To some extent, we know more about distant stellar systems than about our own Galaxy - the Milky Way. It is more difficult to study its structure than the structure of any other galaxies, because you have to study it from the inside, and much is not so easy to see. Interstellar dust clouds absorb light from a myriad of distant stars.
Only with the development of radio astronomy and the advent of infrared telescopes have scientists been able to understand how our Galaxy works. But many details remain unclear to this day. Even the number of stars in the Milky Way is roughly estimated. The latest electronic guides list numbers from 100 to 300 billion stars.
Not so long ago, it was believed that our Galaxy has 4 large arms. But in 2008, astronomers at the University of Wisconsin published the results of processing about 800,000 infrared images taken by the Spitzer Space Telescope. Their analysis showed that the Milky Way has only two arms. As for the other sleeves, they are only narrow lateral branches. So, the Milky Way is a spiral galaxy with two arms. It should be noted that most of the spiral galaxies known to us also have only two arms.
“Thanks to the Spitzer telescope, we have the opportunity to rethink the structure of the Milky Way, - said astronomer Robert Benjamin of the University of Wisconsin, speaking at the conference of the American Astronomical Society. “We are refining our understanding of the Galaxy in the same way as centuries ago, when the discoverers traveled around the globe, they refined and rethought previous ideas about what the Earth looks like.”
Since the early 90s of the XX century, observations carried out in the infrared range have been changing our knowledge of the structure of the Milky Way more and more, because infrared telescopes make it possible to look through clouds of gas and dust and see what is inaccessible to conventional telescopes.
2004 - The age of our Galaxy was estimated at 13.6 billion years. It arose shortly after the Big Bang. In the beginning, it was a diffuse gas bubble containing mainly hydrogen and helium. Over time, it turned into a huge spiral galaxy in which we now live.
general characteristics
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But how did the evolution of our Galaxy proceed? How was it formed - slowly or, on the contrary, very quickly? How was she saturated with heavy elements? How did the shape of the Milky Way and its chemical composition change over billions of years? Scientists have yet to provide detailed answers to these questions.
Our Galaxy is about 100,000 light years long, and the average thickness of the galactic disk is about 3,000 light years (the thickness of its convex part - the bulge - reaches 16,000 light years). However, in 2008 Australian astronomer Brian Gensler, after analyzing the results of observations of pulsars, suggested that the galactic disk is probably twice as thick as is commonly believed.
Is our Galaxy large or small by cosmic standards? By comparison, the Andromeda Nebula, the closest large galaxy to us, is approximately 150,000 light-years across.
At the end of 2008, researchers established by radio astronomy methods that the Milky Way is rotating faster than previously thought. Judging by this indicator, its mass is approximately one and a half times higher than was commonly believed. According to various estimates, it ranges from 1.0 to 1.9 trillion solar masses. Again, for comparison: the mass of the Andromeda nebula is estimated at least 1.2 trillion solar masses.
Structure of galaxies
So, the Milky Way is not inferior in size to the Andromeda nebula. “We should no longer treat our galaxy as the little sister of the Andromeda nebula,” said astronomer Mark Reid of the Smithsonian Center for Astrophysics at Harvard University. At the same time, since the mass of our Galaxy is larger than expected, its gravitational force is also higher, which means that the likelihood of its collision with other galaxies in our vicinity increases.
Black hole
Our Galaxy is surrounded by a globular halo that reaches 165,000 light years across. Astronomers sometimes refer to the halo as the "galactic atmosphere." It contains approximately 150 globular clusters, as well as a small number of ancient stars. The rest of the halo space is filled with rarefied gas, as well as dark matter. The mass of the latter is estimated at about a trillion solar masses.
The spiral arms of the Milky Way contain enormous amounts of hydrogen. This is where stars continue to be born. Over time, young stars leave the arms of galaxies and "migrate" to the galactic disk. However, the most massive and brightest stars do not live long enough, therefore they do not have time to move away from their place of birth. It is no coincidence that the arms of our Galaxy glow so brightly. Most of the Milky Way is made up of small, not very massive stars.
The central part of the Milky Way is located in the constellation Sagittarius. This area is surrounded by dark clouds of gas and dust, beyond which nothing can be seen. Only since the 1950s, using the means of radio astronomy, have scientists been able to gradually see what lies there. In this part of the Galaxy, a powerful radio source was discovered, called Sagittarius A. Observations showed that a mass is concentrated here, exceeding the mass of the Sun by several million times. The most acceptable explanation for this fact is only possible: there is a black hole in the center of our Galaxy.
Now, for some reason, she took a break for herself and does not show much activity. The influx of matter here is very scarce. Maybe over time, the black hole will have an appetite. Then it will again begin to absorb the veil of gas and dust that surrounds it, and the Milky Way will join the list of active galaxies. It is possible that before this in the center of the Galaxy stars will begin to vigorously appear. Such processes are likely to be repeated regularly.
2010 - American astronomers using the Fermi Space Telescope, designed to observe the sources of gamma radiation, discovered two mysterious structures in our Galaxy - two huge bubbles emitting gamma radiation. Each of them averages 25,000 light years in diameter. They scatter from the center of the Galaxy in the north and south directions. Perhaps we are talking about the streams of particles that were once emitted by a black hole in the middle of the Galaxy. Other researchers believe that we are talking about gas clouds that exploded when stars were born.
Several dwarf galaxies are located around the Milky Way. The most famous of these are the Large and Small Magellanic Clouds, which are connected to the Milky Way by a kind of hydrogen bridge, a huge plume of gas that stretches behind these galaxies. It was named "Magellanic Stream". It spans about 300,000 light years. Our Galaxy constantly engulfs the nearest dwarf galaxies, in particular, the Sagitarius galaxy, which is located 50,000 light-years from the galactic center.
It remains to add that the Milky Way and the Andromeda nebula are moving towards each other. Presumably, in 3 billion years, both galaxies will merge together, forming a larger elliptical galaxy, which has already been named "Milky Honey".
The origin of the Milky Way
For a long time, it was believed that the Milky Way formed gradually. 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 was approximately 300,000 light-years across, and consisted primarily of hydrogen and helium. Under the influence of gravity, the protogalaxy shrank and became flat; at the same time, its rotation accelerated noticeably.
Andromeda's nebula
For almost two decades, this model suited scientists. But the new observations showed that the Milky Way could not have arisen the way theorists dictated it.
According to this model, a halo is formed first, and then a galactic disk. But the disk also contains very ancient stars, such as the red giant Arcturus, whose age is more than 10 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 are swallowed up by our later Galaxy.
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.
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. Not so long ago, in the minds of astronomers, her youth 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, a galactic disk was formed from this mixture. New stars began to appear in it. But 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 compiled 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 10 times higher than that of the halo.
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 that have already been read once.
Milky Way. Computer model
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. Already 500-1000 million years after the Big Bang, the gas clusters that surrounded the dark halos became "incubators" of stars. Small protogalaxies appeared here. In dense clouds of gas, the first globular clusters arose, 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, not such stars were 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.
At the same time, dark halos were increasingly colliding with each other. Moreover, 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, it is possible to 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 preserved, because 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. It probably arose from the merger of the largest protogalaxies that held 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 still be found today.
So, for example, not all the stars of the Milky Way revolve around the galactic center. Probably, over the billions of years of its existence, our Galaxy "swallowed up" 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 the once captured dwarf galaxies. A group of British scientists from the Astronomical Institute (Cambridge), led by Gerard Gilmour, calculated that the Milky Way, apparently, could absorb from 40 to 60 dwarf galaxies like Karin.
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. Perhaps it will absorb the dwarf galaxies closest to us - Fornax, Karina and, probably, Sextans, and then merge with the Andromeda nebula. Around the Milky Way - this insatiable "star cannibal" - will become even deserted.
A. Volkov