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Arthur T. Knackerbracket writes in from afar with a story from phys.org:

Astronomers from the University of Cambridge have developed a new, highly accurate method of measuring the distances between stars, which could be used to measure the size of the galaxy, enabling greater understanding of how it evolved.

Using a technique which searches out stellar 'twins', the researchers have been able to measure distances between stars with far greater precision than is possible using typical model-dependent methods. The technique could be a valuable complement to the Gaia satellite - which is creating a three-dimensional map of the sky over five years - and could aid in the understanding of fundamental astrophysical processes at work in the furthest reaches of our galaxy. Details of the new technique are published in the Monthly Notices of the Royal Astronomical Society.

"Determining distances is a key problem in astronomy, because unless we know how far away a star or group of stars is, it is impossible to know the size of the galaxy or understand how it formed and evolved," said Dr Paula Jofre Pfeil of Cambridge's Institute of Astronomy, the paper's lead author. "Every time we make an accurate distance measurement, we take another step on the cosmic distance ladder."

The best way to directly measure a star's distance is by an effect known as parallax, which is the apparent displacement of an object when viewed along two different lines of sight - for example, if you hold out your hand in front of you and look at it with your left eye closed and then with your right eye closed, your hand will appear to move against the background. The same effect can be used to calculate the distance to stars, by measuring the apparent motion of a nearby star compared to more distant background stars. By measuring the angle of inclination between the two observations, astronomers can use the parallax to determine the distance to a particular star.

However, the parallax method can only be applied for stars which are reasonably close to us, since beyond distances of 1600 light years, the angles of inclination are too small to be measured by the Hipparcos satellite, a precursor to Gaia. Consequently, of the 100 billion stars in the Milky Way, we have accurate measurements for just 100,000.

Gaia will be able to measure the angles of inclination with far greater precision than ever before, for stars up to 30,000 light years away. Scientists will soon have precise distance measurements for the one billion stars that Gaia is mapping - but that's still only one percent of the stars in the Milky Way.

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