The stars are the most interesting astronomical objects, and represent the most fundamental building blocks galaxies. The age, distribution and composition of stars in a galaxy allows us to determine its history, dynamics and evolution. In addition, stars are responsible for the production and distribution of heavy elements such as carbon, nitrogen, oxygen in space, and their characteristics are closely related to the planetary systems that they form. Therefore, the study of the process of birth, life and death of stars occupies a central place in the astronomical field.
The Birth of Stars
Stars are born in clouds of dust and gas that are scattered throughout most galaxies. A striking example of the distribution of such a cloud is the Orion Nebula.
The featured image combines visible and infrared wavelength images from the Hubble and Spitzer Space Telescopes. Turbulence in the depths of these clouds leads to the creation of nodes with sufficient mass to begin the process of heating the material in the center of this node. It is this hot core, better known as a protostar, that could one day become a star.
Three-dimensional computer simulations of star formation show that rotating clouds of gas and dust can break into two or three pieces; this explains why most stars in Milky Way are in pairs or small groups.
Not all the material from the gas and dust cloud ends up in the future star. The remaining material may form planets, asteroids, comets, or simply remain as dust.
Main sequence of stars
A star the size of our Sun takes about 50 million years to mature from formation to adulthood. Our Sun will remain in this phase of maturity for approximately 10 billion years.
Stars are powered by the energy released in the process of nuclear fusion of hydrogen with the formation of helium in their depths. The outflow of energy from their central regions of the star provides the necessary pressure to prevent the star from collapsing under the influence of gravity.
As shown in the Hertzsprung-Russell diagram, the main sequence of stars covers wide range luminosity and color of stars, which can be classified according to these characteristics. The smallest stars are known as red dwarfs, have a mass of about 10% of the mass of the Sun and emit only 0.01% of the energy compared to our star. Their surface temperature does not exceed 3000-4000 K. Despite their miniature size, red dwarfs are by far the most numerous type of stars in the Universe and are tens of billions of years old.
On the other hand, the most massive stars, known as hypergiants, can have a mass 100 times or more the mass of the Sun and a surface temperature of more than 30,000 K. Hypergiants release hundreds of thousands of times more energy than the Sun, but have lifetimes of only a few million years. Such extreme stars, scientists believe, were widespread in the early Universe, but today they are extremely rare - only a few hypergiants are known throughout the Milky Way.
Evolution of a star
In general terms, the larger the star, the shorter its lifespan, although all but supermassive stars live for billions of years. When a star has completely produced hydrogen in its core, nuclear reactions in its depths cease. Deprived of the energy the core needs to maintain itself, it begins to collapse into itself and become much hotter. The remaining hydrogen outside the nucleus continues to fuel the nuclear reaction outside the nucleus. The hotter and hotter core begins to push the star's outer layers outward, causing the star to expand and cool, turning it into a red giant.
If the star is massive enough, the process of core collapse can raise its temperature enough to support more exotic nuclear reactions that consume helium and produce various heavy elements, including iron. However, such reactions provide only a temporary reprieve from the global collapse of the star. Gradually, the internal nuclear processes of the star become more and more unstable. These changes cause a pulsation inside the star, which will subsequently lead to the shedding of its outer shell, surrounding itself with a cloud of gas and dust. What happens next depends on the size of the kernel.
The further fate of a star depending on the mass of its core
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For medium-sized stars like the Sun, the process of stripping the core from its outer layers continues until all the surrounding material is ejected. The remaining, highly heated core is called a white dwarf. White dwarfs are comparable in size to Earth and have the mass of a full-fledged star. Until recently, they remained a mystery to astronomers - why further destruction of the core does not occur. Quantum mechanics solved this riddle. The pressure of fast-moving electrons saves the star from collapse. The more massive the core, the denser the dwarf is formed. Thus, than smaller size white dwarf, the more massive it is. These paradoxical stars are quite common in the Universe - our Sun will also turn into a white dwarf in a few billion years. Due to the lack of an internal source of energy, white dwarfs eventually cool down and disappear into the vast expanses of outer space. |
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If a white dwarf formed in a binary or multiple star system, the end of its life may be more intense, known as formation nova. When astronomers gave this name to this event, they really thought that a new star was forming. However, today it is known that in fact we're talking about about very old stars - white dwarfs. If a white dwarf is close enough to its companion star, its gravity can pull hydrogen from the outer layers of its neighbor's atmosphere and create its own surface layer. When enough hydrogen accumulates on the surface of a white dwarf, a nuclear fuel explosion occurs. This causes its brightness to increase and the remaining material to be shed from the surface. Within a few days, the star's brightness decreases and the cycle begins again. Sometimes, especially in massive white dwarfs (whose mass is more than 1.4 solar masses), it can become so overgrown big amount material so that during an explosion they are completely destroyed. This process is known as the birth of a supernova. |
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Main sequence stars with a mass of about 8 solar masses or more are destined to die in a powerful explosion. This process is called the birth of a supernova. A supernova is not just a big nova. In a nova, only the surface layers explode, while in a supernova, the core of the star itself collapses. As a result, a colossal amount of energy is released. In a period of several days to several weeks, a supernova can eclipse an entire galaxy with its light. The terms Nova and Supernova do not accurately describe the essence of the process. As we already know, physically, the formation of new stars does not occur. The destruction of existing stars occurs. This misconception is explained by several historical cases when bright stars appeared in the sky, which until that time were practically or completely invisible. This effect and the appearance of a new star influenced the terminology. |
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If at the center of a supernova there is a core with a mass of 1.4 to 3 solar masses, the destruction of the core will continue until electrons and protons combine and create neutrons, which subsequently form a neutron star. Neutron stars are incredibly dense cosmic objects - their density is comparable to the density of an atomic nucleus. Because a large number of mass packed in a small volume, gravity on the surface neutron star just incredible Neutron stars have large magnetic fields that can accelerate atomic particles around it magnetic poles producing powerful beams of radiation. If such a beam is oriented towards the Earth, then we can detect regular pulses in the X-ray range from this star. In this case, it is called a pulsar. |
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If the core of a star is more than 3 solar masses, then in the process of its collapse a black hole is formed: an infinitely dense object whose gravity is so strong that even light cannot escape it. Since photons are the only tool through which we can study the universe, detecting black holes directly is impossible. Their existence can only be known indirectly. One of the main indirect factors indicating the existence of a black hole in a certain area is its enormous gravity. If there is any material near the black hole - most often companion stars - it will be captured by the black hole and pulled towards it. The attracted matter will move towards the black hole in a spiral, forming a disk around it, which heats up to enormous temperatures, emitting copious amounts of X-rays and gamma rays. It is their detection that indirectly indicates the existence of a black hole next to the star. |
Useful articles that will answer most interesting questions about the stars.
Deep space objects
MAIN SEQUENCE, in astronomy, the region on the HERTSPRUNG RUSSELL DIAGRAM where the most stars, including the Sun, are found. It stretches diagonally from the hot ones bright stars in the upper left to the cool faint stars in the lower right... ... Scientific and technical encyclopedic dictionary
Hertzsprung Russell diagrams, a narrow band on this diagram within which the vast majority of stars are located. Crosses the diagram diagonally (from high to low luminosities and temperatures). Main sequence stars (to... ... encyclopedic Dictionary
A set of stars that are physically similar to the Sun and form a practically one-parameter sequence on the state diagram (Hertzsprung-Russell diagram (See Hertzsprung-Russell diagram)). Along the G. p.w. diagrams... ... Great Soviet Encyclopedia
Hertzsprung-Russell diagrams, a narrow band on this diagram, within which the vast majority of stars are located. Crosses the diagram diagonally (from high to low luminosities and temp p). G. stars (these include, in particular... ... Natural science. encyclopedic Dictionary
MAIN SEQUENCE of the Hertzsprung-Russell diagram is a narrow band on this diagram within which the vast majority of stars are located. Crosses the diagram diagonally (from high to low luminosities and temperatures). Stars… … Big Encyclopedic Dictionary
Main sequence of the Hertzsprung-Russell diagram- the diagram expresses the relationship between the luminosity and temperature of stars (the spectral class or color index of some objective characteristics of stars), on it there are similar physical properties stars occupy separate regions: the main one... ... The beginnings of modern natural science
A set of stars that are physically similar to the Sun and form a single sequence on the luminosity spectrum diagram (see Hertzsprung-Russell diagram) in which the luminosities decrease monotonically with decreasing surface temperature, mass and... ... Astronomical Dictionary
SEQUENCE OF ACTIONS OF THE INTERMEDIATE- – logic of actions of a third party in order to resolve an interpersonal conflict. It includes 17 basic steps. 1. Try to imagine big picture conflict and penetrate into its essence by analyzing the information we have. Estimate… …
SEQUENCE OF SELF-RESOLVATION OF CONFLICT- – the logic of actions taken by a more psychologically competent opponent in order to end the interpersonal conflict. It includes 17 basic steps. 1. Stop fighting with your opponent. Understand that through conflict it will not be possible to protect your... Encyclopedic Dictionary of Psychology and Pedagogy
- ... Wikipedia
main feature
So, the metaphor is that you can draw a similar picture about startups, and it will also turn out that there is a narrow zone of stability - the “main sequence” - and there are unstable states beyond it. The axes can be cash burn (the rate at which investments are spent) and the growth rate of key metrics (each project has its own, of course; in the most typical case, this is the number of users).
On the main sequence are projects that know how to balance one with the other. The ideal situation is a careful, smooth movement through it: expenses gradually increase, and growth rates increase proportionally (namely, growth rates, not the metrics themselves!). In other words, the money invested gives explosive growth - the startup “takes off.”
A huge dwarf cemetery is under the main sequence. These projects are frozen, they do not consume money, or consume a very small, constant amount of it (roughly speaking, hosting costs) - but the metrics are stable, not growing or practically not growing. Maybe someone comes in, registers, even starts using it - but this will not lead to a new round of growth. (From personal experience this is, of course, 9facts).
Above the main sequence are artificially inflated giants. Money burns very quickly (like helium!), but this happens in the wrong place, or simply too early - the market is not yet ready to respond with a corresponding increase in metrics. The spectrogram of such a startup shows very clearly characteristic features: overstaffed, lack of organic user growth (growth only through purchasing traffic), tossing from side to side. The history, as a rule, is of a “wild investor” - someone who believes very strongly in the idea, but at the same time is not involved in professional development of startups, cannot assess the needs of the project at the next stage, and gives too much money. (And this was also all we had with 9facts, by the way).
Very often one can observe how a project goes exactly the same way as a star in the process of its evolution: from the main sequence to giants (they mistakenly decided that they had grabbed the model that would ensure explosive growth and began pumping money in), and then to dwarfs ( ran out of money). Well, several more funny analogies can be seen within this rich metaphor.
And the productivity of this metaphor is this.
1) The main sequence is very narrow. This is a thin path, it is impossible to walk along it without a very clear understanding of how the venture industry works in general (I’ll take this opportunity to once again advertise , and ), without a very clear focus on the essence of your product, without identifying and monitoring your own key metrics. without experienced pilots, without involvement, hard work, even fanaticism. One step to the left, one step to the right - and it will be difficult, almost impossible, to return. If a derailment does occur, you need to drop everything and try to come back. This is the benefit of my metaphor for a startuper.
2) If a project is obviously outside the main sequence, there is no point in investing in it, there is no point in considering it. No chance. In particular, there is no point in considering a project that has not even begun yet, but the main parameters of which from the very beginning imply a deviation from the main sequence (“we will immediately hire 30 people”). This is the benefit of my metaphor for the investor; it really helps save time.
3) And of course, we must not forget that generalizations and dogmas are useful only when you remember their logical basis and can understand for yourself why in a given specific situation generalization will not work, but dogma can be broken.
And finally, a few words about what the main sequence looks like for startups. (Naturally, this can only be discussed in a very generalized form; markets, countries, etc. vary greatly).
It all starts in that part of the schedule where there are no users yet - and at this stage the team cannot have more than 2-3 people, and it cannot burn hundreds of thousands of rubles per month, and it would be better not to burn anything at all. The prototype is ready, the main hypothesis has been formulated, attempts at promotion have begun, seed funding has been raised - the team can have 5-6 people, it can spend a couple of hundred thousand a month, but there must be clients, even in beta testing mode, and a significant Some of the money should not be used for development. The product has been created, customers are using it and started paying the first money, we managed to attract serious funding from business angels - the main thing at this stage is to at some point stop the growth of development costs, focusing on business development and obtaining sustainable metrics; You can't spend millions yet. Stable growth has been achieved, the first venture round of financing has been raised - this is not a reason for uncontrolled staffing and careless handling of money; successful projects here grow to 10-20 people, and keep their costs within 50-100 thousand dollars per month. And so on.
In short, everything is like in space, with only one difference.
There, 90% of the stars are on the main sequence, and it would not be a big exaggeration to say that 90% of startups are trying to find themselves outside of it.
From interviews and pitches just this week:
- startup A has already spent $1.5M in two years on product development, the demand for the solution has not been proven, the user base is not growing, they are trying to attract another $2M - mainly to continue development (who will give it to them? and, most importantly, at what valuation?) ,
- startup B has run out of all the money raised at the seed stage, and the founders continue to tinker with it in parallel with their main work, while competitors have moved ahead at a good pace; at one time the founders did not take decent investments at a good valuation, trying not to be diluted and counting on own strength, and now they already agree to a significantly lower estimate, but...,
- startup B is trying to raise several tens of millions of rubles at the idea stage, planning to assemble a team of about 20 people to create a prototype and test the hypothesis,
... and so on.
Posted on Feb. 17th, 2013 at 02:10 pm |
Stars are huge balls of luminous plasma. There are a huge number of them within our galaxy. Stars played an important role in the development of science. They were also noted in the myths of many peoples and served as navigation tools. When telescopes were invented, and the laws of motion of celestial bodies and gravity were discovered, scientists realized: all stars are similar to the Sun.
Main sequence stars include all those within which hydrogen is converted into helium. Since this process is characteristic of most stars, most of the luminaries observed by humans fall into this category. For example, the Sun also belongs to this group. Alpha Orionis, or, for example, the satellite of Sirius does not belong to the stars of the main sequence.
For the first time, scientists E. Hertzsprung and G. Russell took up the issue of comparing stars with their spectral classes. They created a diagram that showed the spectrum and luminosity of stars. This diagram was subsequently named after them. Most of the luminaries located on it are called main sequence celestial bodies. This category includes stars ranging from blue supergiants to white dwarfs. The luminosity of the Sun in this diagram is taken as unity. The sequence includes stars of different masses. Scientists have identified the following categories of luminaries:
From a structural point of view, the Sun can be divided into four conventional zones, within which various physical processes. The star's radiation energy, as well as internal thermal energy, arises deep inside the star, transmitted to the outer layers. The structure of main sequence stars is similar to the structure of the star solar system. The central part of any luminary, which belongs to this category on the Hertzsprung-Russell diagram, is the nucleus. Nuclear reactions constantly occur there, during which helium is converted into hydrogen. In order for hydrogen nuclei to collide with each other, their energy must be higher than the repulsive energy. Therefore, such reactions occur only under very high temperatures. The temperature inside the Sun reaches 15 million degrees Celsius. As it moves away from the star's core, it decreases. At the outer boundary of the core, the temperature is already half of the value in the central part. The plasma density also decreases.
But not only in their internal structure are main sequence stars similar to the Sun. The luminaries of this category are also distinguished by the fact that nuclear reactions inside them occur through a three-stage process. Otherwise it is called the proton-proton cycle. In the first phase, two protons collide with each other. As a result of this collision, new particles appear: deuterium, positron and neutrino. Next, the proton collides with a neutrino particle, and a nucleus of the helium-3 isotope appears, as well as a gamma-ray quantum. At the third stage of the process, two helium-3 nuclei merge with each other, and ordinary hydrogen is formed.
During these collisions, nuclear reactions continually produce elementary neutrino particles. They overcome the lower layers of the star and fly into interplanetary space. Neutrinos are also detected on earth. The quantity that is recorded by scientists using instruments is disproportionately less than what scientists assume it should be. This problem is one of the biggest mysteries in solar physics.
The next layer in the structure of the Sun and main sequence stars is the radiative zone. Its boundaries extend from the core to a thin layer located on the border of the convective zone - the tachocline. The radiant zone gets its name from the way energy is transferred from the core to the outer layers of the star - radiation. Photons, which are constantly produced in the core, move in this zone, colliding with the plasma nuclei. It is known that the speed of these particles is equal to the speed of light. But despite this, photons take about a million years to reach the boundary of the convective and radiant zones. This delay occurs due to the constant collision of photons with plasma nuclei and their re-emission.
The Sun and main sequence stars also have a thin zone that appears to play an important role in the formation magnetic field luminary It's called tachocline. Scientists suggest that this is where magnetic dynamo processes occur. It lies in the fact that plasma flows pull out magnetic power lines and increase the overall field strength. There are also suggestions that in the tachocline zone there is a sharp change in the chemical composition of the plasma.
This area is the outermost layer. Its lower boundary is located at a depth of 200 thousand km, and its upper boundary reaches the surface of the star. At the beginning of the convective zone, the temperature is still quite high, reaching about 2 million degrees. However, this indicator is no longer sufficient for the process of ionization of carbon, nitrogen, and oxygen atoms to occur. This zone got its name because of the method by which matter is constantly transferred from deep to external layers - convection, or mixing.
In a presentation about main sequence stars, you can point out the fact that the Sun is an ordinary star in our galaxy. Therefore, a number of questions - for example, about the sources of its energy, structure, and the formation of the spectrum - are common to both the Sun and other stars. Our star is unique in terms of its location - it is the closest star to our planet. Therefore, its surface is subjected to detailed study.
The visible shell of the Sun is called the photosphere. It is she who emits almost all the energy that comes to Earth. The photosphere consists of granules, which are elongated clouds of hot gas. Here you can also observe small spots called torches. Their temperature is approximately 200 o C higher than the surrounding mass, so they differ in brightness. Torches can last up to several weeks. This stability arises due to the fact that the magnetic field of the star does not allow vertical flows of ionized gases to deviate in the horizontal direction.
Also, dark areas sometimes appear on the surface of the photosphere—spot nuclei. Often, spots can grow to a diameter that exceeds the diameter of the Earth. As a rule, they appear in groups and then grow. Gradually they are split into smaller sections until they disappear completely. Spots appear on both sides of the solar equator. Every 11 years, their number, as well as the area occupied by the spots, reaches a maximum. From the observed movement of the sunspots, Galileo was able to detect the rotation of the Sun. This rotation was later refined using spectral analysis.
Until now, scientists are puzzling over why the period of increase in sunspots is exactly 11 years. Despite the gaps in knowledge, information about sunspots and the periodicity of other aspects of a star's activity gives scientists the ability to make important predictions. By studying this data, one can make predictions about the occurrence of magnetic storms and disruptions in radio communications.
The amount of energy emitted by a star in one unit of time is called. This value can be calculated from the amount of energy that reaches the surface of our planet, provided that the distance of the star to the Earth is known. Main sequence stars are more luminous than cool, low-mass stars and less luminous than hot stars, which are between 60 and 100 solar masses.
Cool stars are in the lower right corner relative to most luminaries, and hot stars are in the upper left corner. Moreover, for most stars, unlike red giants and white dwarfs, the mass depends on the luminosity index. Each star spends most of its life on the main sequence. Scientists believe that more massive stars live much shorter lives than those with low mass. At first glance, it should be the other way around, because they have more hydrogen to burn, and they have to spend it longer. However, massive stars use up their fuel much faster.
Main sequence stars
Units
Most stellar characteristics are usually expressed in SI, but GHS is also used (for example, luminosity is expressed in ergs per second). Mass, luminosity and radius are usually given in relation to our Sun:
To indicate the distance to stars, units such as light year and parsec are used.
Long distances, such as the radius of giant stars or the semimajor axis of binary star systems are often expressed using
astronomical unit (AU) - the average distance between the Earth and the Sun (150 million km).
Fig. 1 – Hertzsprung-Russell diagram
Types of stars
Classifications of stars began to be built immediately after their spectra began to be obtained. To a first approximation, the spectrum of a star can be described as the spectrum of a black body, but with absorption or emission lines superimposed on it. Based on the composition and strength of these lines, the star was assigned one or another specific class. This is what they do now, however, the current division of stars is much more complex: in addition, it includes absolute stellar magnitude, the presence or absence of variability in brightness and size, and the main spectral classes are divided into subclasses.
At the beginning of the 20th century, Hertzsprung and Russell plotted various stars on a diagram “Absolute magnitude” - “spectral class”, and it turned out that most of them are grouped along a narrow curve. Later this diagram (now called Hertzsprung-Russell diagram) turned out to be the key to understanding and researching the processes occurring inside a star.
Now that there's a theory internal structure stars and the theory of their evolution, it became possible to explain the existence of classes of stars. It turned out that the whole variety of types of stars is nothing more than a reflection quantitative characteristics stars (such as mass and chemical composition) and the evolutionary stage at which the star is currently located.
In catalogs and in writing, the class of stars is written in one word, with letter designation main spectral class (if the class is not precisely defined, a letter range is written, for example, O-B), then the spectral subclass is specified in Arabic numerals, then the luminosity class (region number on the Hertzsprung-Russell diagram) is specified in Roman numerals, and then comes Additional Information. For example, the Sun has a class G2V.
The most numerous class of stars are main sequence stars; our Sun also belongs to this type of star. From an evolutionary point of view, the main sequence is the place on the Hertzsprung-Russell diagram where a star spends most of its life. At this time, energy losses due to radiation are compensated by the energy released during nuclear reactions. The lifetime on the main sequence is determined by the mass and fraction of elements heavier than helium (metallicity).
The modern (Harvard) spectral classification of stars was developed at the Harvard Observatory in 1890 - 1924.
| Basic (Harvard) spectral classification of stars | ||||
| Class | Temperature, K | true color | Visible color | Main features |
| O | 30 000-60 000 | blue | blue | Weak lines of neutral hydrogen, helium, ionized helium, multiply ionized Si, C, N. |
| B | 10 000-30 000 | white-blue | white-blue and white | Absorption lines of helium and hydrogen. Weak H and K lines of Ca II. |
| A | 7500-10 000 | white | white | Strong Balmer series, lines H and K of Ca II intensify towards class F. Also, closer to class F, lines of metals begin to appear |
| F | 6000-7500 | yellow-white | white | The H and K lines of Ca II, the lines of metals, are strong. The hydrogen lines begin to weaken. The Ca I line appears. The G band formed by the Fe, Ca and Ti lines appears and intensifies. |
| G | 5000-6000 | yellow | yellow | The H and K lines of Ca II are intense. Ca I line and numerous metal lines. The hydrogen lines continue to weaken, and bands of CH and CN molecules appear. |
| K | 3500-5000 | orange | yellowish orange | Metal lines and G band are intense. The hydrogen line is almost invisible. TiO absorption bands appear. |
| M | 2000-3500 | red | orange-red | The bands of TiO and other molecules are intense. The G band is weakening. Metal lines are still visible. |
Brown dwarfs
Brown dwarfs are a type of star in which nuclear reactions could never compensate for the energy lost to radiation. For a long time, brown dwarfs were hypothetical objects. Their existence was predicted in the mid-20th century, based on ideas about the processes occurring during the formation of stars. At the same time, a brown dwarf was discovered for the first time in 2004. To date, quite a lot of stars of this type have been discovered. Their spectral class is M - T. In theory, another class is distinguished - designated Y.
Main sequence stars - concept and types. Classification and features of the category "Main Sequence Stars" 2017, 2018.
