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.
For even more distant stars, astronomers will still need to rely on models which look at a star’s temperature, surface gravity and chemical composition, and use the information from the resulting spectrum, together with an evolutionary model, to infer its intrinsic brightness and to determine its distance. However, these models can be off by as much as 30 percent. “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, one of the study’s co-authors. “Therefore stellar distances obtained by such indirect methods should be taken with a pinch of salt.”
The Cambridge researchers have developed a novel method to determine distances between stars by relying on stellar ‘twins’: two stars with identical spectra. Using a set of around 600 stars for which high-resolution spectra are available, the researchers found 175 pairs of twins. For each set of twins, a parallax measurement was available for one of the stars.
The researchers found that the difference of the distances of the twin stars is directly related to the difference in their apparent brightness in the sky, meaning that distances can be accurately measured without having to rely on models. Their method showed just an eight percent difference with known parallax measurements, and the accuracy does not decrease when measuring more distant stars.
“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.”
Since a utilised spectrum for a single star contains as many as 280,000 data points, comparing entire spectra for different stars would be both time and data-consuming, so the researchers chose just 400 spectral lines to make their comparisons. These particular lines are those which give the most distinguishing information about the star – similar to comparing photographs of individuals and looking at a single defining characteristic to tell them apart.
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. While only looking at stars which have twins restricts the method somewhat, thanks to the new generation of high-powered telescopes, high-resolution spectra are available for millions of stars. With even more powerful telescopes under development, spectra may soon be available for stars which are beyond even the reach of Gaia, so the researchers say their method is a powerful complement to Gaia.
“This method provides a robust way to extend the crucially-important cosmic distance ladder in a new special way,” said Professor Gerry Gilmore, the Principal Investigator for UK involvement in the Gaia mission. “It has the promise to become extremely important as new very large telescopes are under construction, allowing the necessary detailed observations of stars at large distances in galaxies far from our Milky Way, building on our local detailed studies from Gaia.”
The research was funded by the European Research Council.
Jofré, P. et. al. Climbing the cosmic ladder with stellar twins. Monthly Notices of the Royal Astronomical Society (2015). DOI: 10.1093/mnras/stv1724.
A new method of measuring the distances between stars enables astronomers to climb the ‘cosmic ladder’ and understand the processes at work in the outer reaches of the galaxy.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 evolvedPaula Jofre PfeilCarolina JofréTwo ‘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
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The Gaia satellite, which orbits the sun at a distance of 1.5million km from the earth, was launched by the European Space Agency in December 2013 with the aim of observing a billion stars and revolutionising our understanding of the Milky Way.
The unique mission is reliant on the work of Cambridge researchers who collect the vast quantities of data transmitted by Gaia to a data processing centre at the university, overseen by a team at the Institute of Astronomy.
Since the start of its observations in August 2014, Gaia has recorded 272 billion positional (or astrometric) measurements and 54.4 billion brightness (or photometric) data points.
Gaia surveys stars and many other astronomical objects as it spins, observing circular swathes of the sky. By repeatedly measuring the positions of the stars with extraordinary accuracy, Gaia can tease out their distances and motions throughout the Milky Way galaxy.
Dr Francesca de Angeli, lead scientist at the Cambridge data centre, said: “The huge Gaia photometric data flow is being processed successfully into scientific information at our processing centre and has already led to many exciting discoveries.”
The Gaia team have spent a busy year processing and analysing data, with the aim of developing enormous public catalogues of the positions, distances, motions and other properties of more than a billion stars. Because of the immense volumes of data and their complex nature, this requires a huge effort from expert scientists and software developers distributed across Europe, combined in Gaia’s Data Processing and Analysis Consortium (DPAC).
“The past twelve months have been very intense, but we are getting to grips with the data, and are looking forward to the next four years of operations,” said Timo Prusti, Gaia project scientist at ESA.
“We are just a year away from Gaia's first scheduled data release, an intermediate catalogue planned for the summer of 2016. With the first year of data in our hands, we are now halfway to this milestone, and we’re able to present a few preliminary snapshots to show that the spacecraft is working well and that the data processing is on the right track.”
As Gaia has been conducting its repeated scans of the sky to measure the motions of stars, it has also been able to detect whether any of them have changed their brightness, and in doing so, has started to discover some very interesting astronomical objects.
Gaia has detected hundreds of transient sources so far, with a supernova being the very first on August 30, 2014. These detections are routinely shared with the community at large as soon as they are spotted in the form of ‘Science Alerts’, enabling rapid follow-up observations to be made using ground-based telescopes in order to determine their nature.
One transient source was seen undergoing a sudden and dramatic outburst that increased its brightness by a factor of five. It turned out that Gaia had discovered a so-called ‘cataclysmic variable’, a system of two stars in which one, a hot white dwarf, is devouring mass from a normal stellar companion, leading to outbursts of light as the material is swallowed. The system also turned out to be an eclipsing binary, in which the relatively larger normal star passes directly in front of the smaller, but brighter white dwarf, periodically obscuring the latter from view as seen from Earth.
Unusually, both stars in this system seem to have plenty of helium and little hydrogen. Gaia’s discovery data and follow-up observations may help astronomers to understand how the two stars lost their hydrogen.
Gaia has also discovered a multitude of stars whose brightness undergoes more regular changes over time. Many of these discoveries were made between July and August 2014, as Gaia performed many subsequent observations of a few patches of the sky.
Closer to home, Gaia has detected a wealth of asteroids, the small rocky bodies that populate our solar system, mainly between the orbits of Mars and Jupiter. Because they are relatively nearby and orbiting the Sun, asteroids appear to move against the stars in astronomical images, appearing in one snapshot of a given field, but not in images of the same field taken at later times.
Gaia scientists have developed special software to look for these ‘outliers’, matching them with the orbits of known asteroids in order to remove them from the data being used to study stars. But in turn, this information will be used to characterise known asteroids and to discover thousands of new ones.
Gerry Gilmore, Professor of Experimental Philosophy, and the Gaia UK Principal Investigator, said: “The early science from Gaia is already supporting major education activity involving UK school children and amateur astronomers across Europe and has established the huge discovery potential of Gaia’s data.
"We are entering a new era of big-data astrophysics, with a revolution in our knowledge of what we see in the sky. We are moving beyond just seeing to knowing about the galaxy in which we live.”
A space mission to create the largest, most-accurate, three-dimensional map of the Milky Way is celebrating its first completed year of observations.We are moving beyond just seeing to knowing about the galaxy in which we live.Gerry Gilmore Credit: Marisa Grove/Institute of AstronomyArtist’s impression of Gaia14aae
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The text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.