What determines the final stage of a star's evolution. Life cycle of stars

Stars, like people, can be newborn, young, old. Every moment, some stars die and others are formed. Usually the youngest of them are similar to the Sun. They are in the process of formation and are actually protostars. Astronomers call them T-Tauri stars after their prototype. According to their properties - for example, luminosity - protostars are variable, since their existence has not yet entered a stable phase. Around many of them there is a large amount of matter. Powerful wind currents emanate from T-type stars.

Protostars: the beginning of the life cycle

If matter falls on the surface of a protostar, it quickly burns up and turns into heat. As a consequence, the temperature of the protostars is constantly increasing. When it rises so much that nuclear reactions start in the center of the star, the protostar acquires the status of an ordinary one. With the onset of nuclear reactions, a star acquires a constant source of energy, which supports its vital activity for a long time. How long the life cycle of a star in the Universe will be depends on its original size. However, it is believed that stars with a diameter of the Sun have enough energy to exist comfortably for about 10 billion years. Despite this, it also happens that even more massive stars live for only a few million years. This is due to the fact that they burn their fuel much faster.

Normal size stars

Each of the stars are clumps of hot gas. In their depths, the process of generating nuclear energy is constantly taking place. However, not all stars are like the Sun. One of the main differences is color. Stars are not only yellow, but also bluish, reddish.

Brightness and luminosity

They also differ in such characteristics as shine and brightness. How bright a star observed from the Earth's surface turns out to be depends not only on its luminosity, but also on its distance from our planet. Given their distance to Earth, stars can have very different brightness. This indicator ranges from one ten-thousandth of the brightness of the Sun to a brightness comparable to more than a million Suns.

Most of the stars are in the lower end of this spectrum, being faint. In many ways, the Sun is an average, typical star. However, compared to others, it has a much higher brightness. A large number of faint stars can be observed even with the naked eye. The reason stars differ in brightness is because of their mass. Color, luster and change in brightness over time are determined by the amount of substance.

Attempts to explain the life cycle of stars

People have long tried to trace the life of stars, but the first attempts of scientists were rather timid. The first achievement was the application of Lane's law to the Helmholtz-Kelvin hypothesis of gravitational contraction. This brought a new understanding to astronomy: theoretically, the temperature of a star should increase (its rate is inversely proportional to the radius of the star) until the increase in density slows down the compression processes. Then the energy consumption will be higher than its arrival. At this moment, the star will begin to cool down rapidly.

Hypotheses about the life of stars

One of the original hypotheses about the life cycle of a star was proposed by the astronomer Norman Lockier. He believed that stars arise from meteoric matter. In this case, the provisions of his hypothesis were based not only on theoretical conclusions available in astronomy, but also on the data of spectral analysis of stars. Lockyer was convinced that the chemical elements that take part in the evolution of celestial bodies are composed of elementary particles - "protoelements". Unlike modern neutrons, protons and electrons, they do not have a general, but individual character. For example, according to Lockyer, hydrogen decomposes into the so-called "protohydrogen"; iron becomes "proto-iron". Other astronomers have also tried to describe the life cycle of a star, for example, James Hopwood, Yakov Zeldovich, Fred Hoyle.

Giant and dwarf stars

Bigger stars are the hottest and brightest. They are usually white or bluish in appearance. Despite the fact that they are gigantic in size, the fuel inside them burns up so quickly that they are deprived of it in just a few million years.

Small stars, as opposed to giant ones, are usually not so bright. They have a red color, live long enough - for billions of years. But among the bright stars in the sky, there are also red and orange ones. An example is the star Aldebaran - the so-called "bull's eye", located in the constellation Taurus; and also in the constellation Scorpio. Why are these cool stars able to compete in brightness with hot stars like Sirius?

This is due to the fact that they once expanded very strongly, and in their diameter began to surpass huge red stars (supergiants). The huge area allows these stars to emit an order of magnitude more energy than the Sun. This is despite the fact that their temperatures are much lower. For example, the diameter of Betelgeuse, located in the constellation Orion, is several hundred times larger than the diameter of the Sun. And the diameter of ordinary red stars is usually less than a tenth of the size of the Sun. Such stars are called dwarfs. Each celestial body can go through these types of life cycle of stars - one and the same star at different intervals of its life can be both a red giant and a dwarf.

As a rule, luminaries like the Sun maintain their existence due to the hydrogen inside. It turns into helium inside the star's nuclear core. The sun has a huge amount of fuel, but even it is not infinite - over the past five billion years, half of its supply has been used up.

The lifetime of the stars. Life cycle of stars

After the hydrogen reserves inside the star are depleted, major changes come. The remaining hydrogen begins to burn not inside its core, but on the surface. In this case, the lifetime of the star is increasingly shrinking. The cycle of stars, at least of most of them, in this segment passes into the stage of the red giant. The size of the star becomes larger, while its temperature, on the contrary, is lower. This is how most red giants appear, as well as supergiants. This process is part of the general sequence of changes occurring with the stars, which scientists have called the evolution of stars. The life cycle of a star includes all its stages: ultimately, all stars age and die, and the duration of their existence is directly determined by the amount of fuel. Big stars end their lives in a huge, spectacular explosion. The more modest ones, on the contrary, die, gradually shrinking to the size of white dwarfs. Then they just fade away.

How long does the average star live? The life cycle of a star can last from less than 1.5 million years to 1 billion years or more. All this, as has been said, depends on its composition and size. Stars like the Sun live from 10 to 16 billion years. Very bright stars like Sirius have a relatively short life span - only a few hundred million years. The life cycle of a star includes the following stages. This molecular cloud - the gravitational collapse of the cloud - the birth of a supernova - the evolution of the protostar - the end of the protostellar phase. Then the stages follow: the beginning of the young star stage - the middle of life - maturity - the red giant stage - the planetary nebula - the white dwarf stage. The last two phases are characteristic of small stars.

The nature of planetary nebulae

So, we briefly reviewed the life cycle of a star. But what is it? Transforming from a huge red giant into a white dwarf, sometimes stars shed their outer layers, and then the star's core becomes exposed. The shell of gas begins to glow under the influence of the energy emitted by the star. This stage got its name due to the fact that luminous gas bubbles in this shell often resemble disks around planets. But in fact, they have nothing to do with the planets. The life cycle of stars for children may not include all the scientific details. One can only describe the main phases of the evolution of celestial bodies.

Star clusters

Astronomers love to explore. There is a hypothesis that all luminaries are born in groups, and not singly. Since stars belonging to the same cluster have similar properties, the differences between them are true, and not due to the distance to the Earth. Whatever changes are accounted for by these stars, they originate at the same time and under equal conditions. Especially a lot of knowledge can be obtained by studying the dependence of their properties on mass. After all, the age of the stars in the clusters and their distance from the Earth are approximately equal, so they differ only in this indicator. The clusters will be of interest not only to professional astronomers - every amateur will be happy to take a beautiful photo, admire their exceptionally beautiful view in the planetarium.

Formed by condensation of the interstellar medium. Through observations, it was possible to determine that the stars arose at different times and arise to this day.

The main problem in the evolution of stars is the question of the origin of their energy, thanks to which they glow and emit a huge amount of energy. Previously, many theories were put forward that were designed to identify the sources of energy in stars. It was believed that the continuous source of stellar energy is continuous compression. This source is, of course, good, but it cannot maintain adequate radiation for a long time. In the middle of the 20th century, the answer to this question was found. The source of radiation is thermonuclear fusion reactions. As a result of these reactions, hydrogen turns into helium, and the released energy passes through the interior of the star, transforms and radiates into space (it should be noted that the higher the temperature, the faster these reactions proceed; that is why hot massive stars leave the main sequence faster).

Now imagine the emergence of a star ...

A cloud of interstellar gas-dust medium began to condense. A rather dense ball of gas is formed from this cloud. The pressure inside the ball is not yet able to balance the forces of gravity, so it will shrink (perhaps at this time clots with a smaller mass are formed around the star, which eventually turn into planets). When compressed, the temperature rises. Thus, the star gradually settles on the main sequence. Then the pressure of the gas inside the star balances out the attraction and the protostar turns into a star.

The early stage of the evolution of the star is very small and the star at this time is immersed in a nebula, so the protostar is very difficult to detect.

The transformation of hydrogen into helium occurs only in the central regions of the star. In the outer layers, the hydrogen content remains practically unchanged. Since the amount of hydrogen is limited, sooner or later it burns out. The release of energy in the center of the star stops and the core of the star begins to shrink, and the envelope swells. Further, if the star is less than 1.2 times the mass of the sun, it sheds the outer layer (the formation of a planetary nebula).

After the shell is separated from the star, its inner very hot layers are opened, and the shell, meanwhile, moves away further and further. After several tens of thousands of years, the envelope will decay and only a very hot and dense star will remain, gradually cooling down it will turn into a white dwarf. Gradually cooling down, they turn into invisible black dwarfs. Black dwarfs are very dense and cold stars, slightly larger than Earth, but with a mass comparable to that of the sun. The cooling process of white dwarfs takes several hundred million years.

If the mass of a star is from 1.2 to 2.5 solar masses, then such a star will explode. This explosion is called supernova... An outburst star increases its luminosity hundreds of millions of times in a few seconds. Such outbreaks are extremely rare. In our Galaxy, a supernova explosion occurs approximately once every hundred years. After such an outburst, a nebula remains, which has a large radio emission, and also very quickly scatters, and the so-called neutron star (more on this later). In addition to the huge radio emission, such a nebula will still be a source of X-ray radiation, but this radiation is absorbed by the earth's atmosphere, so it can only be observed from space.

There are several hypotheses about the cause of the explosions of stars (supernovae), but there is no generally accepted theory yet. There is an assumption that this is due to the too fast fall of the inner layers of the star towards the center. The star is rapidly compressed to a catastrophically small size of about 10 km, and its density in this state is 10 17 kg / m 3, which is close to the density of the atomic nucleus. This star consists of neutrons (while electrons, as it were, are pressed into protons), which is why it is called "NEUTRON"... Its initial temperature is about a billion kelvin, but in the future it will quickly cool down.

Due to its small size and rapid cooling, this star was considered impossible for observation for a long time. But after a while, pulsars were discovered. These pulsars turned out to be neutron stars. They are named so because of the short-term radiation of radio pulses. Those. the star "blinks" as it were. This discovery was made quite by accident and not so long ago, namely in 1967. These periodic impulses are due to the fact that during a very fast rotation, the cone of the magnetic axis constantly flashes past our gaze, which forms an angle with the axis of rotation.

The pulsar can be detected only for us under the conditions of orientation of the magnetic axis, and this is about 5% of their total number. Some of the pulsars are not located in radio nebulae, since the nebulae are scattering relatively quickly. After a hundred thousand years, these nebulae cease to be visible, and the age of the pulsars is estimated in tens of millions of years.

If the mass of a star exceeds 2.5 solar masses, then at the end of its existence it will, as it were, collapse into itself and be crushed by its own weight. In a matter of seconds, it will turn into a point. This phenomenon was called "gravitational collapse", and this object was also called "black hole".

From all of the above, it can be seen that the final stage of the evolution of a star depends on its mass, but it is also necessary to take into account the inevitable loss of this very mass and rotation.

> Life cycle of a star

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

Everything in this world is developing. Any cycle begins with birth, growth and ends with death. Of course, in stars, these cycles take place in a special way. Let's just remember that their time frames are more ambitious and are measured in millions and billions of years. In addition, their death has 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 safely exist in the Universe without any changes. But suddenly a supernova explodes not far from it, or it hits another cloud. Due to this push, the destruction process is activated. It is divided into small parts, each of which is drawn into itself. As you already understood, all these heaps are preparing to become stars. Gravity heats up the temperature, and the stored momentum supports the rotation process. The lower diagram clearly demonstrates the cycle of stars (life, stages of development, transformation options and death of a celestial body with a photo).

Second life cycle of a star: Protostar

The material thickens 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. Part is attracted to the object, increasing its mass. The rest of the debris will group and create a 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, a tremendous amount of energy is released. The new stellar stage was named after the prototype - T Taurus. It is a variable star located 600 light years away.

It can become very bright 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 a celestial body rises to the required level, activating nuclear fusion. All the stars go through this. Hydrogen is transformed into helium, releasing a huge amount of heat and energy.

Energy is released as gamma rays, but due to the slow motion of the star, it falls off with wavelength. Light is pushed outward and comes into confrontation with gravity. It can be considered that perfect balance is being created here.

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

Fifth life cycle of a star: Red giant

During the melting process, hydrogen ends up, and helium accumulates. When there is no hydrogen left at all, all vigorous reactions freeze, and the star begins to shrink due to gravity. The hydrogen shell around the core heats up and ignites, causing the object to grow 1,000 to 10,000 times larger. At a certain moment, our Sun will repeat this fate, increasing to the earth's orbit.

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

Sixth life cycle of a star: White dwarf

A star with a solar mass does not 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.

Like any bodies in nature, stars also cannot remain unchanged. They are born, develop and, finally, "die". The evolution of stars takes billions of years, but there is debate about the time of their formation. Previously, astronomers believed that the process of their "birth" from stardust took millions of years, but not so long ago, photographs of a region of the sky from the Great Orion Nebula were obtained. Over the course of several years, a small

In the 1947 images, 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 disintegrated into separate ones. So for the first time, the process of birth of stars took place literally in front of astronomers.

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

Traditionally, scientists have assumed 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 sphere, dense in structure, is formed from the formed clouds. 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 the 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 stars presupposes a very high temperature in their interior, which contributes to continuous thermonuclear processes (the hydrogen that forms them turns into helium). It is these processes that cause the intense radiation of stars. The time it takes for them to 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 energy release ceases, gravitational forces begin to compress the core. 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 contracting helium nucleus and the transformation of helium nuclei into carbon nuclei.

Our Sun is predicted to turn into a red giant in eight billion years. In this case, its radius will increase by several tens of times, and the luminosity will increase by hundreds of times in comparison with the current indicators.

The lifespan of a star, as already noted, depends on its mass. Objects with a mass that is less than the sun, very economically "spend" their reserves, therefore, 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 from it.

Giant stars tend to quickly deplete their nuclear fuel supply. This is accompanied by a significant loss of mass, in particular due to the ejection of the outer shells. As a result, only the gradually cooling central part remains, in which nuclear reactions have completely stopped. Over time, such stars cease their radiation and become invisible.

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

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

Star evolution begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in a galaxy actually contains 0.1 to 1 molecule per cm 3. The molecular cloud has a density of about a million molecules per cm 3. 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.

Star evolution begins in a giant molecular cloud, also called a stellar cradle.

As long as the cloud orbits freely around the center of its home 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 a collision of two clouds. Another event causing the collapse could be the passage of a cloud through the dense arm of a spiral galaxy. Also, a critical factor may be the explosion of a nearby supernova, the shock wave of which will collide with a 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 collapse in the collision. In general, any discontinuities in the forces acting on the mass of the cloud can trigger the star formation process.

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

In the course of this process, the inhomogeneities of the molecular cloud will contract under the action of their own gravity and gradually take the shape of a ball. When compressed, gravitational energy turns into heat, and the object's temperature rises.

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 are almost entirely dependent 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 in its core runs out. When in the center of the star all the hydrogen turns into helium, a helium core is formed, and the thermonuclear combustion of hydrogen continues at the periphery of the core.

Small and cold red dwarfs slowly burn up hydrogen reserves and remain on the main sequence for tens of billions of years, while massive supergiants leave the main sequence within tens of millions (and some after 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 depths. Since the age of the universe is 13.8 billion years, which is not enough to deplete the supply of hydrogen fuel in such stars, modern theories are based on computer simulations of the processes occurring in such stars.

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

Medium-sized stars like the Sun remain 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 a star depletes its core hydrogen supply, it leaves the main sequence.

As soon as a star depletes its core hydrogen supply, it leaves the main sequence.

Without the pressure that arises in the course of thermonuclear reactions and balances the internal gravity, the star begins to shrink again, as it was earlier in the process of its formation.

Temperature and pressure rise again, but unlike the protostar stage, to much higher levels.

The collapse continues until, at a temperature of about 100 million K, thermonuclear reactions with the participation of 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 about 100 million K, thermonuclear reactions with the participation of helium begin

The thermonuclear “burning” of matter, renewed at a new level, becomes the cause of the monstrous expansion of the star. The star "swells", becoming very "loose", and its size increases by about 100 times.

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

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

In medium stars, the reaction of thermonuclear combustion of helium can lead to an explosive ejection of the outer layers of the star with the formation of planetary nebula... The core of a star, in which thermonuclear reactions stop, while cooling down, 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 Earth's diameter.

For massive and supermassive stars (with a mass of five solar masses or more), the processes occurring in their cores as gravitational compression increases, lead to an explosion supernova with the release of tremendous energy. The explosion is accompanied by the ejection of a significant mass of stellar matter into interstellar space. This substance further participates 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 remaining after the explosion can end its evolution as a neutron star (pulsar) if the mass of the star in its later stages exceeds the Chandrasekhar limit (1.44 solar masses), or as a black hole if the star's mass 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 fade away, new ones light up 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 still no single generally accepted point of view on how life originated.