News in Science

New measure brings galactic neighbour closer

Size of the Universe Scientists have made the most accurate measurements yet of the distance to a neighbouring galaxy, the Large Magellanic Cloud (LMC).

The new data, reported in this week's Nature journal, places the LMC at a distance of 162,983.823 light years.

This brings the LMC about 400 light years closer than previously thought.

The new measure will help scientists refine astronomical calculations for the Hubble constant, the rate at which the universe is expanding out from the big bang.

"Uncertainty [in the Hubble Constant] is dominated by the uncertainty in the distance to the Large Magellanic Cloud," write the authors, including Dr Grzegorz Pietrzynski of the Universidad de Concepcion in Chile, who led the research.

The authors add the LMC, which is our second-closest galaxy, "serves as the best anchor point for the cosmic distance scale".

The new study, uses star systems called eclipsing binaries to determine a more accurate distance to the LMC.

Eclipsing binaries consist of two gravitationally bound stars passing directly in front of each other every orbit, as seen from Earth.

The total brightness of the system drops slightly during each eclipse, allowing scientists to measure the properties of the stars, including their distance from Earth.

Eight years' monitoring

Pietrzynski and colleagues used 16 years of observations to identify eight eclipsing binaries in the LMC that were then monitored for eight years.

Previous studies of eclipsing binaries in the LMC used hot stars that could not be precisely measured.

This resulted in some assumptions being made that limited accuracy to between 5 and 10 per cent.

The new observations involved stars that were harder to find, but which could be measured more precisely, improving accuracy to 2.2 per cent.

"These ... systems provide an opportunity to use the full potential of eclipsing binaries as precise and accurate distance indicators, and to calibrate the cosmic distance scale," the authors write.

"This new measurements will decrease the uncertainty in calculations of the Hubble constant," says Louisiana State University Professor Bradley Schaefer, in an accompanying commentary.

"The better we understand the constant, the better we understand dark energy.

"If the LMC distance is off by 10 per cent, then distances to all galaxies quantified with the Hubble constant will be in error by the same 10 per cent.

"The method used in this new study inspires confidence because it uses simple undergraduate astronomy and physics."

Schaefer raises some concerns over the methodology of the work and says the "distance to the LMC has long been a contentious question".

He suggests however the new measure will have a limited time at the "forefront of astrophysics".

He says the Gaia spacecraft, to be launched later this year, will provide "definitive calibration" as it has "awesome capability for measuring accurate distances" even out to the LMC.

However