A major international project in the field of astronomy is changing the understanding of the structure of our Galaxy.
At the beginning of the 19th century, William Parsons built a telescope that was gigantic for those times (the diameter of the mirror was 183 centimeters). With it, he observed and sketched a star cluster with two spiral arms, which we now call the Whirlpool Galaxy (M51). It is believed to be the first spiral galaxy discovered by astronomers. But this is now considered, then it was not clear at all what it was - a part of our Galaxy or an independent object. The debate on this topic continued for almost a hundred years, until another famous astronomer, Edwin Hubble, put an end to them, proving that the Maelstrom is located far beyond the Milky Way. Thus, the idea that the Milky Way is the entire universe was finally rejected .
Further, astronomers found out that we live in a galaxy of a similar type - spiral (this is generally one of the two main types of galaxies, the second is elliptical). Moreover, even galaxies of the same type, such as the Whirlpool and ours, can have noticeable differences in their structure. For a long time it was believed that this galaxy is a much closer analogue of the Milky Way.
This neighboring galaxy NGC 1300 is a stellar spiral with a bar over 100,000 light years across.
True, questions arose with the number of arms in our galaxy. If we start from the pictures taken ten years ago by the Spitzer space telescope, it turns out that there are two of them. And observations in the radio range of atomic hydrogen and carbon monoxide, which are concentrated in the spiral arms of other galaxies, show that there are four arms.
Recently, an international team of astronomers has made an attempt to map the Milky Way from the inside, which for the first time will allow us to build an accurate model of its structure. This became possible as a result of the parallel execution of several large scientific programs using the most modern radio and optical telescopes, as well as data accumulated in the course of earlier observations.
The two main problems that astronomers have faced in the course of this work are distance and dust. On the one hand, the Milky Way is very large, the light from the stars from the other end flies towards us for 50 thousand years. And at such distances it is very difficult often even to understand which of the two stars located next to our sky is closer to us, and which is further away (what you need to know to build an accurate three-dimensional model). And, as if that were not enough, interstellar space is filled with a large amount of dust, which obscures many distant objects (in the optical range) from an observer from Earth.
They are trying to solve these problems with the help of radio telescopes - radio waves easily pass through the dust and make it possible to explore the entire disk of the galaxy and make a map of its structure.
Now two such programs are running in parallel. The first - VERA ("Radioastrometric research using radio interferometers with an ultra-long base") used four radio telescopes in Japan. The second, the BeSSeL (The Bar And Spiral Structure Legacy) Survey, uses an extra-long baseline antenna array that consists of ten telescopes and covers much of the Western Hemisphere, from Hawaii and New England to Santa -Cruz in the US Virgin Islands. Because telescopes are spaced nearly as large as the diameter of the Earth, these arrays can provide much greater angular resolution than any other telescope operating at any wavelength.
Young, massive stars would be optimal candidates for mapping purposes. They ionize the gas around them, causing it to glow blue, and thus become beacons to track the spiral arm. For the reasons stated above, observing these objects throughout the Milky Way is very difficult. But it is possible to record the radio emission of water and methyl alcohol molecules located in the immediate vicinity of such regions of ionized gas. Which is what astronomers do. As a result, they managed to measure the distance of up to 200 young hot stars in different parts of our galaxy using the parallax method. The data obtained covered about a third of the Milky Way and made it possible to identify four arms in the structure of the galaxy.
The same map also shows that the Sun is located very close to the fifth object, called the Local Arm, which appears to be an isolated fragment of the spiral arm (and not just a fine structure within one of the arms, as previously thought).
Based on the resulting model, its authors estimated the distance from the Sun to the center of the Galaxy at 8150 ± 150 parsecs (or 26.6 thousand light years). This is less than the value of 8.5 thousand parsecs recommended by the International Astronomical Union in 2010 . Also, according to their calculations, it turned out that the Milky Way rotates at a speed of 236 km / s, and the Sun revolves around the center of the Milky Way every 212 million years.
Also, this model made it possible to more accurately determine the location of the solar system in the galaxy. The fact that this part is flat and rather thin has been known for a long time. But the question of the location of the Sun relative to this plane remains controversial. The previous consensus was that it is 82 light-years from the center of this plane; the new model reduces this distance by a factor of four. Now it turns out that the Sun lies almost strictly in the central plane of the Galaxy disk, but far from its center, at a distance of two-thirds of its radius .
Despite the amount of data obtained, the authors of the study admit that there are far more questions than answers. For example, how spiral arms appear in general. Or what is the exact age of the Milky Way and is it possible to establish it at all (according to modern cosmology, the process of formation of galaxies was gradual and stretched over billions of years, protogalaxies merged and separated again, so it is not clear what to take as a starting point).
As for further refinement of the three-dimensional map of the Milky Way, the next step will open up with the implementation of two new mega-signals - the Square Kilometer Array radio telescope systems in Africa and the Next Generation SuperLarge Antenna Array in North America .
Adapted from Scientific American