How stars are born

“How are stars born and how do they die?
Scientists want to know these secrets.”

((Motto of the work presented in 1958 at the competition of the German Society of Naturalists and Physicians and awarded a prize.))

We followed the life of a star from the ignition of hydrogen in its youth to its gray old age. But what happened even earlier? Where do the stars whose fate we observed come from? How do they arise?

Since the lifetime of stars is limited, they must arise in a finite time. How could we learn something about this process? Is it possible to see stars forming in the sky? Are we witnessing their birth? Hundreds of billions of stars form the flat spiral of our Galaxy; Are there any clues here about how stars are formed?

Stars are born today

The key to the solution is provided by facts already known to us. We have seen that massive stars, more than ten solar masses, age rapidly. They frivolously waste their hydrogen and leave the main sequence. Therefore, when we observe a massive main sequence star, we know that it cannot be old. Such a star is distinguished by great brightness: due to the very high surface temperature, it glows blue. Thus, blue bright stars are still young - their age does not exceed a million years. This, of course, is very short compared to the billions of years during which our Sun shines. So, anyone who wants to find where stars are born in the Universe must use the bright blue main sequence stars as their guide. If you find a place where stars have recently formed, it may happen that stars are still being born there today.

In the sky you can find entire clusters of bright blue stars. Why are they wonderful for us? Regions are discovered in which the density of young stars is high - they are located among old stars, but there are still more of them here than anywhere else. It seems that not so long ago new stars arose among the old stars, which are now slowly mixing with their surroundings. While the stars in clusters are located close to each other and do not move apart, held by the force of mutual attraction, these young stars soon “scatter” and “lose sight of each other.” These so-called stellar associations attracted the attention of the Soviet astronomer V. A. Ambartsumyan. Can they tell us how stars are formed? Dense gas and dust accumulations can be seen between the stars here. An example is the Orion Nebula (Fig. 12.1). There are many bright blue stars here, less than a million years old. In the constellation Sagittarius, young stars are hidden by dense dust clouds. Only with observations in the long-wave infrared range were Hans Elsösser and his colleagues from the Spanish-German Observatory in Calar Alto able to take pictures through clouds of dust and study emerging stars for the first time.

Rice. 12.1. Luminous Orion Nebula. In a region about 15 light-years across, the interstellar gas is highly compacted; one cubic centimeter contains up to 10,000 hydrogen atoms. Although this is a very high density by interstellar standards, the gas rarefaction here is much higher than in the best vacuum installations on Earth. The entire mass of the luminous gas is approximately 700 solar. The glow of gas in the nebula is excited by the light of bright blue stars. The Orion Nebula contains stars less than a million years old. The presence of compactions suggests that star formation continues here to this day. The light from the nebula that we receive today was actually emitted by the nebula during the Great Migration. (Photo by US Naval Observatory, Washington.)

We already know that the space between stars is not completely empty: it is filled with gas and dust. The gas density is approximately one hydrogen atom per cubic centimeter, and its temperature corresponds to minus 170 degrees Celsius. Interstellar dust is much colder (minus 260 degrees Celsius). But where there are young stars, the situation is different. Dark dust clouds block the light of the stars behind them. Gas clouds glow: here their density is tens of thousands of atoms per cubic centimeter, and the radiation from nearby young stars heats them up to 10,000 degrees Celsius. In the radio range, one can observe the characteristic frequencies of radiation from complex molecules: alcohol, formic acid. The concentration of interstellar matter in these regions suggests that stars form from interstellar gas.

This is also supported by considerations first expressed by the English astrophysicist James Jeans, a contemporary of Eddington. Let's imagine space filled with interstellar gas. From the side of each of the atoms, the gravitational force of attraction acts on the others, and the gas tends to compress. This is mainly prevented by gas pressure. The equilibrium here is exactly like that found inside stars, where gravitational forces are balanced by gas pressure. Let's take a certain amount of interstellar gas and mentally compress it. When compressed, the atoms come closer together and the force of attraction increases. However, gas pressure increases faster and the compressed gas tends to return to its previous state. The equilibrium of interstellar gas is said to be stable. However, Gine showed that a stable equilibrium can be disrupted. If a sufficiently large amount of matter is compressed at the same time, then gravitational forces can increase faster than gas pressure, and the cloud will begin to compress on its own. For this process to occur under the influence of the cloud's own gravitational forces, a very large amount of matter is needed: at least 10,000 solar masses of interstellar matter are required for instability to develop. This is probably why young stars are always observed only in groups: they are most likely born in large groups. When 10,000 solar masses of interstellar gas and dust begin to compress at an ever-increasing rate, individual condensations appear to form, which further compress themselves. And each such compaction becomes a separate star.

Computer model of star birth

The process of star birth was described in his doctoral dissertation prepared at the California Institute of Technology by the young Canadian astrophysicist Richard Larson in 1969. His dissertation has become a classic of modern astrophysical literature. Larson investigated the formation of a single star from interstellar matter. The solutions he obtained describe in detail the fate of an individual gas cloud.

Larson looked at a spherical cloud with a mass equal to one solar mass, and, using a computer, observed its further development with such precision as was only then possible. The cloud he took was itself a condensation, a fragment of a large collapsing volume of the interstellar medium. Accordingly, its density was higher than the density of interstellar gas: one cubic centimeter contained 60,000 hydrogen atoms. The diameter of the original Larson cloud was 5 million solar radii. The Sun was formed from this cloud, and this process, on an astrophysical scale, takes a very short time: only 500,000 years.

At first the gas is transparent. Each particle of dust constantly emits light and heat, and this radiation is not delayed by the surrounding gas, but goes freely into space. This is the original transparent model; the further fate of the gas ball is as follows: the gas falls freely towards the center; Accordingly, matter accumulates in the central region. An initially homogeneous gas ball forms a core with a higher density in the center, which further increases (Fig. 12.2). The acceleration of gravity near the center becomes greater, and the speed of falling matter increases most strongly near the center. Almost all hydrogen goes into molecular form: hydrogen atoms are bonded in pairs into strong molecules. At this time, the gas temperature is low and does not yet increase. The gas is still so rarefied that all the radiation passes through it to the outside and does not heat up the collapsing ball. Only after a few hundred thousand years does the density in the center increase to such an extent that the gas becomes opaque to the heat-carrying radiation. As a result, a hot core (the radius of which is approximately 1/250 of the original radius of the ball) is formed in the center of our large gas ball, surrounded by falling matter. As the temperature increases, the pressure also increases, and at some point the compression stops. The radius of the compaction region is approximately equal to the radius of Jupiter's orbit; At this time, approximately 0.5% of the mass of all matter participating in the process is concentrated in the core. Matter continues to fall onto the relatively small core. Falling matter carries energy, which when falling turns into radiation. The core contracts and heats up more and more.

Rice. 12.2. Larson's model of the formation of the Sun. The cloud of interstellar dust begins to shrink (a). At first, the density inside it is almost the same everywhere. After 390,000 years, the density in the center of the cloud increases 100 times (b). 423,000 years after the start of the process, a hot core appears in the center of the compaction, which initially stops compressing (c). The figure shows it on an enlarged scale. Its density is 10 million times higher than its original density. The bulk of the mass, however, as before, falls on the surrounding contracting cloud. After a short time, the hydrogen molecules in the core disintegrate into atoms, the core contracts again and a new core is formed, which has the size of the Sun (in the figure doubled) (d). Although at first its mass is small, eventually all the matter of the cloud passes to it. The core in the center heats up to such an extent that hydrogen thermonuclear reaction begins and it becomes a main sequence star with a mass equal to the Sun.

This continues until the temperature reaches approximately 2000 degrees. At this temperature, hydrogen molecules begin to break down into individual atoms. This process has important consequences for the nucleus. The nucleus begins to shrink again and contracts until the energy released turns all the hydrogen molecules into individual atoms. The new core is only slightly larger than our Sun. Remnants of surrounding matter fall onto this core, and it eventually forms a star with a mass equal to the Sun. From now on, only this core is of primary interest.

Because this core will eventually become a star, it is called a protostar. Its radiation is absorbed by matter falling on it; Density and temperature increase, atoms lose their electron shells - as they say, atoms become ionized. Not much can be seen from the outside yet. The protostar is surrounded by a dense shell of gas and dust masses falling on it, which does not allow visible radiation to pass out; it illuminates this shell from the inside. Only when the bulk of the shell's mass falls onto the core will the shell become transparent and we will see the light of the star. While the remnants of the shell fall onto the core, it contracts, and the temperature in its depths rises as a result. When the temperature in the center reaches 10 million degrees, thermonuclear combustion of hydrogen begins. A collapsing cloud, the mass of which is equal to the mass of the Sun, becomes a completely normal main sequence star; this is, so to speak, the ancestral Sun (the young Sun), the further history of which is described at the beginning of this book.

Towards the end of the protostar stage, even before the star reaches the main sequence, convective transfer of energy occurs in its depths to larger regions. Active mixing of solar matter occurs. This provides a clue to the solar lithium paradox discussed in Chap. 5. Atoms of this easily destroyed element are transported deeper into the hot zone, where they turn into helium atoms according to the reactions given in - this happens before the star becomes a main sequence star.

The birth of stars in nature

We became acquainted with Larson's solutions, which were obtained for an idealized problem that can be calculated on a computer. But does the described process correspond to reality? Is it actually realized in nature? Let's return to the sky, to where the stars appear - let's return to the bright, blue, and therefore young stars! We will look for traces of star formation, objects whose existence should be expected based on Larson's solutions.

Bright blue stars are very hot, with surface temperatures reaching 35,000 degrees. Accordingly, their radiation has very high energy. This radiation can strip electrons from hydrogen atoms in interstellar gas, leaving behind positively charged atomic nuclei. Hydrogen is ionized - bright massive stars ionize the surrounding gas masses. In our Galaxy, these regions reveal themselves by their glow, which occurs when ionized hydrogen atoms recapture electrons and emit light. Thermal radiation from these areas can also be detected in the radio range.

The advantage of measurements in the radio range is that radio signals are not distorted by absorbing dust masses. The best example of such participation in the sky, where the glow of interstellar matter is excited by bright massive stars, is again the Orion Nebula (see). Are there objects here that have any relation to the processes calculated by Larson? The lion's share of its life, the protostar is hidden under a dust shell, which slowly settles on it. Dust absorbs radiation from the core; at the same time, it heats up to several hundred degrees and radiates in accordance with this temperature. This thermal radiation should be observed in the IR range.

In 1967, Eric Böcklin and Jerry Neugebauer of the California Institute of Technology in Pasadena discovered an infrared star in the Orion Nebula, the luminosity of which was about 1000 times higher than the luminosity of the Sun, and the radiation temperature was 700 degrees. The diameter of the object was about 1000 times the diameter of the Sun. This is exactly what the gas and dust shell of a protostar should look like. Recently, it has become clear that in those areas of our Milky Way where the formation of new stars is most likely, there are compact sources emitting not only in the infrared, but also in the radio range. In the Orion Nebula, Bonn radio astronomer Peter Metzger and his colleagues discovered regions of high hydrogen density, from which particularly powerful radio emission emanates. In these areas, the concentration of free electrons separated from hydrogen atoms is a hundred times higher than in the surrounding space. Compared to the Orion Nebula, the size of the emitting object is extremely small: it is estimated to be 500,000 times the diameter of the Sun, about four times smaller than the diameter of the cloud falling on the core in Larson's model.

In addition, small objects have been discovered in the Orion Nebula, from which molecular radiation emanates, primarily the radiation of water molecules. Molecules emit radio waves, and this radiation can be received by radio telescopes. It turns out that the spatial dimensions of these objects are only 1000 times the diameter of the Sun. Let us remember that Larson's initial cloud diameter was several million solar radii! Thus, the molecular radiation should apparently come from the core of the protostar.

Of course, one must be careful in interpretations of this kind. We can only say with certainty that in the Orion Nebula there are objects that, without revealing themselves in visible light, have a very significant concentration of gas and dust, which exactly corresponds to the clouds in Larson’s model.

There is, however, other evidence that the observed sources of infrared and radio emission are indeed protostars. Recently, at our institute, a group of Austrian astronomer Werner Charnuter repeated the calculations of the Larson model using improved methods. In particular, the processes associated with the occurrence of IR radiation were calculated. The coincidence with observations turned out to be striking: everything suggests that we are really observing protostars simulated on a computer.

Since we are so close to understanding the origin of stars, we can ask whether this model will be able to explain the formation of all 100 billion stars in our Galaxy. In Fig. Figure 12.3 schematically shows the structure of our star system. Not all stars lie in the same plane: the oldest stars are distributed in a nearly spherical region of space called a halo. The halo stars are very old, as can be deduced from the G-P diagram for the globular clusters present here. Compared to our Sun, they are chemically poorer in elements heavier than helium, often by more than ten times. All young stars are located in the galactic plane and contain more heavy elements. Although elements heavier than helium account for only a small percentage of their mass, they give us the key to the secret of the origin of our Galaxy. Hydrogen and helium have been here since the beginning of the world - these are, so to speak, God-given elements. Heavier elements should have arisen later in the interior of stars and during supernova explosions. Thus, the chemical differences between galactic halo stars and galactic plane stars are associated with nuclear reactions occurring within the stars.

Rice. 12.3. Diagram of the structure of the Milky Way. Most stars are located in a flat disk (in the figure we are looking at it from the side). The arrow indicates the position of the Sun, the light stripe in the middle depicts absorbing dust masses. Globular clusters (bold dots) and very old stars (small dots) form the halo of the Milky Way. These stars have been around for a very long time. Stars being born today are found only in close proximity to dust masses in the central plane of the Galaxy.

Momentum and collapsing clouds

The description of the physical world is significantly simplified with the introduction of a number of “conservation laws”. In everyday life, we use them every now and then, sometimes without realizing it. From school we remember the laws of conservation of mass and energy; We encounter these laws every day. Less obvious, perhaps, is the fact that the angular momentum (angular momentum, angular momentum) of a rotating body, left to its own devices, cannot simply disappear. However, a clear example of the operation of this conservation law is well known to everyone. When a figure skater pirouettes on the ice, she spins slowly at first with her arms extended to the sides. When she bends her arms, the rotation accelerates without any external effort. This occurs due to the law of conservation of angular momentum. The same thing, although not as exciting, is observed when a cloud of interstellar gas rotates. Let the cloud first make one full revolution in 10 million years. When it shrinks to one-tenth of its original diameter, it will spin a hundred times faster, completing a full revolution in one hundred thousand years. As the cloud shrinks further, it will spin even faster. Roughly speaking, the product of the number of revolutions of a cloud per unit time and its surface area (which can be approximately considered spherical) remains constant during collapse. Thus, the smaller the cloud, the faster it rotates.

At the same time, the centrifugal force acting along the equatorial plane against gravity becomes increasingly significant. The collapsing cloud is flattened. This affects the formation of individual stars; This also applies to the formation of our Milky Way.

The history of the Milky Way, reconstructed from its traces

We don't know where it came from. Once upon a time, matter that arose at the beginning of the world and rushed through space formed a cloud of several billion solar masses and began to become denser. Like any substance, this gas, released from the turbulent mass, acquired rotational motion. Gradually the cloud contracted and became denser; Separate areas emerged in it, turning into small, independently condensing gas clouds. The first stars appeared. They consisted only of hydrogen and helium, and thermonuclear combustion of hydrogen took place in them (the reaction of combining two protons). Pretty soon, the most massive stars used up their supply of hydrogen and exploded, becoming supernovae. As a result, the interstellar gas became enriched in elements heavier than helium. This happened everywhere, since the entire galactic cloud still had a spherical shape (Fig. 12.4, a). Therefore, the oldest stars and very old globular clusters are found in the galactic halo. The stars of the galactic halo appeared first, long before the Milky Way took on the shape of a disk, long before our Sun appeared. They contain heavy elements in very small quantities: these stars arose from matter that was still poorly enriched in atoms formed as a result of nuclear reactions in other stars.

Rice. 12.4. Diagram of the formation of the Milky Way. About 10 billion years ago, a cloud formed from primordial matter, which began to become denser due to its own gravity. With increasing density, the first stars (dots) and globular clusters (thick dots) formed (a). Even today they fill the spherical region in which they originated and move relative to the center along the trajectories shown by red arrows (b). Massive stars quickly went through their entire development path and released matter enriched in heavy elements back into the interstellar gas. Stars, already rich in heavy elements, began to form. Due to the rotation, the compacted gas formed a disk. Here, to this day, stars appear (c). This diagram explains the spatial structure of our Galaxy and the chemical differences between the peripheral stars and the stars in the center.

But evolution went further. Interstellar gas was constantly enriched with heavy elements. Dust grains arose in it as a result of collisions of gas particles with condensation nuclei ejected by developing stars. Soon the rotation acquired a noticeable speed. All condensing gas and dust masses took the form of a flat disk, leaving behind a spherical halo of old stars and globular clusters (). New stars were now formed only in an increasingly flattened, lenticular-shaped region from matter containing ever-increasing amounts of heavy elements. Most of the gas had already been consumed, and the last stars were forming in the galactic plane. The first phase of star formation has ended.

This picture explains the basic properties of our Galaxy: the oldest stars belong to a spherical halo and are poor in heavy elements. The youngest stars form today only in the thin disk, since only here there is still a sufficient amount of gas left.

The angular momentum inherited from the cloud from which our Galaxy was formed is responsible for the fact that our star system has the shape of a flat disk. This is why we see our Milky Way in the sky as a narrow strip.

Who commands the formation of stars?

What causes interstellar matter to condense today in certain places in the plane of our Milky Way and form stars? Why don't stars form in other places in our Galaxy? The Milky Way, when viewed from space, would look similar to the Andromeda Nebula: a flat disk with a pronounced spiral structure (see). In other star systems, the spiral structure appears even more clearly (see). In photographs of distant galaxies, spiral arms stand out because they glow from ionized hydrogen. As we already know from the example of the Orion Nebula, bright, massive main sequence stars are responsible for the ionization of hydrogen. Thus, spiral arms are regions where there are young stars, that is, regions where stars have just arisen. And in our Galaxy, young stars line up along the spiral arms.

With the help of radio astronomy, it is possible to study in great detail the distribution of interstellar gas in our Milky Way; It is discovered that in the spiral arms the gas density is higher than in general in the plane of the Galaxy. So, it is given: on the one hand, spiral arms are regions of increased gas density, on the other hand, this is where young stars are located. The question arises: what is responsible for the spiral structure that makes galaxies look like fiery wheels of fireworks?

For a long time, attempts to explain spiral structures encountered great difficulties, and even now their occurrence cannot be considered completely clear. The star system is rotating. The speed of its rotation can be measured (see); it turns out that the system does not rotate like a rigid body. The rotation speed decreases towards the periphery, so that the central part of the galaxy rotates faster.

At first glance, it is not surprising that galaxies exhibit a spiral structure. Spiral structures also appear when stirring coffee with milk in a cup, since at different distances from the center the liquid rotates at different speeds. One would expect that any initial structure of the galaxy would become spiral after some time due to the difference in the speed of rotation at different distances from the center.

Carl Friedrich von Weizsäcker once said that the Milky Way today would have to have a spiral structure, even if it once looked like a cow. Many years ago in Göttingen we took up Weizsäcker's galactic cow; Alfred Baer, ​​who until recently taught in Hamburg, helped us. The result is shown in Fig. 12.5. Even before the bulk of the stars complete their first revolution around the center, the cow galaxy will turn into a beautiful spiral. Unfortunately, there is one problem here.

Rice. 12.5. The Milky Way does not rotate like a rigid body. Therefore, from an arbitrary initial structure, a spiral object is formed after 100 million years. Unfortunately, the spiral arms of our Galaxy defy such an explanation.

It takes less than a hundred million years for our arbitrary initial structure to form a spiral. Our Milky Way is a hundred times older. During this time, the spiral would have to stretch much more: like the grooves on a long-playing record, the threads of the spiral would have to wrap around the center a hundred times or more. But we don't see this. The spiral arms of the galaxy, as seen in , did not stretch into threads, and, therefore, cannot be the remnants of some original structure. Since none of the observed spiral galaxies have a filamentary spiral structure, we must accept that the spiral is not elongated. At the same time, spiral arms consist of stars and gas that participate in rotational motion. How to resolve this contradiction?

There is only one way out. We should abandon the assumption that matter always belongs to the same arms of the spiral, and assume that there is a flow of stars and gas through the arms of the spiral structure. Although stars and gas participate in rotational motion, the arms of the spiral themselves represent only certain states that accept the flow of stars and gas.

Let us illustrate this with an example from everyday experience. The flame of a gas burner does not consist of the same substance. It represents only a certain state of the gas flow: here gas molecules enter into certain chemical reactions. In the same way, spiral arms are regions of the galactic disk in which the flow of stars and gas has a certain state. This state is determined by the peculiarities of the gravitational forces of the matter of the entire galaxy. Let's explain this in more detail.

Spiral arms: what are they?

In nature, jet streams often give rise to regular formations. The interaction of water and wind generates surf waves that rhythmically roll onto the shore. Sandy seabanks run in wavy folds. When liquids of different temperatures and densities are carefully mixed, regular structures can also arise. A regular pattern is observed on the surface of the cooled cocoa in the cup.

Stars orbiting in the plane of the galaxy around a common center and being at the mercy of gravitational attraction and centrifugal force also exhibit a tendency to form structures.

Let us imagine a large number of stars forming a rotating disk. At each point on the disk, centrifugal force and gravity are mutually balanced. This equilibrium is, generally speaking, unstable. If somewhere the density of stars is higher, then they tend to come closer together, like particles of interstellar gas that has become unstable during the formation of stars. However, centrifugal force also plays an important role, and this complicates the process. The situation under consideration can be simulated on a computer. In Fig. Figure 12.6 shows the solution obtained for a rotating disk consisting of 200,000 stars. Long spiral regions of increased density of stars form completely independently: stars form spiral arms! The sleeves, however, do not stretch into threads, since they are not composed of the same stars. A stream of stars flows through the sleeves. When the stars move in their circular orbits, when they fall into the arms, they come closer together. As stars emerge from the arms, the distance between them increases. Thus, spiral arms are areas where stars come closer together, just as a burner flame is an area where gas molecules undergo chemical reactions.

Rice. 12.6. A simplified computer model of the motion of stars in our Galaxy. 200,000 stars move relative to the center of a flat disk, we look at it from above. The numbers under the pictures indicate the number of revolutions that the system has made. It can be seen that the spiral structure is formed very quickly. The interpenetration of the spirals, i.e. the fact that at each moment they consist of different stars, can be seen in the example of the upper arm in pictures 4.5 and 5.5. The arm shifted slightly, but during this time the stars made a full revolution around the center. The solution given here was obtained by American astronomer Frank Hall at NASA Langley Center (Hampton, Virginia, USA).

Spiral arms are regions where the density of stars is higher than elsewhere in the galactic disk. This is clearly visible on, but in a normal galaxy the changes in density are so small that they cannot be observed directly. However, along with the density of stars, the density of interstellar gas, which participates together with the stars in rotational motion, also changes: passing through the spiral arms, the gas becomes denser. As a result of this compaction, the conditions necessary for the formation of stars arise. This is why stars form in spiral arms. Among them there are also massive stars. These bright blue stars excite the glow of the surrounding gas. It is the glowing clouds of ionized hydrogen that create the remarkable spectacle of the spiral arms, not the more closely packed stars.

We have already become acquainted with the galaxy in the constellation Canes Venatici (see). Here we learn even more about star formation in spiral arms. We look at this system from afar: it shines through the nearby stars of our own Galaxy. The light from it travels for twelve million years before it reaches our telescopes. Since we see this galaxy, so to speak, from above, perpendicular to its plane, its spiral arms can be distinguished especially well.

Star formation in the galaxy in the constellation Canes Venatici

Radio emission comes from this galaxy to us. Fast-moving electrons, which have gained enormous speed, apparently as a result of supernova explosions, fly through the star system, emitting radio waves as they do so. These radio waves are received by sensitive radio telescopes. It is even possible to determine from which areas of the galaxy the radiation is stronger and from which it is weaker. In 1971, radio astronomers Donald Mathewson, Piet van der Kruyt and Wim Brouw in Holland obtained a radio image of this galaxy (Fig. 12.7). In this image, the intensity of radio emission is transmitted by areas of different densities: the stronger the radio emission, the lighter the area of ​​the image. Although the radio telescope does not produce as sharp a picture as an optical telescope, the spiral structure is clearly visible in the image. Thus, the spiral arms emit not only visible light, but also radio waves.

Rice. 12.7. A radio image of the galaxy shown in . In this computer image, the galaxy looks as we would see it if our eyes were sensitive to radio emission at a wavelength of 21 cm and, moreover, could “see” as well as the large radio telescope in Westerbork (Holland). Radio emission comes mainly from those regions where the density of interstellar gas is increased. It is also clear that the gas clouds in this galaxy have almost the same spiral structure as the distribution of young stars. (Photograph of the Leiden Observatory.)

Why is the radio emission created by electrons stronger in some places in the galaxy and weaker in others? This is due to the very mechanism of the occurrence of this radiation, the details of which we will not go into here. It is enough to point out that stronger radio emission occurs where the density of interstellar gas is higher. Thus, the radio image of the galaxy in the constellation Canes Venatici proves that in the spiral arms not only are the stars closer to each other, but also the interstellar gas has a higher density.

The Canes Venatici nebula shows us something else, too. It can be noted that the areas of maximum intensity of radio emission do not exactly coincide with the visible arms of the spiral (Fig. 12.8). The region of greatest density of interstellar gas is slightly shifted inward relative to the visible arm. What would that mean? Through the spiral arms there is a flow of stars and interstellar gas, and this flow crosses the arm so that it enters it from the “inner” (facing the center) side and exits from the outside. A comparison of the visible arm, illuminated by newborn stars, and the radio arm, corresponding to the region of maximum compression of interstellar gas, allows us to draw the following picture.

Rice. 12.8. Areas of maximum radio emission (schematically drawn with white lines), superimposed on an optical image of the galaxy in the constellation Canes Venatici. It can be seen that the spiral arms of maximum gas density and the spiral structures formed by young stars do not completely coincide. Thus, one should distinguish between density arms (radio arms) and visible arms of the galaxy.

Stars and interstellar matter revolve around the center of the galaxy (Fig. 12.9). Approaching the spiral arm, the stars come closer to each other, the gas becomes denser, and thus the conditions necessary for the emergence of new stars are created. Clouds of interstellar gas appear; they collapse and the first protostars appear. After some time, the stars and interstellar gas emerge from the region of maximum density (which corresponds to the arm in the radio image of the galaxy). But the process of star formation that began there continues, and after some time the first massive stars emerge from protostars. These bright blue stars excite the glow of the surrounding gas, and we see this as a visible spiral arm.

Rice. 12.9. Star formation in the galaxy in the constellation Canes Venatici. At the top right, the structure of the galaxy is schematically shown (cf.). The area marked with a dashed square is shown enlarged at the bottom of the figure. The matter of a galaxy rotating counterclockwise first passes through density arms (radio arms). In this case, the interstellar gas is compressed. Star formation begins. After some time, the first young stars appear, they illuminate the adjacent masses of gas, which produce visible radiation (visible arms of the galaxy). Since the gas has time to move from the moment of compaction to the moment of star formation, the radio arms and visible arms do not coincide with each other. This explains the situation shown in . The direction of movement of the substance is indicated by red arrows.

So, the substance first passes through a region of increased density. This is where the process of star formation begins. After some time, the first stars light up, and we observe a visible spiral arm. Since we know how fast the stars and gas in the galaxy in Canes Venatici are moving, and we can measure the distance between the radio arm and the visible arm of the galaxy, we can calculate the time it takes from the consolidation of interstellar gas to the appearance of the first stars: it is approximately six million years. In the last 500,000 years of those six million, a process of the type described by Larson's solutions has occurred. It takes five and a half million years for the interstellar matter to form the cloud that Larson based his model on.

Before galactic matter can make a complete revolution around the galactic center, the lifespan of massive stars expires. They return a significant part of their matter to the interstellar gas, and they themselves become white dwarfs or explode, forming supernovae. The matter entering the interstellar gas from them is enriched with atoms of heavy elements that arose in the bowels of stars, and the next time it passes through the spiral arm, it participates in the formation of new stars. Only matter contained in compact objects - white dwarfs or neutron stars, remaining after the death of stars, is excluded from this cycle of matter.

Once upon a time, long after the formation of the stars in the galactic halo, the material of our Sun in the form of interstellar gas passed through the spiral arm, and then many stars were formed. The more massive brothers of our Sun have long since ended their lives, while the less massive ones, like our Sun, during this time, due to uneven rotation in our Galaxy, scattered throughout the Galaxy and disappeared from view.

Notes:

Here and throughout this book, unless otherwise noted, we use the absolute temperature scale, the zero of which corresponds to -273° Celsius. To go from absolute temperature to temperature on the Celsius scale, you need to subtract 273 degrees. The surface temperature of the Sun in Celsius is therefore 5530°

These ideas belong to Isaac Newton! And Gine quotes him in his book. - Approx. Ed.

Osmosis is often called airless space, suggesting that it is empty. However, it is not. In interstellar space there is dust and gas (mainly helium and hydrogen, with much more of the latter). There are entire clouds of dust and gas in the Universe. Thanks to these clouds, we cannot see the center of our Galaxy. These clouds can be hundreds of light years in size, and parts of them can be compressed under the influence of gravity.

During the compression process, part of the cloud will become denser, decreasing in size and at the same time heating up. If the mass of a compressed substance is sufficient for nuclear reactions to begin to occur within it during the compression process, then such a cloud produces star.

It should be noted that usually a whole group is born from one cloud stars , which is usually called stellar cluster. In this cloud, separate compactions are formed (we will also call them clouds in the future), each of which can generate star. As mentioned, the easiest stars have a mass 12 times less than the Sun. If the collapsing cloud is less massive, but is not less massive than the Sun by more than a hundred times, such clouds form so-called brown dwarfs. Brown dwarfs are even cooler than red dwarfs stars. These objects are heated quite strongly by the forces of gravitational compression and emit a lot of heat (infrared radiation), but barely glow. But nuclear reactions do not start in brown dwarfs. In the end, gravitational compression is stopped by gas pressure from the inside, new portions of energy cease to be released, and brown dwarfs cool down in a relatively short time. One of the latest brown dwarfs to be discovered is a dwarf in the constellation Hydra, its magnitude is only 22.3, although it is only 33 light years away from the Sun. The uniqueness of this nearby brown dwarf lies in the fact that all previously discovered similar objects were part of binary systems, and this one is single. It is noticed only due to its proximity to Earth. The planet Jupiter, the largest in the solar system, is 80 times lighter than the lowest mass one stars and only 8-10 times lighter than brown dwarfs. Again we note the role of the mass of an object in its own fate.

If massive enough to form stars the cloud warms up so much that it begins to actively emit heat and, perhaps, faintly glow dark red (even before nuclear fusion begins), such a cloud is usually called a protostar(before- star). As soon as the temperature at the center of the protostar reaches 10,000,000 K, nuclear fusion begins. The compression of the protostar is stopped by light pressure, it becomes star. Again, the mass determines how quickly the protostar will turn into star. Stars type of the Sun spend on this stage of their birth 30,000,000 years, stars three times more massive - 100,000 years, and ten times less massive - 100,000,000 years. So, non-massive stars They do everything more slowly, and they are born and live. As we remember, to such easy to the stars include red stars, which are small in size and are called red dwarfs. Red dwarfs are ten times smaller than the Sun. Star type of the Sun is called a yellow dwarf, such stars are also relatively small. The heaviest and largest normal stars are called blue giants.

In young age star is still surrounded by its parent cloud, which in the form of a gas or gas-dust disk rotates around it. Wherein star wind - a stream of all kinds of particles escaping from the surface stars at high speeds, exerts pressure on the cloud substance, trying to push it away. Since the cloud has a flat disk shape, the movement of particles in its plane under pressure stellar wind is difficult. Matter rushes along the axis of rotation stars and clouds, in two opposite directions. There is little matter in these directions, and cloud particles almost unhindered rush away from stars. This is how the often observed outflows of matter from young stars.

When we hear the word star, we often imagine various celestial bodies visible in the sky. But not all of them are stars; they can be planets, groups of stars, or simply clouds of gas.

Star is a ball of gas. It glows due to its very high temperature. The temperatures of stars range from 2,100 to 50,000 degrees Celsius. The temperature of a star directly affects its color. This can be compared to hot metal that changes color depending on the temperature. The hottest stars appear blue.

The appearance of a star

Scientists have long tried to figure out how stars are formed. Stars can have different sizes. Many of its other characteristics, such as its temperature, color and life expectancy, depend on its size. Stars are made of cosmic dust and gas. Gravity forces compact these components. They increase their rotation speed and temperature, which leads to the formation of a protostar. When the gas at the core of a protostar heats up to 12,000,000 degrees, hydrogen inside it will begin to turn into helium. During this process, the protostar emits a lot of energy, as a result of which it stops contracting.

Life path

The energy emitted by a star makes it bright for many years. For example, a star similar to the Sun lives and shines for an average of 10 billion years. Larger stars have a shorter life span of only a few million years. This is due to the fact that the gas in their depths is processed faster. Stars smaller than our Sun produce less heat and light and live 50 billion years or more.

Groups of stars

In some cases, two or an entire group of stars are formed from the same source material in the form of gas and dust. They are called multiples. Scientists observing such stars noticed that sometimes the light of one star outshines another, and sometimes the light emitted by them is summed up.

• During the conversion of hydrogen into helium, a large amount of energy is released in the core of the star, which stops further compression of the star.
• The so-called Pleiades, groups of stars located quite far from the earth, can be perceived by the naked eye as a foggy spot.
• A star is born from a cloud of gas and dust. The force of gravity compacts this cloud. The temperature of the gas increases, which leads to the release of energy, in particular light.
• The temperature of the gas increases all the time, the light emitted by the star becomes brighter.
• Our sun is currently in the middle of its life path. According to scientists, there is enough gas in it to live for another 5 billion years.

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Stars are born when a cloud of interstellar gas and dust is compressed and compacted by its own gravity.
It is believed that this process leads to the formation of stars. Using optical telescopes, astronomers can see these zones, which look like dark spots against a bright background. They are called "giant molecular cloud complexes" because hydrogen is present in molecular form. These complexes, or systems, along with globular star clusters, are the largest structures in the galaxy, sometimes reaching 1,300 light-years in diameter.
Younger stars, called "stellar population I", were formed from the remnants resulting from the outbursts of older stars, they are called
"stellar population II". An explosive flare causes a shock wave that reaches the nearest nebula and provokes its compression.

Bok globules.

So, part of the nebula is compressed. Simultaneously with this process, the formation of dense dark round-shaped gas and dust clouds begins. They are called "Bock globules". Bok, an American astronomer of Dutch origin (1906-1983), was the first to describe globules. The mass of the globules is approximately
200 times the mass of our Sun.
As the Bok globule continues to condense, its mass increases, attracting matter from neighboring regions due to gravity. Due to the fact that the inner part of the globule condenses faster than the outer part, the globule begins to heat up and rotate. After several hundred thousand years, during which compression occurs, a protostar is formed.

Evolution of a protostar.

Due to the increase in mass, more and more matter is attracted to the center of the protostar. The energy released from the gas compressed inside is transformed into heat. The pressure, density and temperature of the protostar increase. Due to the increase in temperature, the star begins to glow dark red.
The protostar is very large, and although thermal energy is distributed over its entire surface, it still remains relatively cold. In the core, the temperature rises and reaches several million degrees Celsius. The rotation and round shape of the protostar change somewhat, it becomes flatter. This process lasts millions of years.
It is difficult to see young stars, since they are still surrounded by a dark dust cloud, due to which the brightness of the star is practically invisible. But they can be viewed using special infrared telescopes. The hot core of a protostar is surrounded by a rotating disk of matter with a strong gravitational force. The core gets so hot that it begins to eject matter from the two poles, where resistance is minimal. When these emissions collide with the interstellar medium, they slow down and disperse on either side, forming a teardrop-shaped or arched structure known as a Herbic-Haro object.

Star or planet?

The temperature of a protostar reaches several thousand degrees. Further developments depend on the dimensions of this celestial body; if the mass is small and is less than 10% of the mass of the Sun, this means that there are no conditions for nuclear reactions to occur. Such a protostar will not be able to turn into a real star.
Scientists have calculated that for a contracting celestial body to transform into a star, its minimum mass must be at least 0.08 of the mass of our Sun. A gas-containing cloud of smaller sizes, condensing, will gradually cool and turn into a transitional object, something between a star and a planet, this is the so-called “brown dwarf”.
The planet Jupiter is a celestial object too small to become a star. If it were larger, perhaps nuclear reactions would begin in its depths, and it, along with the Sun, would contribute to the emergence of a system of double stars.

Nuclear reactions.

If the mass of a protostar is large, it continues to condense under the influence of its own gravity. The pressure and temperature in the core increase, the temperature gradually reaches 10 million degrees. This is enough to combine hydrogen and helium atoms.
Next, the “nuclear reactor” of the protostar is activated, and it turns into an ordinary star. A strong wind is then released, which disperses the surrounding shell of dust. Light can then be seen emanating from the resulting star. This stage is called the "T-Taurus phase" and can last 30 million years. The formation of planets is possible from the remnants of gas and dust surrounding the star.
The birth of a new star can cause a shock wave. Having reached the nebula, it provokes the condensation of new matter, and the star formation process will continue through gas and dust clouds. Small stars are faint and cold, while large ones are hot and bright. For most of its existence, the star balances in the equilibrium stage.

Municipal budgetary educational institution "Gymnasium"

Abstract on the topic: How stars are formed

Completed by 4th grade student Wolf Vladislav

G. Chernogorsk, RH

1. Introduction
2. A star is born
3. Star Bonds
4. Birth of the earth
5. Sun
6. Moon
7. Constellations
8. Conclusion

Introduction

Just recently, my mother gave me the book “The Great Schoolchild’s Encyclopedia.” I was very happy. When I started studying it, I realized how fascinating and interesting it was in content. Including stories about space, about the solar system, about the birth of new stars or planets. I really liked it, and I decided to make a small report so that other children would know too.

How stars are formed

When people talk about stars, they usually mean all the luminous bodies that can be seen in the night sky. Many of them, however, are not stars, but planets, groups of stars, or simply clouds of gas.

A star is a ball of gas heated to such a temperature that it glows. The temperature of stars ranges from 2100*C to 50,000*C. The color of a star depends on its temperature. Imagine that a piece of metal is heated on a fire. First the metal turns bright red. Then it becomes white hot. White stars are hotter than red stars, but the hottest stars are blue.

A STAR IS BIRTH

For many years, scientists have been looking for an answer to the question of how stars are born. Stars come in different sizes. The lifespan of a star, brightness and other characteristics depend on its size. Stars are born from clouds of cosmic gas and dust. Under the influence of gravitational forces, the cloud becomes denser, its rotation speed and temperature gradually increase, and it turns into a protostar. When the temperature in the center of a protostar reaches approximately 12,000,000 * C, thermonuclear reactions begin in its depths, converting hydrogen and helium. In this case, such a huge amount of energy is released that the star stops contracting under the influence of its own gravitational forces. This is where star formation ends.

The released energy not only prevents the star from shrinking, but also makes it glow for a very long time. A star the size of our Sun can live about 10 billion years. Larger stars burn gas faster and only live a few million years. Stars smaller than the Sun and cooler can live more than 50 billion years.

STAR TIES

Sometimes two stars are born nearby from one rotating cloud of gas and dust. Moreover, newborns often differ in color and size and do not look like twins at all. They are connected by forces of mutual attraction and move in orbits, revolving around each other, just as the Moon revolves around the Earth. Such stars are called double stars. If there are more than two stars in a group, they are called multiples. Astronomers compare the brightness of such stars by observing them at different periods: when the light of one star outshines another or when their radiations are summed up.

There are the Pleiades - an open star cluster, which includes more than 100 stars. They are very far from the ground, so most of them are not visible to the naked eye and are perceived collectively as a foggy spot.

THE BIRTH OF THE EARTH

The Earth was apparently formed about 4.6 billion years ago (about 8.5 - 10.5 billion years after the birth of the Universe as a result of a colossal release of energy called the Big Bang). It was formed as protoplanetary matter gathered into a clot and heated up. Heavy particles of iron and nickel were concentrated in the center of this ball, and an outer, probably molten layer was formed from lighter materials. After millions of years, the outer layer began to cool and harden. In the depths of the Earth, the substance is still hot, and some of it is molten. From space, our planet appears blue because most of it is covered by oceans and the Earth is surrounded by an atmosphere - a shell of air. It protects against cosmic radiation and regulates the temperature of the Earth. Higher up, the atmosphere becomes thinner until it becomes airless space. It is held by the force of gravity. The Earth is shaped like a ball, although somewhat flattened at the poles and wider at the equator, in the middle. Our planet's magnetic field is generated by streams of charged particles in the earth's iron-rich core.

SUN

Our star. It is now in the middle of its life cycle, and its gas reserves will last for another 5 billion years. Nine cosmic bodies called planets revolve around the Sun in the same direction - counterclockwise when viewed from above. Together with the Sun they make up the Solar System. The Earth makes a full revolution around the Sun in one year (365 days). The Sun is at a distance of 150 million km from the Earth. The Sun is approximately 333,000 times heavier than the Earth. With the precision of volumetric vision, about 1,300,000 planets like Earth would fit inside the Sun. Like all stars, the Sun is a ball of hot gases, mainly hydrogen and helium. A thermonuclear reaction occurs in the solar core, converting hydrogen into helium. A huge amount of energy is released, due to which the temperature of the core reaches 15,000,000 * C and the Sun glows.

MOON

This is the cosmic body closest to Earth and the only satellite of our

Planets. Astronomers call the Moon a satellite because it orbits the Earth every 27.3 days. At the same time, it manages to turn around its axis, so the Moon always faces the Earth with the same side. The moon shines with light reflected from the sun. During the new moon, the side of the Moon facing us is not illuminated by the Sun, and we cannot see it at all. Sometimes the Moon appears between the Earth and the Sun, obscuring the Sun. Then a solar eclipse occurs on Earth. Lunar eclipses occur when the Earth passes between the Sun and the Moon, casting a shadow on the Moon's surface. They occur more often than solar ones. Some scientists believe that 4 billion years ago the Earth collided with a solid celestial body called a planetesimal. Upon impact, pieces broke off from the Earth's surface. Moving around it in orbit, they gradually came closer, forming the Moon. There is no atmosphere on the moon and all meteorites fall onto its surface without burning, forming craters. The temperature on the surface of the Moon is from -170*C to 100*C.

Earth planetesimal

CONSTELLATIONS

Thousands of stars are visible in the night sky. The stars form various patterns and shapes. Groups of stars that create a specific pattern are called constellations. Even in ancient times, people noticed that all the stars seemed to revolve around the North Star. She always stands in her place, motionless. Located directly above the North Pole. In the Southern Hemisphere, it is convenient to navigate by the Southern Cross constellation. The shape of the constellations does not change, but the planets change their position as they move among the constellations. Ancient astronomers called the mysterious moving objects "planets", which means "wanderers" in ancient Greek.

ASTRONOMY

Science about space and cosmic bodies. Every year we learn more and more about the Solar System, our Galaxy (Milky Way) and many other objects and phenomena in the Universe. Astronomers use the most modern scientific equipment to penetrate the secrets of space. Thanks to their research, we understand the structure of the solar system and the universe. Peering into the depths of space, astronomers work together with chemists, physicists and other scientists, exchanging knowledge and ideas.