ScienceShot: A Better Cosmic Yardstick

Estimates of the distance to a nearby galaxy orbiting the Milky Way, a key parameter that astronomers use to gauge the scale of the universe, are now more accurate than ever—a feat that may help cosmologists in their search for dark matter. Previous studies calculating the distance to the Large Magellanic Cloud (close-up shown), a group of stars that together weigh about 1% of our galaxy, are accurate only to somewhere within 5% or 10%. Now, astronomers have used observations of eight pairs of binary stars in the distant cluster to develop a new figure. The orbits of these pairs are aligned such that one star passes in front of the other as seen from Earth, which allowed the researchers to approximate the size of each member from the durations of the eclipses. The spectra of these cool, mature stars allowed the team to determine their surface temperatures. Together, those bits of data enabled the astronomers to estimate the amount of energy emitted by the stars, and that, in conjunction with observations of their actual brightness as seen from Earth, allowed the researchers to estimate the distance to each pair. The average of the distances to those eight binary systems, which conveniently are all located near the center of the Large Magellanic Cloud, is a shade under 163,000 light-years—a figure that's accurate to about 2.2%, the team reports online today in Nature. The new, better estimate of distance to the Large Magellanic Cloud, in conjunction with observations of other stars in the cluster, will serve as a yardstick to better approximate a parameter called the Hubble constant, which helps cosmologists estimate how quickly the universe is expanding. A more accurate value for the Hubble constant could also aid researchers who are searching for dark energy and dark matter, the mysterious components of the universe that remain invisible to Earthbound instruments.

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