Astronomers at the University of Cambridge have discovered a new and surprisingly simple method of measuring distances between stars, which is far more accurate than the current model-dependent techniques and can help scientists obtain data about stellar bodies as far as 30,000 light-years away.
The method involves comparing the brightness of “twin” stars and is already hailed as a major advance in astrometry and a valuable complement to the Gaia satellite, which is currently on a five-year mission to establish the positions of a billion stars in the Milky Way as part of a project to create a three-dimensional map of our native 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 the university’s Institute of Astronomy, and the lead author on the paper. “Every time we make an accurate distance measurement, we take another step on the cosmic distance ladder.”
Currently, the best way to measure the distance between two stars is by making use of an effect know as parallax, which is the apparent displacement of an object when viewed along two different lines of sight (an example of that would be the illusory movement of your hand when looked at with one eye closed and then the other). By measuring the angle of inclination between the two observations, astronomers can use the parallax to determine the distance to a particular star.
The problem with this method, however, is that it can only be used for measuring the distances between stars no further away than 1,600 light-years, meaning that of the 100 billion stars in our galaxy, we can only measure around 100,000.
To measure stellar distances beyond this limit, astronomers are forced to employ models that infer a star’s brightness and distance from other astronomical bodies by looking at its temperature, surface gravity and chemical composition. Useful as they are, these techniques rely on a number of simplifying assumptions and can sometimes be off by as many as 30 percent.
The new method, on the other hand, is much more direct – comparing the spectra of 175 pairs of stellar “twins”, the researchers found that the distances between them are directly related to their brightness in the sky. Available parallax values for the same stars showed only an 8 percent difference, and the accuracy of measurement did not drop with increasing distances.
“It’s a remarkably simple idea – so simple that it’s hard to believe no one thought of it before,” said Jofre Pfeil. “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.”
Given that a utilised spectrum for a single star contains around 280,000 data-points, the team chose only 400 spectral lines for each comparison.
Although the new technique only works with stars that have “twins”, with more powerful telescopes under development, we may soon have spectra for stars beyond even the reach of Gaia, expanding our knowledge of the cosmos well outside the Milky Way.