Our Galaxy. Mysteries of the Milky Way
To some extent, we know more about distant star systems than about our home Galaxy - the Milky Way. It is more difficult to study its structure than the structure of any other galaxies, because it has to be studied from the inside, and many things are not so easy to see. Interstellar dust clouds absorb the light emitted by myriads of distant stars.
Only with the development of radio astronomy and the advent of infrared telescopes were scientists 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 estimated rather roughly. The latest electronic reference books give figures from 100 to 300 billion stars.
Not so long ago, it was believed that our Galaxy has 4 large arms. But in 2008, astronomers from the University of Wisconsin published the results of processing about 800,000 infrared images that were taken by the Spitzer Space Telescope. Their analysis showed that the Milky Way has only two arms. As for the other branches, they are only narrow side branches. So, the Milky Way is a spiral galaxy with two arms. It should be noted that most 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 a conference of the American Astronomical Society. “We are refining our understanding of the Galaxy in the same way that centuries ago, pioneers, traveling around the globe, refined and rethought previous ideas about what the Earth looks like.”
Since the early 90s of the 20th century, observations carried out in the infrared range have increasingly changed our knowledge of the structure of the Milky Way, because infrared telescopes make it possible to look through gas and dust clouds 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. At first it was a diffuse gas bubble containing mainly hydrogen and helium. Over time, it turned into the huge spiral galaxy in which we now live.
general characteristics
But how did the evolution of our Galaxy proceed? How was it formed - slowly or, on the contrary, very quickly? How did it become saturated with heavy elements? How has the shape of the Milky Way and its chemical composition changed over billions of years? Scientists have yet to provide detailed answers to these questions.
The extent of our Galaxy is about 100,000 light years, 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, our closest large galaxy, is approximately 150,000 light years across.
At the end of 2008, researchers established using 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 varies from 1.0 to 1.9 trillion solar masses. Again, for comparison: the mass of the Andromeda nebula is estimated at at least 1.2 trillion solar masses.
Structure of galaxies
Black hole
So, the Milky Way is not inferior in size to the Andromeda nebula. “We should no longer think of 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 greater than expected, its gravitational force is also greater, which means that the likelihood of it colliding with other galaxies in our vicinity increases.
Our Galaxy is surrounded by a spherical halo, reaching a diameter of 165,000 light years. Astronomers sometimes call the halo a “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 approximately 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 “move” into the galactic disk. However, the most massive and bright stars do not live long enough, so 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 consists 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 gas and dust clouds, behind which nothing can be seen. Only since the 1950s, using radio astronomy, have scientists been able to gradually discern what lies there. In this part of the Galaxy, a powerful radio source was discovered, called Sagittarius A. As observations have shown, a mass is concentrated here that exceeds the mass of the Sun by several million times. The most acceptable explanation for this fact is only one: in the center of our Galaxy is located.
Now, for some reason, she has taken a break for herself and is not particularly active. The flow of matter here is very poor. Maybe over time the black hole will develop 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, stars will begin to rapidly form in the center of the Galaxy. Similar processes are likely to be repeated regularly.
2010 - American astronomers, using the Fermi Space Telescope, designed to observe sources of gamma radiation, discovered two mysterious structures in our Galaxy - two huge bubbles emitting gamma radiation. The diameter of each of them is on average 25,000 light years. They fly away from the center of the Galaxy in northern and southern directions. Perhaps we are talking about streams of particles that were once emitted by a black hole located in the middle of the Galaxy. Other researchers believe that we are talking about gas clouds that exploded during the birth of stars.
There are several dwarf galaxies around the Milky Way. The most famous of them 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 called the Magellanic Stream. Its extent is about 300,000 light years. Our Galaxy constantly absorbs the dwarf galaxies closest to it, in particular the Sagitarius Galaxy, which is located at a distance of 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, after 3 billion years, both galaxies will merge together, forming a larger elliptical galaxy, which has already been called Milkyhoney.
Origin of the Milky Way
Andromeda's nebula
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 it, a homogeneous cloud of gas once slowly rotated in place of the Milky Way. It resembled a ball and reached approximately 300,000 light years in diameter, and consisted mainly of hydrogen and helium. Under the influence of gravity, the protogalaxy shrank and became flat; at the same time, its rotation noticeably accelerated.
For almost two decades, this model suited scientists. But new observational results show that the Milky Way could not have arisen in the way theorists predicted.
According to this model, a halo forms first, and then a galactic disk. But the disk also contains very ancient stars, for example, the red giant Arcturus, whose age is more than 10 billion years, or numerous white dwarfs of the same age.
Globular clusters have been discovered in both the galactic disk and halo that are younger than the ELS model allows. Obviously, they are absorbed by our late Galaxy.
Many stars in the halo rotate in a different direction than the Milky Way. Maybe they, too, were once outside the Galaxy, but then they were drawn into this “stellar vortex” - like a random swimmer in a whirlpool.
1978 - Leonard Searle and Robert Zinn proposed their model of the formation of the Milky Way. It was designated as "Model SZ". Now the history of the Galaxy has become noticeably more complicated. Not so long ago, its youth, in the opinion of astronomers, was described as simply as in the opinion of physicists - rectilinear translational motion. The mechanics of what was happening were clearly visible: there was a homogeneous cloud; it consisted only of evenly spread gas. Nothing by its presence complicated the theorists' calculations.
Now, instead of one huge cloud in the visions of scientists, several small, intricately scattered clouds appeared at once. Stars were visible among them; however, they were located only in the halo. Inside the halo everything was seething: clouds collided; 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 subsequently criticized.
It was impossible to understand what connected the halo and the galactic disk. This condensed disk and the sparse stellar shell around it had little in common. 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 the halo.
The whole secret is that both models contain a grain of truth. The trouble is that they are too simple and one-sided. Both now seem to be fragments of the same recipe that created the Milky Way. Eggen and his colleagues read a few lines from this recipe, Searle and Zinn read a few others. Therefore, trying to re-imagine the history of our Galaxy, we now and then notice familiar lines that we have already 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 German astronomer Andreas Burkert, author of a new model of the birth of the Galaxy.
Dark halos became embryos - nuclei - of future galaxies. Gas accumulated around them under the influence of gravity. A homogeneous collapse occurred, as described by the ELS model. Already 500-1000 million years after the Big Bang, gas accumulations surrounding dark halos became “incubators” of stars. Small protogalaxies appeared here. The first globular clusters arose 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, ruins of a universe more than 12 billion years old.
There were many very massive stars in the protogalaxies. Less than a few tens of millions of years passed before most of them exploded. These explosions enriched the gas clouds with heavy chemical elements. Therefore, the stars that were born in the galactic disk were not the same 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 beyond the protogalaxies. A separation of gas masses and dark matter 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 increasingly collided with each other. Moreover, the protogalaxies stretched out or disintegrated. These catastrophes are reminiscent of the chains of stars 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, these stars formed a vast sphere - the halo we see. As it cooled, gas clouds penetrated inside it. Their angular momentum was conserved, so they did not collapse into one single point, but formed a rotating disk. All this happened more than 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 formed - its middle part, reminiscent of an ellipsoid. The bulge is made up of very old stars. It probably arose during the merger of the largest protogalaxies that held gas clouds for the longest time. In the middle of 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 the huge black hole that now resides in the center of our Galaxy was born.
The history of the Milky Way is much more chaotic than previously thought. Our native Galaxy, impressive even by cosmic standards, was formed after a series of impacts and mergers - after a series of cosmic disasters. Traces of those ancient events can still be found today.
For example, not all stars in the Milky Way revolve around the galactic center. Probably, over the billions of years of its existence, our Galaxy has “absorbed” many fellow travelers. Every tenth star in the galactic halo is less than 10 billion years old. By that time, the Milky Way had already formed. Perhaps these are the remnants of once captured dwarf galaxies. A group of English scientists from the Astronomical Institute (Cambridge), led by Gerard Gilmour, calculated that the Milky Way could apparently absorb from 40 to 60 Carina-type dwarf galaxies.
In addition, the Milky Way attracts huge masses of gas. Thus, 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 were rushing towards the galactic disk.
Our Galaxy will not restrain its appetite in the future. Perhaps it will absorb the dwarf galaxies closest to us - Fornax, Carina and, probably, Sextans, and then merge with the Andromeda nebula. Around the Milky Way – this insatiable “stellar cannibal” – it will become even more deserted.
Divided into social groups, our Milky Way galaxy will belong to a strong “middle class”. Thus, it belongs to the most common type of galaxy, but at the same time it is not average in size or mass. Galaxies that are smaller than the Milky Way are larger than those that are larger than it. Our “star island” also has at least 14 satellites - other dwarf galaxies. They are doomed to circle around the Milky Way until they are absorbed by it, or fly away from an intergalactic collision. Well, for now this is the only place where life probably exists - that is, you and me.
But the Milky Way remains the most mysterious galaxy in the Universe: being on the very edge of the “star island”, we see only a part of its billions of stars. And the galaxy is completely invisible - it is covered with dense arms of stars, gas and dust. Today we will talk about the facts and secrets of the Milky Way.
> >> How many stars are there in the Milky WayHow many stars are there in the Milky Way galaxy?: how to determine the number, Hubble telescope research, the structure of a spiral galaxy, observation methods.
If you have the opportunity to admire the dark sky, then you have an incredible collection of stars in front of you. From any place you can view 2500 stars of the Milky Way without the use of technology and 5800-8000 if you have binoculars or a telescope hidden at hand. But this is only a small part of their number. So, how many stars are in the Milky Way galaxy?
Scientists believe that the total number of stars in the Milky Way ranges from 100-400 billion, although there are those who rise to the trillion mark. Why such differences? The fact is that we have an open view from the inside and there are places hidden from the earth’s visibility zone.
Galactic structure and its influence on the number of stars
Let's start with the fact that the Solar system is located in a spiral-type galactic disk, with a length of 100,000 light years. We are 30,000 light years away from the center. That is, there is a huge gap between us and the opposite side.
Then another observational difficulty arises. Some stars are brighter than others and sometimes their light outshines their neighbors. The most distant stars visible to the naked eye are located at a distance of 1000 light years. The Milky Way is filled with dazzling lights, but many of them are hidden behind a haze of gas and dust. It is this elongated trace that is called “milk”.
The stars in our galactic “region” are open to observation. Imagine that you are at a party in a room where the entire area is packed with people. You stand in one corner and are asked to name the exact number of people present. But that's not all. One of the guests turns on the smoke machine, and the entire room is filled with thick fog, blocking everyone standing further from you. Now count!
Methods for visualizing the number of stars
But there is no need to panic, because there are always loopholes. Infrared cameras allow you to get through dust and smoke. Similar projects include the Spitzer telescope, COBE, WISE and the German Space Observatory.
All of them have emerged in the last ten years to study space at infrared wavelengths. This helps to find hidden stars. But even this does not allow us to see everything, so scientists are forced to make calculations and put forward speculative figures. Observations begin from stellar orbits on the galactic disk. Thanks to this, the orbital speed and period of rotation (motion) of the Milky Way are calculated.
Conclusions about how many stars are in the Milky Way
It takes the Solar System 225-250 million years to complete one rotation around the galactic center. That is, the speed of the galaxy is 600 km/s.
Next, the mass is determined (dark matter halo - 90%) and the average mass is calculated (the masses and types of stars are studied). As a result, it turns out that the average estimate of the number of stars in the Milky Way galaxy is 200-400 billion celestial bodies.
Future technologies will make it possible to find every star. Or probes will be able to reach incredible distances and photograph the galaxy from the “north” - above the center. For now, we can only rely on mathematical calculations.
Planet Earth, the Solar System, billions of other stars and celestial bodies - all this is our Milky Way galaxy - a huge intergalactic formation, where everything obeys the laws of gravity. Data on the true size of the galaxy are only approximate. And the most interesting thing is that there are hundreds, maybe even thousands, of such formations, larger or smaller, in the Universe.
The Milky Way Galaxy and what surrounds it
All celestial bodies, including the Milky Way planets, satellites, asteroids, comets and stars, are constantly in motion. Born in the cosmic vortex of the Big Bang, all these objects are on the path of their development. Some are older, others are clearly younger.
The gravitational formation rotates around the center, with individual parts of the galaxy rotating at different speeds. If in the center the rotation speed of the galactic disk is quite moderate, then at the periphery this parameter reaches values of 200-250 km/s. The Sun is located in one of these areas, closer to the center of the galactic disk. The distance from it to the center of the galaxy is 25-28 thousand light years. The Sun and the Solar System complete a full revolution around the central axis of the gravitational formation in 225-250 million years. Accordingly, in the entire history of its existence, the Solar System has flown around the center only 30 times.
Place of the galaxy in the Universe
One notable feature should be noted. The position of the Sun and, accordingly, the planet Earth is very convenient. The galactic disk is constantly undergoing a process of compaction. This mechanism is caused by the discrepancy between the speed of rotation of the spiral branches and the movement of stars, which move within the galactic disk according to their own laws. During compaction, violent processes occur, accompanied by powerful ultraviolet radiation. The Sun and the Earth are comfortably located in the corotational circle, where such vigorous activity is absent: between two spiral branches on the border of the Milky Way arms - Sagittarius and Perseus. This explains the calm in which we have been for such a long time. For more than 4.5 billion years, we have not been affected by cosmic disasters.
Structure of the Milky Way galaxy
The galactic disk is not homogeneous in its composition. Like other spiral gravitational systems, the Milky Way has three distinguishable regions:
- a core formed by a dense star cluster containing a billion stars of varying ages;
- the galactic disk itself, formed from clusters of stars, stellar gas and dust;
- corona, spherical halo - the region in which globular clusters, dwarf galaxies, individual groups of stars, cosmic dust and gas are located.
Near the plane of the galactic disk there are young stars collected in clusters. The density of star clusters in the center of the disk is higher. Near the center, the density is 10,000 stars per cubic parsec. In the region where the Solar System is located, the density of stars is already 1-2 stars per 16 cubic parsecs. As a rule, the age of these celestial bodies is no more than several billion years.
Interstellar gas also concentrates around the plane of the disk, subject to centrifugal forces. Despite the constant speed of rotation of the spiral branches, the interstellar gas is distributed unevenly, forming large and small zones of clouds and nebulae. However, the main galactic building material is dark matter. Its mass prevails over the total mass of all celestial bodies that make up the Milky Way galaxy.
If in the diagram the structure of the galaxy is quite clear and transparent, then in reality it is almost impossible to examine the central regions of the galactic disk. Gas and dust clouds and clusters of stellar gas hide from our view the light from the center of the Milky Way, in which lives a real space monster - a supermassive black hole. The mass of this supergiant is approximately 4.3 million M☉. Next to the supergiant is a smaller black hole. This gloomy company is complemented by hundreds of dwarf black holes. The black holes of the Milky Way are not only devourers of stellar matter, but also act as a maternity hospital, throwing huge bunches of protons, neutrons and electrons into space. It is from them that atomic hydrogen is formed - the main fuel of the star tribe.
The jumper bar is located in the region of the galactic core. Its length is 27 thousand light years. Old stars reign here, red giants, whose stellar matter feeds black holes. The bulk of molecular hydrogen is concentrated in this region, which acts as the main building material for the star formation process.
Geometrically, the structure of the galaxy looks quite simple. Each spiral arm, and there are four of them in the Milky Way, originates from a gas ring. The sleeves diverge at an angle of 20⁰. At the outer boundaries of the galactic disk, the main element is atomic hydrogen, which spreads from the center of the galaxy to the periphery. The thickness of the hydrogen layer on the outskirts of the Milky Way is much wider than in the center, while its density is extremely low. The discharge of the hydrogen layer is facilitated by the influence of dwarf galaxies, which have been closely following our galaxy for tens of billions of years.
Theoretical models of our galaxy
Even ancient astronomers tried to prove that the visible stripe in the sky is part of a huge stellar disk rotating around its center. This statement was supported by the mathematical calculations carried out. It was possible to get an idea of our galaxy only thousands of years later, when instrumental methods of space exploration came to the aid of science. A breakthrough in the study of the nature of the Milky Way was the work of the Englishman William Herschel. In 1700, he was able to experimentally prove that our galaxy is disk-shaped.
Already in our time, research has taken a different turn. Scientists relied on comparing the movements of stars between which there were different distances. Using the parallax method, Jacob Kaptein was able to approximately determine the diameter of the galaxy, which, according to his calculations, is 60-70 thousand light years. Accordingly, the place of the Sun was determined. It turned out that it is located relatively far from the raging center of the galaxy and at a considerable distance from the periphery of the Milky Way.
The fundamental theory of the existence of galaxies is that of the American astrophysicist Edwin Hubble. He came up with the idea to classify all gravitational formations, dividing them into elliptical galaxies and spiral-type formations. The latter, spiral galaxies, represent the largest group, which includes formations of various sizes. The largest recently discovered spiral galaxy is NGC 6872, with a diameter of more than 552 thousand light years.
Expected future and forecasts
The Milky Way Galaxy appears to be a compact and orderly gravitational formation. Unlike our neighbors, our intergalactic home is quite calm. Black holes systematically affect the galactic disk, reducing it in size. This process has already lasted tens of billions of years and how much longer it will continue is unknown. The only threat looming over our galaxy comes from its nearest neighbor. The Andromeda Galaxy is rapidly approaching us. Scientists suggest that a collision of two gravitational systems could occur in 4.5 billion years.
Such a meeting-merger will mean the end of the world in which we are accustomed to living. The Milky Way, which is smaller in size, will be absorbed by the larger formation. Instead of two large spiral formations, a new elliptical galaxy will appear in the Universe. Until this time, our galaxy will be able to deal with its satellites. Two dwarf galaxies - the Large and Small Magellanic Clouds - will be absorbed by the Milky Way in 4 billion years.
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People have been interested in the age of the Universe since ancient times. And although you cannot ask her for a passport to see her date of birth, modern science has been able to answer this question. True, only quite recently.
Passport to the Universe Astronomers have studied in detail the early biography of the Universe. But they had doubts about her exact age, which were only dispelled in the last couple of decades.
Alexey Levin
The sages of Babylon and Greece considered the universe eternal and unchanging, and Hindu chroniclers in 150 BC. determined that he was exactly 1,972,949,091 years old (by the way, in terms of the order of magnitude, they were not much mistaken!). In 1642, the English theologian John Lightfoot, through a scrupulous analysis of biblical texts, calculated that the creation of the world occurred in 3929 BC; a few years later, Irish Bishop James Ussher moved it to 4004. The founders of modern science, Johannes Kepler and Isaac Newton, also did not ignore this topic. Although they appealed not only to the Bible, but also to astronomy, their results turned out to be similar to the calculations of theologians - 3993 and 3988 BC. In our enlightened times, the age of the Universe is determined in other ways. To see them in a historical perspective, let’s first take a look at our own planet and its cosmic environment.
Astronomers have studied in detail the early biography of the Universe. But they had doubts about her exact age, which were only dispelled in the last couple of decades.
Fortune telling by stones
Since the second half of the 18th century, scientists began to estimate the age of the Earth and the Sun based on physical models. Thus, in 1787, the French naturalist Georges-Louis Leclerc came to the conclusion that if our planet was a ball of molten iron at birth, it would need from 75 to 168 thousand years to cool to its current temperature. After 108 years, the Irish mathematician and engineer John Perry re-calculated the thermal history of the Earth and determined its age at 2-3 billion years. At the very beginning of the 20th century, Lord Kelvin came to the conclusion that if the Sun gradually contracts and shines solely due to the release of gravitational energy, then its age (and, consequently, the maximum age of the Earth and other planets) could be several hundred million years. But at that time, geologists could neither confirm nor refute these estimates due to the lack of reliable geochronological methods.
In the middle of the first decade of the twentieth century, Ernest Rutherford and the American chemist Bertram Boltwood developed the basis of radiometric dating of earth rocks, which showed that Perry was much closer to the truth. In the 1920s, mineral samples were found whose radiometric age was close to 2 billion years. Later, geologists increased this value more than once, and by now it has more than doubled - to 4.4 billion. Additional data is provided by the study of “heavenly stones” - meteorites. Almost all radiometric estimates of their age fall within the range of 4.4−4.6 billion years.
Modern helioseismology makes it possible to directly determine the age of the Sun, which, according to the latest data, is 4.56 - 4.58 billion years. Since the duration of the gravitational condensation of the protosolar cloud was measured in only millions of years, we can confidently say that no more than 4.6 billion years have passed from the beginning of this process to the present day. At the same time, solar matter contains many elements heavier than helium, which were formed in the thermonuclear furnaces of massive stars of previous generations that burned out and exploded in supernovae. This means that the existence of the Universe greatly exceeds the age of the Solar System. To determine the extent of this excess, you need to go first into our Galaxy, and then beyond its limits.
Following white dwarfs
The lifetime of our Galaxy can be determined in different ways, but we will limit ourselves to the two most reliable ones. The first method is based on monitoring the glow of white dwarfs. These compact (about Earth-sized) and initially very hot celestial bodies represent the final stage of life for all but the most massive stars. To transform into a white dwarf, a star must completely burn all its thermonuclear fuel and undergo several cataclysms - for example, become a red giant for some time.
Natural clock
According to radiometric dating, the oldest rocks on Earth are now considered to be the gray gneisses of the Great Slave Lake coast in northwestern Canada - their age is determined to be 4.03 billion years. Even earlier (4.4 billion years ago), tiny grains of the mineral zircon, a natural zirconium silicate found in gneisses in western Australia, crystallized. And since the earth’s crust already existed in those days, our planet should be somewhat older.
As for meteorites, the most accurate information is provided by the dating of calcium-aluminum inclusions in the material of Carboniferous chondritic meteorites, which remained virtually unchanged after its formation from the gas-dust cloud that surrounded the newborn Sun. The radiometric age of similar structures in the Efremovka meteorite, found in 1962 in the Pavlodar region of Kazakhstan, is 4 billion 567 million years.
A typical white dwarf is composed almost entirely of carbon and oxygen ions embedded in degenerate electron gas, and has a thin atmosphere dominated by hydrogen or helium. Its surface temperature ranges from 8,000 to 40,000 K, while the central zone is heated to millions and even tens of millions of degrees. According to theoretical models, dwarfs consisting predominantly of oxygen, neon and magnesium (which, under certain conditions, transform into stars with a mass of 8 to 10.5 or even up to 12 solar masses) may also be born, but their existence has not yet been proven. The theory also states that stars with at least half the mass of the Sun end up as helium white dwarfs. Such stars are very numerous, but they burn hydrogen extremely slowly and therefore live for many tens and hundreds of millions of years. So far, they simply haven’t had enough time to exhaust their hydrogen fuel (the very few helium dwarfs discovered to date live in binary systems and arose in a completely different way).
Since a white dwarf cannot support thermonuclear fusion reactions, it shines due to the accumulated energy and therefore slowly cools. The rate of this cooling can be calculated and, on this basis, determine the time required to reduce the surface temperature from the initial one (for a typical dwarf this is about 150,000 K) to the observed one. Since we are interested in the age of the Galaxy, we should look for the longest-lived, and therefore the coldest, white dwarfs. Modern telescopes make it possible to detect intragalactic dwarfs with a surface temperature of less than 4000 K, the luminosity of which is 30,000 times lower than that of the Sun. So far they have not been found - either they are not there at all, or there are very few of them. It follows that our Galaxy cannot be older than 15 billion years, otherwise they would be present in noticeable quantities.
To date rocks, an analysis of the content of decay products of various radioactive isotopes in them is used. Depending on the type of rock and dating time, different pairs of isotopes are used.
This is the upper age limit. What can we say about the bottom? The coolest white dwarfs currently known were detected by the Hubble Space Telescope in 2002 and 2007. Calculations showed that their age is 11.5 - 12 billion years. To this we must also add the age of the predecessor stars (from half a billion to a billion years). It follows that the Milky Way is no younger than 13 billion years old. So the final estimate of its age, obtained from observations of white dwarfs, is approximately 13 - 15 billion years.
Ball certificates
The second method is based on the study of spherical star clusters located in the peripheral zone of the Milky Way and orbiting its core. They contain from hundreds of thousands to more than a million stars bound by mutual attraction.
Globular clusters are found in almost all large galaxies, and their number sometimes reaches many thousands. Almost no new stars are born there, but older stars are present in abundance. About 160 such globular clusters have been registered in our Galaxy, and perhaps two to three dozen more will be discovered. The mechanisms of their formation are not entirely clear, however, most likely, many of them arose soon after the birth of the Galaxy itself. Therefore, dating the formation of the oldest globular clusters makes it possible to establish a lower limit on the galactic age.
This dating is very technically complex, but it is based on a very simple idea. All stars in the cluster (from supermassive to the lightest) are formed from the same gas cloud and therefore are born almost simultaneously. Over time, they burn out the main reserves of hydrogen - some earlier, others later. At this stage, the star leaves the main sequence and undergoes a series of transformations that culminate in either complete gravitational collapse (followed by the formation of a neutron star or black hole) or the emergence of a white dwarf. Therefore, studying the composition of a globular cluster makes it possible to determine its age quite accurately. For reliable statistics, the number of clusters studied should be at least several dozen.
This work was carried out three years ago by a team of astronomers using the ACS (Advanced Camera for Survey) camera of the Hubble Space Telescope. Monitoring of 41 globular clusters in our Galaxy showed that their average age is 12.8 billion years. The record holders were the clusters NGC 6937 and NGC 6752, located 7,200 and 13,000 light years from the Sun. They are almost certainly no younger than 13 billion years, with the most probable lifetime of the second cluster being 13.4 billion years (although with an error of plus or minus a billion).
Stars with a mass on the order of the Sun, as their hydrogen reserves are depleted, swell and become red dwarfs, after which their helium core heats up during compression and helium combustion begins. After some time, the star sheds its shell, forming a planetary nebula, and then becomes a white dwarf and then cools down.
However, our Galaxy must be older than its clusters. Its first supermassive stars exploded as supernovae and ejected the nuclei of many elements into space, in particular the nuclei of the stable isotope beryllium-beryllium-9. When globular clusters began to form, their newborn stars already contained beryllium, and more so the later they arose. Based on the beryllium content in their atmospheres, one can determine how much younger the clusters are than the Galaxy. As evidenced by data on the NGC 6937 cluster, this difference is 200 - 300 million years. So, without much of a stretch, we can say that the age of the Milky Way exceeds 13 billion years and perhaps reaches 13.3 - 13.4 billion. This is almost the same estimate as that made on the basis of observations of white dwarfs, but it was obtained in a completely different way way.
Hubble's Law
The scientific formulation of the question about the age of the Universe became possible only at the beginning of the second quarter of the last century. In the late 1920s, Edwin Hubble and his assistant Milton Humason began to clarify the distances to dozens of nebulae outside the Milky Way, which only a few years earlier had become independent galaxies.
These galaxies are moving away from the Sun at radial velocities that were measured by the redshift of their spectra. Although the distances to most of these galaxies could be determined with a large error, Hubble still found that they were approximately proportional to the radial velocities, as he wrote about in an article published in early 1929. Two years later, Hubble and Humason confirmed this conclusion based on observations of other galaxies - some of them more than 100 million light years away.
These data formed the basis of the famous formula v=H0d, known as Hubble's law. Here v is the radial velocity of the galaxy relative to the Earth, d is the distance, H0 is the proportionality coefficient, whose dimension, as is easy to see, is the inverse of the dimension of time (previously it was called the Hubble constant, which is incorrect, since in previous epochs the value of H0 was different than Nowadays). Hubble himself and many other astronomers for a long time rejected assumptions about the physical meaning of this parameter. However, Georges Lemaitre showed back in 1927 that the general theory of relativity allows us to interpret the expansion of galaxies as evidence of the expansion of the Universe. Four years later, he had the courage to take this conclusion to its logical conclusion, putting forward the hypothesis that the Universe arose from an almost point-like embryo, which he, for lack of a better term, called the atom. This primordial atom could remain in a static state for any time up to infinity, but its “explosion” gave birth to an expanding space filled with matter and radiation, which in a finite time gave rise to the present Universe. Already in his first article, Lemaitre derived a complete analogue of the Hubble formula and, having the data known by that time on the velocities and distances of a number of galaxies, he obtained approximately the same value of the coefficient of proportionality between distances and velocities as Hubble. However, his article was published in French in a little-known Belgian magazine and initially went unnoticed. It became known to most astronomers only in 1931 after the publication of its English translation.
The evolution of the Universe is determined by the initial rate of its expansion, as well as the effects of gravity (including dark matter) and antigravity (dark energy). Depending on the relationship between these factors, the graph of the size of the Universe has a different shape both in the future and in the past, which affects the estimate of its age. Current observations show that the Universe is expanding exponentially (red graph).
Hubble time
From this work by Lemaître and the later works of both Hubble himself and other cosmologists it directly followed that the age of the Universe (naturally, measured from the initial moment of its expansion) depends on the value 1/H0, which is now called Hubble time. The nature of this dependence is determined by the specific model of the universe. If we assume that we live in a flat Universe filled with gravitating matter and radiation, then to calculate its age 1/H0 must be multiplied by 2/3.
This is where the snag arose. From the measurements of Hubble and Humason it follows that the numerical value of 1/H0 is approximately equal to 1.8 billion years. It followed that the Universe was born 1.2 billion years ago, which clearly contradicted even the greatly underestimated estimates of the age of the Earth at that time. One could get out of this difficulty by assuming that galaxies are moving away more slowly than Hubble thought. Over time, this assumption was confirmed, but it did not solve the problem. According to data obtained by the end of the last century using optical astronomy, 1/H0 ranges from 13 to 15 billion years. So the discrepancy still remained, since the space of the Universe was and is considered flat, and two-thirds of Hubble time is much less than even the most modest estimates of the age of the Galaxy.
Empty world
According to the latest measurements of the Hubble parameter, the lower limit of Hubble time is 13.5 billion years, and the upper limit is 14 billion. It turns out that the current age of the Universe is approximately equal to the current Hubble time. Such equality must be strictly and invariably observed for an absolutely empty Universe, where there is neither gravitating matter nor anti-gravitating fields. But in our world there is enough of both. The fact is that space first expanded slowly, then the speed of its expansion began to increase, and in the current era these opposing trends almost compensated for each other.
In general, this contradiction was eliminated in 1998 - 1999, when two teams of astronomers proved that over the last 5 - 6 billion years, outer space has been expanding not at a decreasing, but an increasing rate. This acceleration is usually explained by the fact that in our Universe the influence of the anti-gravity factor, the so-called dark energy, the density of which does not change over time, is growing. Since the density of gravitating matter decreases as the Cosmos expands, dark energy competes more and more successfully with gravity. The duration of the existence of a Universe with an antigravitational component does not have to be equal to two-thirds of Hubble time. Therefore, the discovery of the accelerating expansion of the Universe (noted in 2011 by the Nobel Prize) made it possible to eliminate the discrepancy between cosmological and astronomical estimates of its lifetime. It was also a prelude to the development of a new method for dating her birth.
Cosmic rhythms
On June 30, 2001, NASA sent Explorer 80 into space, two years later renamed WMAP, the Wilkinson Microwave Anisotropy Probe. His equipment made it possible to record temperature fluctuations of the microwave cosmic microwave background radiation with an angular resolution of less than three tenths of a degree. It was already known then that the spectrum of this radiation almost completely coincides with the spectrum of an ideal black body heated to 2.725 K, and its temperature fluctuations in “coarse-grained” measurements with an angular resolution of 10 degrees do not exceed 0.000036 K. However, in “fine-grained” measurements on the scale of the WMAP probe, the amplitudes of such fluctuations were six times larger (about 0.0002 K). The cosmic microwave background radiation turned out to be spotty, closely dotted with slightly more and slightly less heated areas.
Fluctuations in the cosmic microwave background radiation are generated by fluctuations in the density of the electron-photon gas that once filled outer space. It dropped to almost zero about 380,000 years after the Big Bang, when virtually all the free electrons combined with the nuclei of hydrogen, helium and lithium, thereby giving rise to neutral atoms. Until this happened, sound waves propagated in the electron-photon gas, influenced by the gravitational fields of dark matter particles. These waves, or, as astrophysicists say, acoustic oscillations, left their mark on the spectrum of the cosmic microwave background radiation. This spectrum can be deciphered using the theoretical apparatus of cosmology and magnetic hydrodynamics, which makes it possible to re-evaluate the age of the Universe. As the latest calculations show, its most probable extent is 13.72 billion years. It is now considered the standard estimate of the lifetime of the Universe. If we take into account all possible inaccuracies, tolerances and approximations, we can conclude that, according to the results of the WMAP probe, the Universe has existed for between 13.5 and 14 billion years.
Thus, astronomers, estimating the age of the Universe in three different ways, obtained quite compatible results. Therefore, we now know (or, to put it more cautiously, we think that we know) when our universe arose - at least to an accuracy of several hundred million years. Probably, descendants will add the solution to this age-old riddle to the list of the most remarkable achievements of astronomy and astrophysics.
