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Gaia’s early data release 3 charts positions of 1.8 billion stars - and reveals secrets of Milky Way and beyond




A catalogue of 1.8 billion stars that describes their positions and movements with exquisite accuracy has been unveiled by an international team of astronomers led by the University of Cambridge.

The third early data release from the European Space Agency’s Gaia space observatory was launched at a briefing hosted by the Royal Astronomical Society last Thursday and represents a publicly available treasure trove from which astronomers worldwide will extract discoveries about our galaxy’s past and future.

Data from more than 1.8 billion stars have been used to create this map of the entire sky. It shows the total brightness and colour of stars observed by ESA’s Gaia satellite and released as part of Gaia’s early data release 3. Picture: ESA/Gaia/DPAC
Data from more than 1.8 billion stars have been used to create this map of the entire sky. It shows the total brightness and colour of stars observed by ESA’s Gaia satellite and released as part of Gaia’s early data release 3. Picture: ESA/Gaia/DPAC

Stellar positions, movement, brightness and colours are described in the data, along with the first optical measurement of the acceleration of the Solar system in its orbit around the Milky Way.

Researchers found that the Sun accelerates towards the centre of the galaxy by 7mm per second, as it speeds along its orbit at about 230km a second (about 515,000mph). This gentle acceleration is expected from a system in a circular orbit.

To put it another way, the findings show the velocity of the Solar System changes by 0.23 nm/s every second, meaning its trajectory is deflected by the diameter of an atom every second. In a year this adds up to around 115km.

The concept for Gaia was originally proposed by Prof Gerry Gilmore and colleagues at the Institute of Astronomy at the University of Cambridge before the turn of the century.

With a billion euro budget, 1,000 people involved and numerous industrial partners, it is one of ESA’s flagship missions, designed to measure billions of stars 70 times each over five years to 2022 - with the possibility of a mission extension to 2025..

Approved in 2000, and launched in 2013, Gaia now operates in an orbit around the Lagrange 2 (L2) point, 1.5 million kilometres behind the Earth in a direction away from the Sun.

Professor Gerry Gilmore, one of the original proposers of the Gaia mission, with a model of Gaia. Picture: Keith Heppell
Professor Gerry Gilmore, one of the original proposers of the Gaia mission, with a model of Gaia. Picture: Keith Heppell

At this point, the gravitational forces between the Earth and Sun are balanced meaning the spacecraft stays in a stable position, allowing essentially unobstructed views of the sky.

Dr Floor van Leeuwen of Cambridge’s Institute of Astronomy said: “Gaia is measuring the distances of hundreds of millions of objects that are many thousands of light years away, at an accuracy equivalent to measuring the thickness of hair at a distance of more than 2000 kilometres.

“These data are one of the backbones of astrophysics, allowing us to forensically analyse our stellar neighbourhood, and tackle crucial questions about the origin and future of our Galaxy.”

Gaia’s primary objective is to measure stellar distances using the parallax method.

This measures the apparent change in the position of stars over time, resulting from the Earth’s movement around the Sun.

The trails on this image show how 40 000 stars, all located within 100 parsecs (326 light years) of the Solar System, will move across the sky in the next 400 thousand years. These proper motions are released as part of the Gaia early data release 3. Picture: ESA/Gaia/DPAC
The trails on this image show how 40 000 stars, all located within 100 parsecs (326 light years) of the Solar System, will move across the sky in the next 400 thousand years. These proper motions are released as part of the Gaia early data release 3. Picture: ESA/Gaia/DPAC

These shifts are tiny, but understanding them enables their distances to be calculated asstars that are closer appear to draw larger shapes in the sky than those that are more distant.

On Earth, blurring caused by the atmosphere makes this difficult, but in space the measurements are only limited by the optics of the telescope - or in Gaia’s case, telescopes. as it has two.

Two previous data releases charted the positions of nearly 1.7 billion stars, but the latest release takes that to more than 1.8 billion, and their positions are significantly more accurate than before as they have been measured more times.

Included is a catalogue of 331,312 ‘nearby’ stars - thought to be 92 per cent of the stars within 326 light years of the Sun.

Gaia also tracks the changing brightness and the positions of the stars over time across the line of sight - known as proper motion. By splitting their light into spectra, it measures how fast they are moving towards or away from the Sun and assesses their chemical composition.

The Large and Small Magellanic Clouds (LMC and SMC, respectively) are two dwarf galaxies that orbit the Milky Way. This image shows the stellar density of the satellite galaxies as seen by Gaia in its Early Data Release 3, which was made public on December 3, 2020. It is composed of red, green and blue layers, which trace mostly the older, intermediate age, and younger stars respectively. Picture: ESA/Gaia/DPAC
The Large and Small Magellanic Clouds (LMC and SMC, respectively) are two dwarf galaxies that orbit the Milky Way. This image shows the stellar density of the satellite galaxies as seen by Gaia in its Early Data Release 3, which was made public on December 3, 2020. It is composed of red, green and blue layers, which trace mostly the older, intermediate age, and younger stars respectively. Picture: ESA/Gaia/DPAC

Astronomers have also been able to use the data to trace populations of older and younger stars towards the very edge of our galaxy, known as the galactic anticentre.

Gaia enables us to see relics of the 10 billion-year-old disc of the Milky Way, and understand its smaller extent - something predicted by computer models, which indicated that the galaxy will grow larger with time as new stars are born.

And beyond our galaxy, Gaia data gives us a better understanding of the stellar populations fo the two largest companion galaxies to the Milky Way - the Small and Large Magellanic, and visualise the ‘bridge’ of stars between them, as gravity pulls a stellar stream from the smaller of the two,

Gaia’s final data releases are expected to reveal stellar positions 1.9 times more accurate than those released so far, with proper motions more than seven times more accurate, in a catalogue of more than two billion objects.

Science minister Amanda Solloway said: “The mysteries of the Milky Way and our Solar System have captured the imagination of generations of scientists and astronomers across the world – all eager to learn more about the origins of the Universe.

“Through this remarkable government-backed mission, UK scientists have taken us a giant leap closer to advancing our knowledge of how our Solar System began by painting the most detailed picture yet that could help to redefine astronomy as we know it.”

More evidence of a close encounter

Gaia's early data release 3 in numbers. Graphic: ESA
Gaia's early data release 3 in numbers. Graphic: ESA

The Gaia data strengthens evidence for a major event in the recent past of the Milky Way.

The findings show that in the outer regions of the galactic disc, there are some slow-moving stars above the plane of our galaxy heading downwards towards the plane, while fast-moving stars below the plane are moving upwards.

This pattern had not been anticipated, but could be the result of the near-collision between the Milky Way and the Sagittarius dwarf galaxy thought to have taken place between 300 and 900 million years ago.

Some astronomers believe that the gravity of Sagittarius’ close pass would have perturbed some of the stars in our galaxy like a stone dropping into water.

ESA’s Gaia satellite is a space telescope designed to measure the positions of billions of stars with unprecedented precision. Gaia was launched on 19 December 2013 and is located at the L2 Lagrange point — the same location that the upcoming NASA/ESA/CSA James Webb Space Telescope will have. Image: ESA/ATG medialab
ESA’s Gaia satellite is a space telescope designed to measure the positions of billions of stars with unprecedented precision. Gaia was launched on 19 December 2013 and is located at the L2 Lagrange point — the same location that the upcoming NASA/ESA/CSA James Webb Space Telescope will have. Image: ESA/ATG medialab

Read more

Prof Gerry Gilmore of Cambridge’s Institute of Astronomy on Gaia’s astonishing 3D map of the galaxy

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