The Gaia telescope is the most advanced astrometric instrument that has tracked the motion of billions of space objects from 2013 to 2025. The results of its work are difficult to comprehend, especially since the actual work on processing the data it obtained will continue for many years. However, we will try to do so.

How many stars are in the sky
On January 15, 2025, the European Space Agency announces that the Gaia space telescope will stop observing the starry sky. For the next few weeks, it will be used to study spacecraft control, and then it will be moved away from Earth so that it cannot collide with anything.
Gaia has been in orbit since December 2013, but it didn’t make the news very often. And when it did, they contained a large number of numbers that characterized the number of stars, galaxies, and asteroids it observed. People are very bad at perceiving numbers, especially large ones, so it is not easy to explain what this instrument did that was so important.
To do this, you have to start from afar. When something is plentiful, it is said of it as a vision in the sky. Indeed, if you go outside on a cloudless night somewhere where there is no light pollution, the sheer number of tiny lights in the sky can make you dizzy.

The naked eye can see them quite a lot: about 6 thousand. However, back in the 17th century, astronomers found out that if you look through a telescope, there are many more of them. Because they are not attached to the celestial sphere but form a giant three-dimensional structure – the Milky Way Galaxy.
Galaxy Map
At the same time, the position of stars in space is not constant. Each of them has its trajectory of motion, which, from the point of view of an Earth observer, although slow, but perceptible. But still, if scientists could determine the position of each star and determine the vector of its motion, it would be possible to make something like a map of the Galaxy, as it is.
Unfortunately, back in the 19th century, the number of stars that astronomers included in their catalogs reached tens and hundreds of thousands. And even then it was clear – this is only the part that telescopes allow us to see.

In the 20th century, after establishing the true size of the Milky Way, it became possible to calculate the number of stars in a certain area of the sky, determine the distances to them, and extrapolate the result to the entire Galaxy, having learned their number. It turned out that this number should be from 200 to 400 billion, that is, hundreds of thousands of luminaries in astronomical catalogs – a tiny particle of a huge system, to comprehend which the human mind is almost impossible, for even what we know is difficult for most scientists, not to mention other people.
It also turned out that the Milky Way itself is only one of hundreds of millions of galaxies in the Universe. However, this fact was somehow lost against the background of the fact that even about our star system we have learned almost nothing for thousands of years of observation of the sky.
Still, astronomers decided to continue compiling more complete catalogs. For this purpose, it was necessary to use astrometric and photometric methods. The first involves determining the angular coordinates of an object in the sky. The second is to determine how much energy an object emits at different wavelengths. All of this has to be done over a long period to capture the movement of stars.

Space telescopes
The problem is that the atmosphere of our planet distorts the light from stars. Therefore, there is a certain limit beyond which the effectiveness of the telescope’s mirror diameter magnification decreases dramatically. In addition, usually the more powerful the power of the astronomical instrument, the narrower its field of view. This means that the time to view the entire sky increases.
The logical way out of this situation is to create a telescope of compromise design, in which both the field of view and magnification are large. Then it should be launched into space, where it will automatically rotate, observing the entire sky.
The first such space telescope was Hipparcos. This space telescope was put into orbit in 1989 and it collected data until 1997. Then scientists spent a long time processing its research, comparing it with what was seen by ground-based telescopes. The result was several catalogs, the most advanced of which was Tycho-2.

Tycho-2 contains data on the extremely precise position in the sky, its motion, and the stellar magnitude of 2.5 million stars. Here, again, the evaluation of the result is hindered by the poor ability of the human brain to operate with large numbers. It can be illustrated as follows.
If the Tycho-2 catalog were a feed of posts on a social network, and if someone were to scroll through it all and just give each luminary a like, spending an average of three seconds, the whole process would take 87 days, without taking into account that a person has to eat, sleep, and meet other needs.
And yet the catalogs created from the observations of the Hipparcos satellite, even the most conservative estimates of the number of stars in the Milky Way, contained only 0.001% of their total number. This ratio raises doubts as to whether what we observe is a typical picture of the Milky Way galaxy at all.
Gaia telescope design
Obviously, for better results, a more powerful telescope had to be launched into space. That’s what Gaia became. The design of the telescope is quite simple. It is based on the largest photographic matrix with 938 million pixels at the beginning of the 2010s.

Two telescopes, each consisting of three pentagonal mirrors, the largest of which has a size of 1.46 by 0.51 meters, focus the collected image on it. Each of them sees a different part of the sky. However, because the telescope is rotating all the time, each of the mirrors views the same section of the sky over a precisely defined period. Therefore, it is possible to determine exactly how the position of all the stars in the sky and their luminosities have changed.
Also on board, Gaia is a spectroscope to identify luminaries from absorption and emission lines, a star finder, and the equipment to make it all work. And then there are the engines to keep it spinning and the fuel tanks to fuel them.
To get rid of any backlighting, the telescope was equipped with a large reflecting shield and placed at the Lagrange point L2 of the Earth-Sun system.

However, because of this arrangement, Gaia could not purely by trajectory change its field of view following the program. Therefore, the engines had to spin up the telescope additionally. The fuel used for this was extremely economical, but eventually, over the decades, its reserves were exhausted, which was the reason for stopping the observations.
Telescope results
However, just because the spacecraft has completed its observations does not mean that the science work with it is also over. While Gaia was active, it produced a continuous stream of huge amounts of raw data, which were sent to Earth. Individual observations of a star or galaxy had to be summarized and their characteristics determined. All this is an extremely laborious process, so it was divided into several stages.

First, in September 2016, almost three years after the launch, the first Gaia data set was released, which was based on the first 14 months of observations, i.e. a little more than the first year of the spacecraft’s operation. And it already contained data on about 1.1 billion stars in the Milky Way. That is, compared to Tycho-2 the number of luminaries for which the coordinates and luminosity were determined increased by 440 times. Here, if you like at the same speed, your whole life will not be enough to reach the end of the list.
By the way the catalog of Tycho-2 scientists working with Gaia, also went through and clarified their characteristics.
Next up was 2018. The scientists released a second set of data. This time it was based on 22 months of telescope observations until May 2016. The number of stars in the catalog is 1.3 billion. In addition, radial velocities were determined for 7 million of them. Also, for the first time in the framework of this project recalled other astronomical objects and published the orbital data of 14 thousand asteroids and comets of the solar system.
It was in 2018 that the first version of what Gaia was created for was released – a three-dimensional map of the stars that the telescope was observing. Of course, it showed only about half a percent of all the stars in our Galaxy, but at the time, it was the best representation of how the stars in the Galaxy were actually arranged.

Finally, in 2022, the third data release from Gaia was presented. It was the first one in which the telescope showed everything it was capable of. The number of observed stars reached 1.8 billion. Most of them determined not only the coordinates, but also the luminosity in visible and blue light, and color. Masses were also determined for tens of millions of them.
In addition, this release contained data on tens of thousands of asteroids in the solar system, hundreds of millions of galaxies, and many other objects. It would be possible to cite many more figures, that are still not perceived by the human brain, but they can be replaced by one: astronomers learned about the existence of 997 out of every 1000 currently known stars after 2013 and it happened thanks to the Gaia space telescope.
It has also been used to make some easier-to-understand discoveries, such as new black holes and exoplanets. The telescope has been particularly effective in studying the Milky Way’s satellite galaxies. One of them, Antlia 2, was discovered with this telescope.

About the other two – the Large and Small Magellanic Clouds – it was found that the stream of stars that is associated with them is much closer to the Milky Way than previously thought. That is, they are much more closely related to us than previously thought.
The most interesting thing happened with the dwarf galaxy Sculptor. It’s the closest star system to us. It’s in the process of merging with the Milky Way. Thanks to data from the Gaia telescope, it was found out that it has already passed through the disk of our Galaxy several times, and the last time it happened was only 100-200 million years ago, so a lot of stars and even clusters in it before that belonged to the galaxy, Sculptor.
The future of the mission
The most interesting thing about the third release of Gaia data is that while it was published in full in 2022, it relied on data collected during only 34 months of operation: from July 2014 to May 2017. That is, most of the data collected by the tool had not yet been processed.
That is why the termination of Gaia does not mean the end of the mission itself. Such an incredible amount of data has been collected that scientists will be processing it for many more years.

The fourth release, which will be based on data collected over 5 years, will not be available until 2026. It will contain complete astrometric, photometric, and radial-velocity catalogs, astrophysical models for variable and multiple stars, classifications of emission sources, as well as numerous astrophysical parameters of stars, unshared binaries, galaxies, and quasars, a list of exoplanets, and epoch and transit data for all sources.
Finally, in the 2030s, astronomers will have to process all the data collected by Gaia. This will mainly lead to an increase in the accuracy of measurements, but some new stars may be discovered.
Is it possible to grasp the vastness? When we look at the starry sky, we think of it as the embodiment of that which our mind cannot control. However, the number of stars that can be seen in it has been known for centuries. However, it turns out that there are many more than we previously thought.
Scientists are faced with a new vastness, and right now they are trying to conquer it with the power of their minds. The Gaia space telescope is the most successful attempt to date to find out how many stars there really are in the sky and to learn more about each of them.