larger distances.

The hope is the greater confidence will now lead to an improvement in our understanding of the expansion rate of the Universe which is described in a value astronomers refer to as the Hubble Constant.

"All other techniques are affected by the zero point," said Grzegorz Pietrzynski from the Universidad de Concepción, Chile. "And after so many years trying to calculate the Hubble Constant, it is clear the biggest contribution to its error is related to the determination error in the distance to the Large Magellanic Cloud. So getting the LMC right is a very important part of the whole procedure," he told BBC News.

Dr Pietrzynski and colleagues made their calculation by observing eight so-called eclipsing binaries.

These are pairs of stars that orbit each other, and, because of their orientation to the Earth observer, pass in front of one another as they do so.

With each eclipse, the brightness of the pair falls away, and by different amounts depending on which star is Earth-side.

By tracking these luminosity changes together with the velocities of the stars, it is possible to determine the binary system's properties and, crucially, its distance. The work, reported in Nature magazine, took almost a decade to complete.

The eight pairs were all sited broadly in the centre of the LMC. Averaged together, they give a separation from Earth of around 49.97 kiloparsecs. A kiloparsec is an astronomical unit that encompasses about 3,300 light-years.

The 49.97 kiloparsecs figure is said to be accurate to 2.2% (previous measures had an accuracy of around 5-10%).

image captionArtist's impression of eclipsing stars: The eight pairs were found in a survey of 35 million stars

Measuring distances is critically important in modern astronomy.

Knowing how far away something is can help to establish its properties. It is also fundamental in mapping the distribution of matter, and for understanding the evolution of the cosmos.

But measuring distances very precisely is actually one of the hardest things to do in astronomy.

For the closest objects, like a star in our Milky Way, it is possible to use trigonometric parallax, by exploiting the way the star's angle on the sky appears to shift as the Earth goes around the Sun. However, this approach only works within the Milky Way.

Scientists have developed a suite of smart techniques that takes them further out into the Universe but each step on this "ladder" is only as good as the errors on the rung below.

The highest step is the Hubble Constant which describes a linear relationship between the distances of objects and the degree to which their light is stretched by the expansion of the Universe. But even this constant (which uses the units of kilometres per second per megaparsec) suffers from the same calibration issue.

Dr Pietrzynski and his team plan to extend their work by finding and investigating yet more eclipsing binaries, both within the LMC and other nearby galaxies. There is high hope also for the European Space Agency's Gaia satellite due to launch later this year.

This will produce an ultra-precise map of more than a billion stars in and near the Milky Way. Some should even be sited within the LMC itself and available to trigonometric measurement.

"Gaia will obtain parallax data for many stars in the LMC with the resultant average accurate to 1%, and will map out its structure," said Prof Bradley Schaefer from Louisiana State University, US. "So the time spent by the LMC's eclipsing binaries at the forefront of astrophysics will be limited to only the next few years," he told Nature.

image captionGaia is a European Space Agency mission due to launch at the end of the year and follow me on Twitter: @BBCAmos

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