News story

Star surveyor completes first year of science observations

Last Friday, 21 August 2015, Europe’s billion-star surveyor, Gaia, completed its first year of science observations in its main survey mode.

Stellar density map
Stellar density map. Credit: ESA/Gaia – CC BY-SA 3.0 IGO.

After launch on 19 December 2013, and a six-month in-orbit commissioning period, the satellite started routine scientific operations on 25 July 2014. Located at the Lagrange point L2, 1.5 million km from Earth, 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 through the Milky Way.

What’s the big deal about Gaia?

Since the start of its routine phase, the satellite has recorded 272 billion positional or astrometric measurements, 54.4 billion brightness or photometric data points, and 5.4 billion spectra. This successful first year of science operations is excellent news for the UK which is a major contributor to the mission.

The Gaia team has spent a busy year processing and analysing these data, en route towards the development of Gaia’s main scientific products, consisting of 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). UK Gaia teams and the Cambridge Gaia Data Processing Centre– part of the DPAC – are playing a key role in processing Gaia’s images and data and will continue processing Gaia data to turn it into a calibrated set of measurements that can be freely used by the astronomical community.

Prof Gerry Gilmore, UK Gaia PI, said:

Exceptional efforts by many people are delivering a revolution in quantitative knowledge of our Milky Way. We are making the transition from seeing to knowing, and enhancing understanding.

As one example of Gaia’s ongoing validation, the Gaia team has been able to measure the parallax for an initial sample of two million stars. Parallax is the apparent motion of a star against a distant background observed over the period of a year and resulting from the Earth’s real motion around the Sun; this is also observed by Gaia as it orbits the Sun alongside Earth. But parallax is not the only movement seen by Gaia: the stars are also really moving through space, which is called proper motion.

Our first Hertzsprung-Russell diagram, with absolute luminosities based on Gaia’s first year and the Tycho-2 catalogue, and colour information from ground-based observations, gives us a taste of what the mission will deliver in the coming years,

says Lennart Lindegren, professor at the University of Lund and one of the original proposers of the Gaia mission.

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. This discovery and publication of Gaia Science Alerts is the responsibility of the Cambridge data processing team, whose results are published, with much explanatory material, on the UK GAIA website.

Gaia has detected hundreds of transient sources so far, with a supernova being the very first on 30 August 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. A description of this rare and interesting system is available at http://gaia/ac/uk/news.

Gaia has also discovered a multitude of stars whose brightness undergoes more regular changes over time, observed the Large Magellanic Cloud (LMC), a dwarf galaxy and close companion of our own galaxy, and the Cat’s Eye Nebula, a planetary nebula also known as NGC 6543, which lies close to the north ecliptic pole.

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.

Finally, in addition to the astrometric and photometric measurements being made by Gaia, it has been collecting spectra for many stars.

These early proof-of-concept studies demonstrate the quality of the data collected with Gaia so far and the capabilities of the processing pipeline. The final data products are not quite ready yet, but we are working hard to provide the first of them to the community next year. Watch this space,

concludes Timo.

Published 28 August 2015