Life cycle of stars. Star evolutions

Although the stars seem to be eternal on the human time scale, they, like all things in nature, are born, live and die. According to the generally accepted hypothesis of a gas and dust cloud, a star is born as a result of gravitational compression of an interstellar gas and dust cloud. As such a cloud becomes denser, it first forms protostar, the temperature at its center rises steadily until it reaches the limit necessary for the speed thermal motion particles has exceeded the threshold, after which the protons are able to overcome the macroscopic forces of mutual electrostatic repulsion ( cm. Coulomb's law) and enter into a thermonuclear fusion reaction ( cm. Nuclear decay and fusion).

As a result of a multi-stage thermonuclear fusion reaction of four protons, a helium nucleus (2 protons + 2 neutrons) is eventually formed and a whole fountain of various elementary particles is released. In the final state, the total mass of the formed particles less masses of the four original protons, which means that free energy is released during the reaction ( cm. Theory of relativity). Because of this, the inner core of a newborn star quickly warms up to ultra-high temperatures, and its excess energy begins to splash out towards its less hot surface - and out. At the same time, the pressure in the center of the star begins to increase ( cm. The equation of state for an ideal gas). Thus, by "burning" hydrogen in the process of a thermonuclear reaction, the star does not allow the forces of gravitational attraction to compress itself to a superdense state, countering the gravitational collapse with a continuously renewed internal thermal pressure, resulting in a stable energy balance. Stars in the stage of active hydrogen burning are said to be in the "main phase" of their life cycle or evolution ( cm. Hertzsprung-Russell diagram). The transformation of one chemical element into another inside a star is called nuclear fusion or nucleosynthesis.

In particular, the Sun has been at the active stage of burning hydrogen in the process of active nucleosynthesis for about 5 billion years, and the reserves of hydrogen in the core for its continuation should be enough for our luminary for another 5.5 billion years. The more massive the star, the more hydrogen fuel it has, but to counteract the forces of gravitational collapse, it has to burn hydrogen at a rate that exceeds the growth rate of hydrogen reserves as the mass of the star increases. Thus, the more massive the star, the shorter its lifetime, determined by the depletion of hydrogen reserves, and the largest stars literally burn out in "some" tens of millions of years. The smallest stars, on the other hand, live comfortably for hundreds of billions of years. So, according to this scale, our Sun belongs to the “strong middle peasants”.

Sooner or later, however, any star will use up all the hydrogen available for combustion in its fusion furnace. What's next? It also depends on the mass of the star. The sun (and all stars less than eight times its mass) end their lives in a very banal way. As the reserves of hydrogen in the interior of the star are depleted, the forces of gravitational contraction, which have been patiently waiting for this hour since the very moment of the birth of the star, begin to prevail - and under their influence the star begins to shrink and condense. This process has a twofold effect: The temperature in the layers immediately around the core of the star rises to a level at which the hydrogen contained there finally enters into a fusion reaction with the formation of helium. At the same time, the temperature in the core itself, which now consists of practically one helium, rises so much that helium itself - a kind of "ash" of the decaying primary nucleosynthesis reaction - enters into a new thermonuclear fusion reaction: one carbon nucleus is formed from three helium nuclei. This secondary reaction process of thermonuclear fusion, fueled by the products of the primary reaction, is one of the key points life cycle of stars.

During the secondary combustion of helium in the core of a star, so much energy is released that the star begins to literally swell. In particular, the envelope of the Sun at this stage of life will expand beyond the orbit of Venus. In this case, the total energy of the star's radiation remains approximately at the same level as during the main phase of its life, but since this energy is now radiated through a much larger surface area, the outer layer of the star cools to the red part of the spectrum. The star turns into red giant.

For stars like the Sun, after the depletion of the fuel that feeds the secondary reaction of nucleosynthesis, the stage of gravitational collapse again sets in - this time the final one. The temperature inside the core is no longer able to rise to the level necessary to start the next level of fusion. Therefore, the star contracts until the forces of gravitational attraction are balanced by the next force barrier. In his role is degenerate electron gas pressure(cm. Chandrasekhar limit). Electrons, which until this stage played the role of unemployed extras in the evolution of the star, do not participate in nuclear fusion reactions and freely move between the nuclei that are in the process of fusion, at a certain stage of compression, they are deprived of "living space" and begin to "resist" further gravitational compression of the star. The state of the star stabilizes, and it turns into a degenerate white dwarf, which will radiate residual heat into space until it cools down completely.

Stars more massive than the Sun are waiting for a much more spectacular end. After the combustion of helium, their mass during compression is sufficient to heat the core and shell to the temperatures necessary to start the next nucleosynthesis reactions - carbon, then silicon, magnesium - and so on, as the nuclear masses increase. At the same time, at the beginning of each new reaction in the core of the star, the previous one continues in its shell. In fact, everything chemical elements up to the iron that makes up the Universe, were formed precisely as a result of nucleosynthesis in the interiors of dying stars of this type. But iron is the limit; it cannot serve as a fuel for nuclear fusion or decay reactions at any temperature and pressure, since both its decay and the addition of additional nucleons to it require an influx of external energy. As a result, a massive star gradually accumulates an iron core inside itself, unable to serve as fuel for any further nuclear reactions.

As soon as the temperature and pressure inside the nucleus reach a certain level, the electrons begin to interact with the protons of the iron nuclei, resulting in the formation of neutrons. And in a very short period of time - some theorists believe that it takes a matter of seconds - the electrons free throughout the previous evolution of the star literally dissolve in the protons of iron nuclei, all the matter of the star's core turns into a continuous bunch of neutrons and begins to rapidly shrink in gravitational collapse , since the pressure of the degenerate electron gas opposing it drops to zero. The outer shell of the star, from under which any support is knocked out, collapses towards the center. The collision energy of the collapsed outer shell with the neutron core is so high that it bounces off with great speed and scatters in all directions from the core - and the star literally explodes in a blinding flash supernova stars. In a matter of seconds, during a supernova explosion, more energy can be released into space than all the stars of the galaxy put together during the same time.

After a supernova explosion and the expansion of the shell in stars with a mass of the order of 10-30 solar masses, the ongoing gravitational collapse leads to the formation of a neutron star, the substance of which is compressed until it begins to make itself felt pressure of degenerate neutrons - in other words, now neutrons (just as electrons did earlier) begin to resist further compression, requiring yourself living space. This usually occurs when the star reaches a size of about 15 km in diameter. As a result, a rapidly rotating neutron star is formed, emitting electromagnetic impulses with the frequency of its rotation; such stars are called pulsars. Finally, if the mass of the star's core exceeds 30 solar masses, nothing can stop its further gravitational collapse, and as a result of a supernova explosion,

Like any body in nature, the stars also cannot remain unchanged. They are born, develop and finally “die”. The evolution of stars takes billions of years, but there are disputes about the time of their formation. Previously, astronomers believed that the process of their "birth" from stardust requires millions of years, but not so long ago, photographs of a region of the sky from the Great Nebula of Orion were obtained. In a few years there has been a small

In the 1947 photographs, a small group of star-like objects was recorded in this place. By 1954, some of them had already become oblong, and after another five years, these objects broke up into separate ones. So for the first time the process of the birth of stars took place literally in front of astronomers.

Let's take a closer look at how the structure and evolution of stars goes, how they begin and end their endless, by human standards, life.

Traditionally, scientists assume that stars are formed as a result of the condensation of clouds of a gas-dust environment. Under the action of gravitational forces, an opaque gas ball is formed from the formed clouds, dense in structure. Its internal pressure cannot balance the gravitational forces compressing it. Gradually, the ball contracts so much that the temperature of the stellar interior rises, and the pressure of the hot gas inside the ball balances external forces. After that, the compression stops. The duration of this process depends on the mass of the star and usually ranges from two to several hundred million years.

The structure of the stars suggests a very high temperature in their depths, which contributes to continuous thermonuclear processes (the hydrogen that forms them turns into helium). It is these processes that are the cause of the intense radiation of stars. The time for which they consume the available supply of hydrogen is determined by their mass. The duration of the radiation also depends on this.

When the reserves of hydrogen are depleted, the evolution of stars approaches the stage of formation. This happens as follows. After the cessation of energy release, gravitational forces begin to compress the nucleus. In this case, the star increases significantly in size. The luminosity also increases as the process continues, but only in a thin layer at the core boundary.

This process is accompanied by an increase in the temperature of the shrinking helium core and the transformation of helium nuclei into carbon nuclei.

Our Sun is predicted to become a red giant in eight billion years. At the same time, its radius will increase by several tens of times, and the luminosity will increase hundreds of times compared to current indicators.

The lifespan of a star, as already noted, depends on its mass. Objects with a mass that is less than the sun "expend" their reserves very economically, so they can shine for tens of billions of years.

The evolution of stars ends with the formation. This happens with those of them whose mass is close to the mass of the Sun, i.e. does not exceed 1.2 of it.

giant stars, as a rule, quickly deplete their supply of nuclear fuel. This is accompanied by a significant loss of mass, in particular, due to the shedding of the outer shells. As a result, only a gradually cooling central part, wherein nuclear reactions stopped completely. Over time, such stars stop their radiation and become invisible.

But sometimes the normal evolution and structure of stars is disturbed. Most often this concerns massive objects that have exhausted all types of thermonuclear fuel. Then they can be converted into neutron ones, or And the more scientists learn about these objects, the more new questions arise.

Our Sun has been shining for more than 4.5 billion years. At the same time, it constantly consumes hydrogen. It is absolutely clear that no matter how great its reserves were, but someday they will be exhausted. And what will happen to the light? There is an answer to this question. The life cycle of a star can be studied from other similar space formations. Indeed, in space there are real patriarchs, whose age is 9-10 billion years. And there are very young stars. They are no more than a few tens of millions of years old.

Therefore, by observing the state of the various stars with which the Universe is "strewn", one can understand how they behave over time. Here we can draw an analogy with an alien observer. He flew to Earth and began to study people: children, adults, old people. Thus, for absolutely short period time he understood what changes occur to people during their lives.

The Sun is currently a yellow dwarf
Billions of years will pass, and it will become a red giant - 2
And then turn into a white dwarf - 3

Therefore, it can be said with certainty that when the hydrogen reserves in the central part of the Sun are exhausted, the thermonuclear reaction will not stop. The zone where this process will continue will begin to move towards the surface of our luminary. But at the same time, gravitational forces will no longer be able to influence the pressure that is formed as a result of a thermonuclear reaction.

Consequently, the star will begin to grow in size and gradually turn into a red giant. This is a space object of a late stage of evolution. But it happens the same way early stage during star formation. Only in the second case does the red giant shrink and turn into main sequence star. That is, in one in which the reaction of the synthesis of helium from hydrogen takes place. In a word, with what the life cycle of a star begins, so it ends.

Our Sun will increase in size so much that it will swallow the nearest planets. These are Mercury, Venus and Earth. But you don't have to be afraid. The luminary will begin to die in a few billion years. During this time, dozens, and maybe hundreds of civilizations will change. A person will pick up a club more than once, and after millennia, he will again sit down at a computer. This is the usual cyclicity on which the entire universe is based.

But becoming a red giant doesn't mean the end. The thermonuclear reaction will throw the outer shell into space. And in the center there will be a helium core devoid of energy. Under the influence of gravitational forces, it will shrink and, in the end, will turn into an extremely dense space formation with a large mass. Such remnants of extinct and slowly cooling stars are called white dwarfs.

Our white dwarf will have a radius 100 times smaller than the radius of the Sun, and the luminosity will decrease by 10 thousand times. At the same time, the mass will be comparable to the current solar one, and the density will be more than a million times. There are a lot of such white dwarfs in our galaxy. Their number is 10% of total number stars.

It should be noted that white dwarfs are hydrogen and helium. But we will not climb into the wilds, but only note that with strong compression, gravitational collapse can occur. And this is fraught with a colossal explosion. At the same time, a supernova explosion is observed. The term "supernova" characterizes not the age, but the brightness of the flash. It's just that the white dwarf was not visible in the cosmic abyss for a long time, and suddenly a bright glow appeared.

Most of the exploding supernova scatters in space with great speed. And the remaining central part is compressed into an even denser formation and is called neutron star. It is the end product of stellar evolution. Its mass is comparable to that of the sun, and its radius reaches only a few tens of kilometers. One cube see a neutron star can weigh millions of tons. There are quite a lot of such formations in space. Their number is about a thousand times less than ordinary suns, which are strewn with the night sky of the Earth.

I must say that the life cycle of a star is directly related to its mass. If it corresponds to the mass of our Sun or less than it, then at the end of life a white dwarf appears. However, there are luminaries that are tens and hundreds of times larger than the Sun.

When such giants shrink in the process of aging, they distort space and time in such a way that instead of a white dwarf, black hole . Its gravitational attraction is so strong that even those objects that move at the speed of light cannot overcome it. The size of the hole characterizes gravity radius. This is the radius of the sphere bounded by event horizon. It represents the space-time limit. Any cosmic body, having overcome it, disappears forever and never comes back.

There are many theories about black holes. All of them are based on the theory of gravity, since gravity is one of the most important forces in the universe. And its main quality is versatility. At least, today not a single space object has been discovered that does not have gravitational interaction.

There is an assumption that through a black hole you can get into a parallel world. That is, it is a channel to another dimension. Everything is possible, but any statement requires practical evidence. However, no mortal has yet been able to carry out such an experiment.

Thus, the life cycle of a star consists of several stages. In each of them, the luminary acts in a certain capacity, which is fundamentally different from the previous and future ones. This is the uniqueness and mystery of outer space. When you get to know him, you involuntarily begin to think that a person also goes through several stages in his development. And the shell in which we exist now is only a transitional stage to some other state. But this conclusion, again, requires practical confirmation..

> Life cycle of a star

Description life and death of stars: evolutionary stages with photo, molecular clouds, protostar, T Taurus, main sequence, red giant, white dwarf.

Everything in this world is evolving. Any cycle begins with birth, growth and ends with death. Of course, the stars have these cycles in a special way. Let us recall, for example, that they have a larger time frame and are measured in millions and billions of years. In addition, their death carries certain consequences. What does it look like life cycle of stars?

The first life cycle of a star: Molecular clouds

Let's start with the birth of a star. Imagine a huge cloud of cold molecular gas that can easily exist in the universe without any changes. But suddenly a supernova explodes not far from it, or it collides with another cloud. Because of this push, the process of destruction is activated. It is divided into small parts, each of which is drawn into itself. As you already understood, all these bunches are preparing to become stars. Gravity heats up the temperature, and the stored momentum keeps the rotation going. The lower diagram clearly demonstrates the cycle of stars (life, stages of development, transformation options and death of a celestial body with a photo).

The second life cycle of a star: protostar

The material condenses more densely, heats up and is repelled by gravitational collapse. Such an object is called a protostar, around which a disk of material is formed. The part is attracted to the object, increasing its mass. The rest of the debris will be grouped and create planetary system. Further development of the star all depends on the mass.

Third life cycle of a star: T Taurus

When material hits a star, it is released great amount energy. The new stellar stage was named after the prototype, T Taurus. This is a variable star located 600 light years away (not far from).

It can reach great brightness because the material breaks down and releases energy. But in the central part there is not enough temperature to support nuclear fusion. This phase lasts 100 million years.

The fourth life cycle of a star:Main sequence

At a certain moment, the temperature of the celestial body rises to the required level, activating nuclear fusion. All stars go through this. Hydrogen is transformed into helium, releasing a huge thermal reserve and energy.

The energy is released as gamma rays, but due to the star's slow motion, it falls off with wavelength. Light is pushed outward and confronts gravity. We can assume that a perfect balance is created here.

How long will she be in the main sequence? You need to start from the mass of the star. Red dwarfs (half the solar mass) are capable of spending hundreds of billions (trillions) of years on their fuel supply. Average stars (like) live 10-15 billion. But the largest ones are billions or millions of years old. See how the evolution and death of stars of various classes looks like in the diagram.

Fifth life cycle of a star: red giant

During the melting process, hydrogen ends and helium accumulates. When there is no hydrogen left at all, all nuclear reactions stop, and the star begins to shrink due to gravity. The hydrogen shell around the core heats up and ignites, causing the object to grow 1000-10000 times. At a certain moment, our Sun will repeat this fate, having increased to the earth's orbit.

Temperature and pressure reach a maximum, and helium fuses into carbon. At this point, the star contracts and ceases to be a red giant. With greater massiveness, the object will burn other heavy elements.

The sixth life cycle of a star: white dwarf

A solar-mass star doesn't have enough gravitational pressure to fuse carbon. Therefore, death occurs with the end of helium. The outer layers are ejected and a white dwarf appears. At first it is hot, but after hundreds of billions of years it will cool down.

Stellar evolution in astronomy is the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years, while it radiates light and heat. during such colossal periods of time, the changes are very significant.

The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in the galaxy actually contains 0.1 to 1 molecule per cm3. A molecular cloud, on the other hand, has a density of about a million molecules per cm3. The mass of such a cloud exceeds the mass of the Sun by 100,000–10,000,000 times due to its size: from 50 to 300 light-years across.

The evolution of a star begins in a giant molecular cloud, also called a stellar cradle.

As long as the cloud circulates freely around the center of the native galaxy, nothing happens. However, due to the inhomogeneity of the gravitational field, disturbances can arise in it, leading to local mass concentrations. Such perturbations cause the gravitational collapse of the cloud. One of the scenarios leading to this is the collision of two clouds. Another collapse-causing event could be the passage of a cloud through a dense arm spiral galaxy. Also a critical factor may be the explosion of a nearby supernova, the shock wave of which will collide with the molecular cloud at great speed. In addition, a collision of galaxies is possible, capable of causing a burst of star formation, as the gas clouds in each of the galaxies are compressed by the collision. In general, any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.

any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.

In the course of this process, the inhomogeneities of the molecular cloud will be compressed under the influence of their own gravity and gradually take the shape of a ball. When compressed, the gravitational energy is converted into heat, and the temperature of the object increases.

When the temperature in the center reaches 15–20 million K, thermonuclear reactions begin and the compression stops. The object becomes a full-fledged star.

The subsequent stages of a star's evolution depend almost entirely on its mass, and only at the very end of a star's evolution can its chemical composition play its role.

The first stage of a star's life is similar to that of the sun - it is dominated by the reactions of the hydrogen cycle.

It remains in this state for most of its life, being on the main sequence of the Hertzsprung-Russell diagram, until the fuel reserves in its core run out. When all the hydrogen in the center of the star turns into helium, a helium core is formed, and the thermonuclear combustion of hydrogen continues on the periphery of the core.

Small and cold red dwarfs slowly burn their hydrogen reserves and remain on the main sequence for tens of billions of years, while massive supergiants leave the main sequence after only a few tens of millions (and some only a few million) years after formation.

At present, it is not known for certain what happens to light stars after the depletion of the supply of hydrogen in their interiors. Since the universe is 13.8 billion years old, which is not enough to deplete the supply of hydrogen fuel in such stars, modern theories are based on computer simulation of the processes occurring in such stars.

According to theoretical concepts, some of the light stars, losing their substance (stellar wind), will gradually evaporate, becoming smaller and smaller. Others, red dwarfs, will slowly cool down over billions of years, continuing to radiate weakly in the infrared and microwave ranges of the electromagnetic spectrum.

Medium-sized stars like the Sun stay on the main sequence for an average of 10 billion years.

It is believed that the Sun is still on it, as it is in the middle of its life cycle. As soon as the star depletes the supply of hydrogen in the core, it leaves the main sequence.

As soon as the star depletes the supply of hydrogen in the core, it leaves the main sequence.

Without the pressure generated by the fusion reactions to balance the internal gravity, the star begins to contract again, as it did earlier in the process of its formation.

The temperature and pressure rise again, but, unlike in the protostar stage, to a much higher level.

The collapse continues until, at a temperature of approximately 100 million K, thermonuclear reactions involving helium begin, during which helium is converted into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally silicon to iron).

The collapse continues until, at a temperature of approximately 100 million K, thermonuclear reactions involving helium begin.

The thermonuclear "burning" of matter resumed at a new level causes a monstrous expansion of the star. The star "swells up", becoming very "loose", and its size increases by about 100 times.

The star becomes a red giant, and the helium burning phase continues for about several million years.

What happens next also depends on the mass of the star.

In medium-sized stars, the reaction of thermonuclear burning of helium can lead to an explosive ejection of the outer layers of the star, forming from them planetary nebula. The core of the star, in which thermonuclear reactions stop, cools down and turns into a helium white dwarf, as a rule, having a mass of up to 0.5-0.6 solar masses and a diameter of the order of the diameter of the Earth.

For massive and supermassive stars (with a mass of five solar masses or more), the processes occurring in their core, as gravitational compression increases, lead to an explosion supernova with the release of enormous energy. The explosion is accompanied by the ejection of a significant mass of the star's matter into interstellar space. This substance is further involved in the formation of new stars, planets or satellites. It is thanks to supernovae that the Universe as a whole and each galaxy in particular chemically evolves. The core of the star left after the explosion can end its evolution as a neutron star (pulsar), if the mass of the star in the later stages exceeds the Chandrasekhar limit (1.44 solar masses), or as a black hole, if the mass of the star exceeds the Oppenheimer-Volkov limit (estimated values ​​2 ,5-3 solar masses).

The process of stellar evolution in the Universe is continuous and cyclical - old stars die out, new ones are lit to replace them.

According to modern scientific concepts, the elements necessary for the emergence of planets and life on Earth were formed from stellar matter. Although there is no single generally accepted point of view on how life arose.