Goddess Gift: The Gaia Space Telescope has created the best 3D map of the Milky Way ever

The Gaia Space Telescope collected information on nearly two billion stars. The result is the richest catalog and the most detailed three-dimensional map of the Milky Way ever observed.

About 1.8 billion stars, millions of galaxies and many tens of thousands of asteroids are a summary of the Gaia telescope’s recently published third data release. Using this data, the project team has produced a three-dimensional map covering a large part of the Galaxy. It is the largest catalog of stars and the best 3D map of the Milky Way to date. In addition, the age, temperature, mass and other parameters have been determined for hundreds of millions of luminaries. So far, it is difficult even to speculate what a wave of discoveries such rich observational material can lead to.

Roulette wheel for stars

It is important for astronomers to determine distances to celestial bodies. Only knowing the distance to the object, you can calculate how much radiation it emits. Otherwise you can, roughly speaking, confuse a distant star with a nearby light bulb.

The most reliable way to mark the vastness of the Universe with verst posts is the parallax method. Its idea is very simple. Go out on the road and note some distant object away from it, such as a tall house. It is necessary to mark in which direction from us it is located. Now walk a mile or two along the road (the distance traveled is called the baseline). The direction to the tall house will change by a certain angle, half of which is called the parallax. Knowing the baseline and parallax, we can determine the distance to the high-rise. This is a high school triangle problem.

This method does not require any hypotheses about celestial bodies. It relies solely on good old-fashioned geometry. That’s why scientists appreciate it.

But if the distance to the object is very large, we need to get very far down the road before the direction to the object changes noticeably. It’s easy to get away from the high-rise on the horizon, but try to get away from the moon in the sky! As all children notice with amazement, the Moon and the Sun “chase” us. It is because the distance to the Moon (380 thousand kilometers) and especially to the Sun (150 million kilometers) is incommensurably greater than the distance we travel on the surface of our planet. What to say about the stars, which are many light years away?

The maximum baseline on the globe is, as is easy to guess, its diameter. Less than 13 thousand kilometers is negligible by cosmic standards. We can’t see the parallax of stars just by traveling around the Earth.

Fortunately, our planet itself is not standing still. Note the direction to the star. Let’s repeat this observation in six months, when the Earth will be at the opposite point of its orbit. We will get a baseline equal to the diameter of the orbit: 300 million kilometers!

Is this enough to determine the distance to the stars by parallax? The correct answer is “it depends on which stars. The key factor here is the accuracy with which our telescopes measure the angular displacement of the luminaries. Let’s say that if we can measure reliably at 10 microsecond angular parallaxes, we can determine the distance to stars within 100 parsecs (about 300 light years).

A few hundred light years is practically the limit for ground-based measurements. Achieving greater accuracy is hampered by the eternal enemy of astronomers – the atmosphere. But the diameter of the Galaxy – a hundred thousand light years. It turns out that we can make a three-dimensional map of only a tiny corner of it.

But a space telescope can measure the position of stars with much greater accuracy, especially if it is specifically designed for this purpose. Humanity first tried this approach when it launched Hipparcos in 1989. Its typical measurement error was two angular microseconds. In 2013, the turn came for a new space mapper – Gaia (“Gaia” or “Gaia,” if you recall that the apparatus is still named after the goddess). It is two hundred times more accurate. Because of this, it can measure distances of tens of thousands of light years, which is comparable with the size of the Milky Way. However, at the extreme distances Gaia, of course, sees only the brightest stars, and not all that are there.

Heaven in Diamonds

So, what did the Gaia team publish?

A total of 1.8 billion stars are in the new catalog – and this, we repeat, is a historical record. For each of them, the apparent brightness and two-dimensional coordinates linking the luminary to a certain point in the sky are defined. This is the minimum minimorum of information about the celestial body that makes sense to enter into the catalog.

Of these, for about 1.5 billion stars, the color and – most importantly – the distance to Earth are also determined. In this way, these luminaries are mapped in three dimensions. Is this a lot or a little? Only about one percent of the stellar population of the Galaxy. But even this is a huge breakthrough compared with a hundred thousand stars, the parallaxes of which were determined by Hipparcos.

It begs the question, is it possible to look at this remarkable map. Yes and no. All the data, including the three-dimensional coordinates of the stars, are publicly available. But, of course, in the form of boring machine-readable files. And hardly anyone bothered to present them as a picture, and even in 3D. Do you really expect that 1.5 billion points can fit on the screen of your device in a reasonable resolution?

Better take a look at the movement of 26 million stars in the same catalog. And, by the way, it’s a good reason to talk about motion.

Where stars fly

For the same 1.5 billion stars, along with distances, the proper motion is defined. What is it?

Stars in the Galaxy do not stand still. Each of them flies somewhere (at least – participates in the general motion around the center of the Milky Way). And the Sun, too, in turn, flies somewhere. Because of this, the luminaries, when viewed from Earth (or from “Gaia”), gradually move across the celestial sphere. This motion remains, even when we discard parallax, precession of the Earth’s axis, and other effects that shift the stars to the observer. This is what is called the proper motion of the star.

The proper motion is the motion on a two-dimensional celestial sphere. To reconstruct the motion of a star in three-dimensional space, another parameter is needed. This is radial velocity, the speed at which the object approaches or moves away from the observer. Radial velocity cannot be calculated by measuring angles. You need the spectrum of the star and the Doppler effect. The spectrum must be of very high quality. Not surprisingly, Gaia determined the radial velocities of “only” 33 million stars. For these luminaries, not only their positions, but also their motion through the Galaxy is plotted on a three-dimensional map.

However, the low-quality spectrum is also very useful. It allows us to determine mass, temperature, age and other characteristics. This work was done for 470 million stars.

What else have we learned thanks to Gaia?

What else is interesting about the published data? Ten million variable stars, for example. And more than 800,000 binary star systems, for which the masses and orbits of their partners have been determined.

The telescope has also detected thousands of star shocks, even though it was not designed for this purpose. Star shocks are relatively rare to observe, yet they provide important information about the inner workings of stars. Surprisingly, the seismicity was detected on those luminaries, which, according to present theories, should not be inclined to it at all. This proves once again that we still know less about stars than we would like.

Also in the lenses of “Gaia” (it has two of them, by the way) got almost five million galaxies and about 160 thousand asteroids. For Galactic cartographers, this is a byproduct, but colleagues from other fields of astronomy will thank them.

Note that the current batch of Gaia data is the third. The first release (data release 1 or DR1) was published in 2016, the second in 2018. The current DR3 catalog contains 100 million more stars than DR2. In addition, due to the greater number of observations, the accuracy of parallax detection has doubled.

By the way, the preliminary version of DR3 was published in late 2020. It already had two-dimensional coordinates, parallaxes, and proper motions, but no spectroscopic observations.

By the way, Gaia’s current ability to map the Milky Way is far from the limit. Its success in measuring distances stems from the accuracy of its angle measurements, not its baseline length. Gaia is only 1% farther from the Sun than Earth. Why wasn’t it launched into a longer orbit? First of all, because this orbiter transmits much larger amounts of data to Earth than an interplanetary probe. It would be much more difficult to do this from a long distance. In addition, it is obvious that Gaia has determined the parallax for almost all the stars it has discerned at all (1.5 billion versus 1.8 billion). That is, the capabilities of the project are limited not by the length of the baseline, but by the sensitivity of the telescope. Therefore, it did not make much sense to launch a space observatory somewhere between the Earth and Mars: it would be more trouble than it was worth. But technology is evolving, and already in the foreseeable future we are likely to see both more sensitive telescopes and more productive communication systems. And in that case, the space mappers of the next generations will probably be launched into larger orbits with a larger radius. And that means they will give us even more impressive maps of the Galaxy.

Anton Berg

Written by Anton Berg

Editor-in-chief of the website since 2012.
Two higher educations: programming and economics.
Inquisitive person with extensive knowledge in various fields. Erudite person.
Supervises all the materials of the site. Dives deeply into each of the various sections of the site.

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