# Best-ever 3D map of the Milky Way created

(ORDO NEWS) — The Gaia space telescope has collected information on almost two billion stars. The result was the richest catalog and the most detailed three-dimensional map of the Milky Way in the entire history of observations.

ome 1.8 billion stars, millions of galaxies, and many tens of thousands of asteroids are a brief description of Gaia ‘s recently published third data release. Based on these data, the project team compiled a three-dimensional map covering a significant part of the Galaxy.

This is the largest catalog of stars and the best 3D map of the Milky Way to date. In addition, age, temperature, mass and other parameters have been determined for hundreds of millions of luminaries. So far, it is difficult even to imagine what a wave of discoveries such a rich observational material can lead to.

#### Roulette for the stars

Let us briefly recall what is the matter. Only by knowing the distance to an object can we 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 milestones is the parallax method. His idea is extremely simple. Let’s go out onto the road and take some distant object away from it, for example, a tall house.

It should be noted in which direction from us it is located. Now let’s go along the road for a kilometer or two (the distance traveled is called basis ).

The direction to the skyscraper will change by some angle, half of which is called parallax . Knowing the basis and parallax, we can determine the distance to the high-rise building. This is a triangular puzzle.

This method does not require any hypotheses about celestial bodies. It relies solely on good old geometry. For this, scientists appreciate him.

But if the distance to the object is very large, we need to go very far on the road before the direction to the object noticeably changes. It’s easy to get away from the skyscraper on the horizon, but try to get away from the moon in the sky!

As all children are surprised to notice, the Moon and the Sun “chase” us. Precisely because the distance to the Moon (380 thousand kilometers) and even more so to the Sun (150 million kilometers) is incommensurably greater than the distance that we travel on the surface of our planet. What can we say about the stars, which are many light years away?

The maximum basis on the globe is, as you might guess, its diameter. Less than 13 thousand kilometers is an insignificant value by cosmic standards. We cannot see the parallax of stars just by traveling the Earth.

Fortunately, our planet itself does not stand still. Note the direction to the star. Let’s repeat this observation in half a year, when the Earth will be at the opposite point of the orbit. We get a basis equal to the diameter of the orbit: 300 million kilometers!

Is this enough to determine the distance to the stars using the parallax method? The correct answer is “depending on which stars.” The key factor here is the accuracy with which our telescopes measure the angular displacement of the star.

Say, if we can reliably measure parallaxes of 10 microarcseconds, then 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 hindered by the eternal enemy of astronomers – the atmosphere. But the diameter of the Galaxy is one 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 specially designed for this. Humanity first tried this approach with the launch of Hipparcos in 1989. The typical error of its measurements was two microseconds of arc.

In 2013, it was the turn of a new space cartographer – Gaia (“Gaia” or “Gaia”, if you remember that the device is named after the goddess). It’s more accurate by a factor of two hundred.

In this regard, it can measure distances of tens of thousands of light years, which is comparable to the size of the Milky Way. True, at extreme distances, Gaia, of course, sees only the brightest stars, and not all that are there.

#### Sky in diamonds

So what did the Gaia team publish ?

In total, there are 1.8 billion stars in the new catalog – and this, we repeat once again, is a historical record.

For each of them, the apparent brightness and two-dimensional coordinates are determined, tying the luminary to a certain point in the sky. This is the minimum minimorum of information about a celestial body that makes sense to catalog.

Of these, for about 1.5 billion stars, the color and, most importantly, the distance to the Earth are also determined. Thus, these luminaries are plotted on a three-dimensional map. Is it a lot or a little?

Only about one percent of the stellar population of the Galaxy. But even this is a huge breakthrough compared to the hundred thousand stars whose parallaxes were determined by Hipparcos.

This begs the question, is it possible to look at this wonderful map. Yes and no. All data, including the three-dimensional coordinates of the stars, are made publicly available.

But, of course, in the form of boring machine-readable files. And hardly anyone bothered to present them in the form of a picture, and even in 3D. Do you really hope that 1.5 billion dots will fit on the screen of your device in a digestible resolution?

Take a better look at the movement of 26 million stars according to the same catalog. And by the way, this is a good reason to talk about the movement.

#### Where the stars go

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

The stars in the galaxy do not stand still. Each of them flies somewhere (at least – participates in the general movement around the center of the Milky Way). And the Sun, too, in turn, flies somewhere. Because of this, the luminaries, when viewed from the Earth (or from Gaia), gradually move around the celestial sphere.

This movement remains even when we discard parallax, precession of the earth’s axis, and other effects that shift the stars for the observer. This is what is called the proper motion of the star.

Proper motion is motion in a two-dimensional celestial sphere. To restore the motion of a star in three-dimensional space, one more parameter is needed. This is the radial velocity – the speed at which an object approaches or moves away from the observer.

Radial velocity cannot be calculated by measuring angles. This requires the spectrum of the star and the Doppler effect. In this case, the spectrum must be of very high quality.

Not surprisingly, Gaia has determined radial velocities for “only” 33 million stars. For these luminaries, not only their position is plotted on a three-dimensional map, but also their movement through the Galaxy.

However, the spectrum of low quality is very useful. It allows you to determine the mass, temperature, age and other characteristics. This work has been done for 470 million stars.

#### What else have we learned from Gaia?

What else is interesting in the published data? For example, ten million variable stars. And more than 800 thousand binary star systems for which the masses and orbits of partners have been determined.

The telescope also helped to detect thousands of starquakes, although it was not intended for this. The concussions of stars are quite rarely observed, and meanwhile they carry important information about their internal structure.

Surprisingly, seismicity was also found on those luminaries that, according to current theories, should not be prone to it at all. This proves once again that we still know less about the stars than we would like.

Also, almost five million galaxies and about 160 thousand asteroids got into the Gaia lenses (she has two, by the way). For galactic cartographers, this is a by-product, but colleagues from other fields of astronomy will thank them.

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

By the way, the preliminary version of DR3 was published at the end of 2020. It already had 2D coordinates, parallaxes, and proper motions, but no spectroscopic observations.

By the way, the current capabilities of Gaia in mapping the Milky Way are far from the limit. Her success in measuring distances stems from the accuracy of angle measurements, not from the length of the basis. 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 any interplanetary probe. From a distance, this would be much more difficult.

In addition, it is obvious that Gaia has determined the parallax for almost all the stars that she has ever seen (1.5 billion versus 1.8). That is, the possibilities of the project are limited not by the length of the basis, but by the sensitivity of the telescope.

Therefore, launching a space observatory somewhere between the Earth and Mars did not make much sense: it would be more trouble than good. But technology is evolving and in the foreseeable future we are likely to see both more sensitive telescopes and more efficient communication systems.

And in this case, space cartographers of the next generations will most likely be launched into orbits with a larger radius. And, therefore, they will give us even more impressive maps of the Galaxy.

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