For a long time, it was believed that the Milky Way formed gradually. In 1962, Olin Eggen, Donald Linden-Bell, and Allan Sandage proposed a hypothesis that became known as the ELS model (named after the initial letters of their last names). According to her, a homogeneous cloud of gas once slowly revolved in place of the Milky Way. It resembled a ball and reached about 300 thousand light-years across, and consisted mainly of hydrogen and helium. Under the influence of gravity, the protogalaxy shrank and became flat; at the same time, its rotation accelerated noticeably.
For nearly two decades, this model suited scientists. However, new observational results showed that the Milky Way could not have emerged as the theorists dictated.
According to this model, a halo is formed first, and then a galactic disk. However, the disk also contains very ancient stars, for example, the red giant Arcturus, whose age is more than ten billion years, or numerous white dwarfs of the same age.
Globular clusters have been found in both the galactic disk and the halo that are younger than the ELS model suggests. Obviously they were absorbed by our Galaxy later.
Many stars in the halo rotate in a different direction than the Milky Way. Perhaps they, too, were once outside the Galaxy, but then they were drawn into this "star vortex" - like an accidental swimmer in a whirlpool.
In 1978, Leonard Searle and Robert Zinn proposed their own model for the formation of the Milky Way. She was designated as "model SZ". Now the history of the Galaxy has become much more complicated. Until recently, her youth, in the minds of astronomers, was described as simply as in the opinion of physicists - rectilinear translational motion. The mechanics of what was happening was clearly visible: there was a homogeneous cloud; it consisted only of evenly spreading gas. Nothing, by its presence, complicated the calculations of theorists.
Now, instead of one huge cloud in the visions of scientists, several small, fancifully scattered clouds appeared at once. Among them were the stars; however, they were located only in the halo. Everything inside the halo was seething: the clouds collided; the gas masses were mixed and compacted. Over time, this mixture formed a galactic disk. New stars began to appear in it. However, this model was later criticized.
It was impossible to understand what connected the halo and the galactic disk. This thickening disk and the sparse stellar envelope around it had little in common. Already after Searle and Zinn made their model, it turned out that the halo rotates too slowly to form a galactic disk. Judging by the distribution of chemical elements, the latter arose from protogalactic gas. Finally, the angular momentum of the disk turned out to be ten times higher than the halo.
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The secret is that both models contain a grain of truth. The trouble is that they are too simple and one-sided. Both of them now seem to be fragments of the same recipe by which the Milky Way was created. Eggen and his colleagues read a few lines from this recipe, Searle and Zinn a few others. Therefore, trying to re-imagine the history of our Galaxy, we now and then notice familiar lines, already read once.
So it all started shortly after the Big Bang. “Today it is generally accepted that fluctuations in the density of dark matter gave rise to the first structures - the so-called dark halos. Thanks to the force of gravity, these structures did not disintegrate,”notes the German astronomer Andreas Burkert, the author of a new model of the birth of the Galaxy.
Dark halos became the embryos - nuclei - of future galaxies. Gas accumulated around them under the influence of gravity. A homogeneous collapse occurred, as the ELS model describes it. Within 500-1000 million years after the Big Bang, the gas clusters that surrounded the dark halos became the "incubators" of stars. Here small protogalaxies arose. The first globular clusters appeared in dense clouds of gas, because stars were born here hundreds of times more often than anywhere else. Protogalaxies collided and merged with each other - this is how large galaxies were formed, including our Milky Way. Today it is surrounded by dark matter and a halo of single stars and their globular clusters, these ruins of the universe, whose age exceeds 12 billion years.
There were many very massive stars in the protogalaxies. In less than a few tens of millions of years, most of them exploded. These explosions enriched the gas clouds with heavy chemical elements. Therefore, stars were not born in the galactic disk as in the halo - they contained hundreds of times more metals. In addition, these explosions generated powerful galactic vortices that heated the gas and swept it out of the protogalaxies. The separation of gas masses and dark matter has occurred. This was the most important stage in the formation of galaxies, not previously taken into account in any model.
Meanwhile, dark halos were increasingly colliding with each other. In this case, the protogalaxies stretched out or disintegrated. These catastrophes are reminiscent of the chains of stars that have been preserved in the halo of the Milky Way since the days of "youth". By studying their location, one can assess the events that took place in that era. Gradually, a vast sphere formed from these stars - the halo we see. As it cooled down, gas clouds penetrated into it. Their angular momentum was conserved, so they did not collapse into a single point, but formed a rotating disk. All this happened over 12 billion years ago. The gas was now compressed as described in the ELS model.
At this time, the "bulge" of the Milky Way is also formed - its middle part, which resembles an ellipsoid. Bulge is composed of very old stars. Obviously, it arose at the merger of the largest protogalaxies, which held the gas clouds for the longest time. Among it were neutron stars and tiny black holes - relics of exploding supernovae. They merged with each other, simultaneously absorbing gas streams. Perhaps this is how a huge black hole originated, which now resides in the center of our Galaxy.
The history of the Milky Way is much more chaotic than previously thought. Our home Galaxy, impressive even by cosmic standards, was formed after a series of impacts and mergers - after a series of cosmic catastrophes. Traces of those old events can be found today.
For example, not all stars in the Milky Way revolve around the galactic center. Obviously, over the billions of years of its existence, our Galaxy has "swallowed" many fellow travelers. Every tenth star in the galactic halo is less than 10 billion years old. By then, the Milky Way had already formed. Perhaps these are the remnants of once captured dwarf galaxies. A group of British scientists from the Astronomical Institute (Cambridge), led by Gerard Gilmour, calculated that the Milky Way apparently swallowed up from 40 to 60 dwarf galaxies of the Karin type.
In addition, the Milky Way attracts huge masses of gas. So, in 1958, Dutch astronomers noticed many small spots in the halo. In fact, they turned out to be gas clouds, which consisted mainly of hydrogen atoms and rushed towards the galactic disk.
Our Galaxy will not moderate its appetite in the future. Obviously, it will swallow the dwarf galaxies closest to us - Fornax, Karina and, perhaps, Sextane, and then merge with the Andromeda nebula. Around the Milky Way - this insatiable "star cannibal" - will become even more desolate.