Surveying the Galaxy The Gaia satellite was launched in December 2013, and over its five-year mission it will measure the positions, motions and distance indicators of more than a billion stars. The ESSOG project aims to exploit this huge body of information, helping to build a more detailed picture of our galaxy, as Professor James Binney explains. Astrometry, the measurement
of the movement of stars across the sky, lies at the heart of astronomy. Upon this foundation astrophysicists build our physical picture of the universe. The deployment of the European Space Agency’s (ESA) Hipparcos satellite in the ‘90s revolutionised astrometry by, for the first time, making astrometric measurements in space and by pioneering a novel technology. “Doing it from space is a game-changer, as the Earth’s atmosphere does two very bad things from the perspective of measuring stars. First, it dithers the image of a star, which makes it difficult to identify the centre of the image. Then light is also refracted by the atmosphere, so it actually moves the apparent position of a star,” explains James Binney, Professor of Physics at the University of Oxford. Moreover, from space it’s possible to look in two quite different directions at once, and doing this resolves the classical difficulty in measuring `parallax’, the tiny variation over the seasons in the direction to a star as the Earth moves around the Sun. “When you look in one direction with a telescope, the parallactic motions of all stars occur in phase. So the angles between stars, which is what you can measure, do not change very much – the stars kind of dance in formation,” outlines Professor Binney. The Hipparcos satellite solved this problem by looking simultaneously in two directions separated by more than 90 degrees, and imaging these two star fields on a single detector. The parallactic motions of the stars in one field were out of phase with those in
the other field, so the distances between their images on the detector changed significantly, and the annual variation of these distances could be measured with precision. Hipparcos was a great success, so soon after its mission was complete, ESA funded a second astrometric satellite called Gaia, which will gather huge volumes of data. “Gaia started its five-year programme of activity in the summer of 2014, and it is measuring the motion of over a billion stars with a precision that is in the order of a hundred times greater than what Hipparcos achieved,” says Professor Binney. Moreover, Gaia works in a different way to
ESSOG project April 2018 saw the release of the first significant set of data from the Gaia mission. The release contains the parallaxes and sky motions of more than a billion stars, and Doppler shifts for several million stars. As Principal Investigator of the ESSOG project, Professor Binney aims to extract scientific insights by combining this trove of data from Gaia with results from massive spectroscopic surveys using large ground-based telescopes. “The goal in ESSOG was to develop the conceptual tools required to exploit this enormous body of information on the kinematics and chemical compositions
Gaia sweeps its two telescopes systematically over the skies, in a complicated pattern, and finds what stars are there. It then measures their positions repeatedly – almost 70 times over the 5 year-period - and from those positions it figures out how they are moving. Hipparcos. “Hipparcos was sent into orbit with a list of roughly 100,000 stars. It was instructed to measure the parallaxes and motions of these stars,” says Professor Binney. “Gaia was not sent with a list, it simply monitors everything in the sky brighter than a faint threshold. It sweeps its two telescopes systematically over the sky, in a complicated pattern, and finds what stars are there. It then measures their positions repeatedly – almost 70 times over the 5 year-period – and from those positions it figures out how they are moving.”
of these stars,” he explains. “We do that by fitting the data into a dynamical model of the galaxy. Such a model specifies the distribution of the mysterious dark matter that holds together galaxies and clusters of galaxies. We have shown that just over half the force that holds the Sun in its orbit around the Galaxy comes from dark matter rather than stars or interstellar gas. The model also specifies how different types of star are distributed in `phase space’ – where the coordinates are position and velocity. Some stars in the galaxy have a
Gaia’s all-sky view of our Milky Way Galaxy and neighbouring galaxies, based on measurements of nearly 1.7 billion stars. The map shows the total brightness and colour of stars observed by the ESA satellite in each portion of the sky between July 2014 and May 2016. Copyright: ESA/Gaia/DPAC.
ESSOG has developed a new type of perturbation theory that yields accurate fits to complex orbits. Left/ top; an orbit in real space; right/ bottom a cross section of the orbit is phase space.
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