Two ‘twin’ stars with identical spectra observed by the La Silla Telescope. Since it is known that one star is 40 parsecs away, the difference in their apparent brightnesses allows calculation of the second star’s distance. Credit: Carolina Jofre
A new, highly accurate method of measuring distances between stars, 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’, they have been able to measure distances between stars with far greater precision than is possible using typical model-dependent methods. It will complement to Gaia satellite, which is creating a 3D map of the sky over 5 yrs, and could aid in the understanding of fundamental astrophysical processes at work far away.
“..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 Pfeil, CambridgeU.
The best way to directly measure a star’s distance is via parallax: apparent displacement of an object when viewed along 2 different lines of sight ie measuring apparent motion of a nearby star compared to more distant background stars. By measuring the angle of inclination between the 2 observations, astronomers can determine the distance to a star.
BUT parallax method can only be applied for stars which are close to us. Beyond 1600 light years, angles of inclination are too small to be measured by Hipparcos satellite, precursor to Gaia. So of 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 1 billion stars that Gaia is mapping – but that’s still 1% of the Milky Way.
For distant stars, they use models which look at star’s temperature, surface gravity and chemical composition, and use the information from the resulting spectrum + evolutionary model, to infer its intrinsic brightness and to determine its distance, but this can be off by as much as 30%. “Using a model also means using a number of simplifying assumptions — like for example assuming stars don’t rotate, which of course they do,” said Dr Thomas Mädler.
Cambridge researchers can now use stellar ‘twins’: 2 stars with identical spectra. Using 600 stars for which high-res spectra are available, they found 175 pairs of twins. For each set of twins, a parallax measurement was available for one of the stars. The difference of distances of the twin stars is related to difference in their apparent brightness, ie distances can be accurately measured without models. Their method showed just 8% difference with known parallax measurements, and accuracy does not decrease for distant stars. “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 next step for the researchers is to compile a ‘catalogue’ of stars for which accurate distances are available, and then search for twins among other stellar catalogues for which no distances are available.With even more powerful telescopes under development, spectra may soon be available for stars which are beyond even the reach of Gaia. http://www.cam.ac.uk/research/news/using-stellar-twins-to-reach-the-outer-limits-of-the-galaxy
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