Scientists have developed a novel method to calculate the distances to stars, and it could be useful in helping map the size of galaxies. The study is published in the Monthly Notices of the Royal Astronomical Society.

The researchers from the University of Cambridge examined what are known as “stellar twins.” These are stars that are identical, with exactly the same chemical composition, which can be worked out from their spectra – the type of light they emit. If they were both placed at the same distance from Earth, they would shine with equal brightness.

So the team realized that if the distance to just one of the stars was known, the other could be calculated relatively easily based on how brightly it was shining. The dimmer it is, the further away it is, and vice versa. The method can be used to accurately measure the distance.

"It's a remarkably simple idea – so simple that it's hard to believe no one thought of it before," said lead author Dr Paula Jofre Pfeil, from Cambridge’s Institute of Astronomy, in a statement. "The further away a star is, the fainter it appears in the sky, and so if two stars have identical spectra, we can use the difference in brightness to calculate the distance."

^{The method relies on two stars with identical spectra. Carolina Jofré.}

Usually, distances to stars are measured using the parallax method. This essentially involves looking at a star from two different positions on Earth. By measuring the distance between those two locations, and their angle of inclination to the star, its distance from Earth can be measured.

But the parallax method is only useful for stars within 1,600 light-years, as if they are further then the angle of inclination is too small to be measured. This makes the new method incredibly useful in working out the distances to stars much farther away.

At the moment, the distances to remote stars are measured by using models that calculate their brightness. These models are fairly inaccurate though, with an error as high as 30%. This new method greatly reduces that inaccuracy. And it could also help better measure the size of galaxies, by calculating the distances to different stellar twins at a galaxy’s extremes.

"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," added Pfeil in the statement. "Every time we make an accurate distance measurement, we take another step on the cosmic distance ladder."