This is not a photo of the Milkyway or an artist’s impression, this a map made up of approximately 1.8 billion data points collected by the Gaia satellite, each one not just representing a star but also comets, asteroids and other astronomical objects in the Milkyway and beyond.
But its not just taking images of the night sky, its also measuring the speed, direction of travel and colour of each star to build up a 3d space catalogue of the Milkyway, not only up to 30,000 light years in all directions from the earth but other more distant objects liken other galaxies and quasars will also be mapped.
With this data and applying the laws of celestial mechanics, we can run time backwards and forwards to see how the Milkyway formed and how it and our little part of it will move and interact with the rest of the nearby stars for millions of years into the future.
So how does one satellite orbiting the sun along with the earth build such a huge amount of data and let us see things on a grand scale that we have never seen before?
The Gaia space observatory was launched by the European space agency in 2013 and its primary role is to map the visible section of the milk way and other celestial objects using astrometry.
Astrometry precisely measures stars, comets, asteroids and other celestial bodies. It is fundamental for fields like celestial mechanics, stellar dynamics and galactic astronomy and can be dated back to the ancient Greeks.
The problem with stars and other celestial objects is that working out their distance from the earth can be difficult. A star could be very bright but far away whereas a dimmer star could be much closer, yet they could look very similar to an earth-based observer.
However, there is a way to work out the distance using parallax, similar to our own stereoscopic vision. We have two eyes that see the same image from different positions and our brains use this difference to calculate the distance to an object.
That’s fine if the object is up to about 10 meters away but our eyes are far too close together to work at the distance of stars. To do that we need to have eyes or the camera equivalent, tens of millions of kilometres apart.
Astrometry takes advantage of the fact that the earth is 148 million km from the sun and rotates around it once per year.
So for example in December, it will be on one side of the sun and in June it will be on the other and the combined distance between these two points is 298 million km. If an astronomer takes two images of the same star, one in December and one in June, the results will show that the star has moved relative to the background stars and by doing some trigonometry we can work out the distance to the star.
This is how Gaia works but instead of taking just two images it takes a continuous stream of them, this way it can scan two million stars an hour as it orbits around the sun and then select which stars it going to track and analyse.
Each star’s position is recorded relative to its background, colour, brightness and spectral shift. The spectral shift shows if it moving towards us or away from us. As Gaia moves along its orbit it takes new images of the same stars and using this data the distance to each can be calculated as well as its direction of travel.
Over its original operational lifetime, Gaia was expected to scan each star over 70 times to build up a 3D model of how each star moves relative to us and all the other stars. This allows us to see where they were in the past and where they will be in the future, something we have never been able to see accurately before.
Gaia actually orbits at the Lagrange point L2, approximately 1.5 million kilometres from Earth. Here it orbits around the earth as the earth rotates around the sun, this way it can scan the space around the sun and maintain a direct line of sight with the earth to send the data back to.
Gaia uses two telescopes that look out from the main body, separated by 106 degrees. The light from these two is focused through a series of mirrors on to largest sensor array so far put into space.
Many of the stars it sees are much fainter than can be seen by the human eye, some 400,000 times fainter in fact. The array is made up of 106 separate CCD devices each having 4500 × 1966 pixels with a combined pixel count of 937.8 megapixels pixels and covering almost half a square meter in size, the CCD’s are also a hundred times more sensitive than a normal camera CCD.
The mirrors are mounted on a frame made from silicon carbide, a very thermally stable material that is as hard as diamond and provides a highly stable mounting basis for the optical system. Along with the 10-meter diameter sun-shield to protect the optics and sensors and thermally stable Lagrange point in space, the accuracy of the measurements Gaia can make is incredible.
The angular level of precision is 10 micro-arc seconds, which is an angle equivalent to the width of 23mm diameter coin on the surface of the moon, measured from the earth.
To enable the repeatability of the extreme precision, Gaia uses eight micro thrusters which produce just 1 micro-newton of pressure, about 1 million times less than a typical thruster. Gaia has no moving parts like reaction wheels or gyroscopes other than actuators to align the mirrors and valves for the thrusters.
As Gaia rotates it looks at two patches of the sky simultaneously. These are separated by 106 degrees and the light from the stars travels across the sensors as Gaia turns providing a continuous moving image strip of space the telescopes see.
The light is folded and reflected by the mirrors onto the sensors, which are split into several parts that do different jobs to analyse the images it captures.
The sensors on the right determine which stars are to be tracked. As the image travels across the main astrometric field sensor, the nine strips of CCDs analyse the selected stars for their brightness and position. To cut down on the huge amount of data this generates only a small window around each star is recorded as it traverses the sensor.
To the immediate left of that are the two photometers which receive light via a couple of special prisms which detect the star’s colour, size and chemical composition before the image ends by traversing the radial velocity spectrometer. This detects the amount of red or blue shift in the light from the star or object to determine if it is travelling towards us or moving away and how fast.
All this is repeated for each star as Gaia turns on it axis every 6 hours.
All the data is then compressed and stored before being sent back to ESA at 1Mbit per second, equivalent to 50Gb per day. After 5 years of operation, some 60 Tb of compressed data was sent, equating to 200 Tb of uncompressed data to analyse back on earth.
All of this tells us a lot about the evolution of the Milky Way and how we came to be where we are in it.
It has shown that stars which are born within the stellar nurseries tend to stay together and rotate around the galactic plane together. But there are other groups of stars which move in very different ways indicating that they were captured from other smaller galaxies which have collided with the Milky Way over time.
It also shows that our galaxy’s disc is warped and that it processes or wobbles on its axis like a buckled bike wheel rather than being a flat plane, again something that would have been affected by interactions with other galaxies in the past.
These collisions trigger new star formation as new gas is introduced from external sources and areas with higher gas densities create new stars. One of these collisions is believed to have occurred about six billion years ago and led to a wave of new star formation, including our sun, solar system and ultimately the earth and us.
Such is the precision of the measurement that we can also track stars in the milky Way’s satellite galaxies, the large and small Magellanic clouds. We can also see into the Andromeda galaxy some two million light years away which is on a collision course with the Milky Way but not for another four billion years or so.
It can also track asteroids over 14,000 so far and as time goes by more are added to the catalogue. As Gaia scans the whole sky and not just in the plane of the asteroid belt it has found ones which orbit at much higher angles indicating that they are not native to the solar system and were flying by when they were captured by the sun’s gravity.
All of this provides us with an amazing insight into how our galaxy formed and will continue to evolve for millions of years into the future, something that no other single satellite has been able to do.
