The birth of the blue giants: Nothing is simpler than a star. - Arthur Stanley Eddington
Today we know much more about stars than Comte or Socrates. But if now even a schoolboy in general terms understands what a star is, then the question “where do the stars come from” is covered in darkness. Really, where from? This article is just about how the so-called blue giants are born - massive stars that are visible in the sky with the naked eye.
Young stars, that is, stars from a million to several hundred million years old, are mainly composed of hydrogen. Hydrogen is the most popular building material of the Universe, a molecular brick, which is laid in the foundation of the most diverse objects of the starry world: from intergalactic gas to blue giants. So, in order to ensure the birth of a star, you need to stock up a considerable amount of hydrogen. But how to collect in one place a large mass of this building material? Where will it come from in the vast expanses of the universe?
Phase One - Interstellar Gas
The space between the stars is not at all an absolute vacuum, it is filled with atoms of calcium, sodium, oxygen, carbon, rather complex molecules that form dust particles, but mostly hydrogen and helium. This is the so-called interstellar gas that fills our entire galaxy. The highest concentration of gas is near its plane, in a very thin layer with a thickness of 70 light years (and the diameter of the Galaxy is about 60 thousand light years). So, the basis for the star was found. In the future, we will talk specifically about our Galaxy as the closest and best studied region of the Universe.
The second stage is thermal instability
What is the mechanism of turning gas into a star? If Sir Maxwell were here, he would say that a homogeneous gas would be in a state of unstable thermal equilibrium, which means that both dense regions (condensations) and more rarefied ones will inevitably appear in it. Although the region is called dense, this name is very arbitrary, since the gas in it is not so dense: literally several dozen atoms in one cubic centimeter. Condensations in a gas are called gas clouds, and we observe them as nebulae. Gas clouds move, with an average speed of 8 km / s, and the fastest accelerate to 80 km / s. And this is not a typo! A huge mass of gas with a diameter of several parsec (1pc = 3.26 light years or 30 thousand billion kilometers) is carried in a much more rarefied medium at a speed exceeding the speed of our spacecraft. And since there are a lot of such clouds in the Galaxy, then at one fine moment (on a galactic scale this moment lasts several thousand years) one gas cloud collides with another. The shock wave arising from this collision causes the gas in the colliding clouds to become denser, giving rise to the next stage of star birth.
Third Stage - Magnetic Field
Gas clouds are huge, but nevertheless their mass is not enough for the birth of a star. The substances in them are as much as in our Sun, and it is necessary - several dozen, hundreds of times more. What makes interstellar clouds come together? It turns out that this task is performed by magnetic galactic fields. The magnetic field of our galaxy was discovered in the late forties of the last century. The cause of this field is still not known exactly. As befits any self-respecting field, it has lines of force, that is, lines of tension. Gas clouds can usually only move along these lines. To understand how interstellar clouds are stacked, imagine a magnetic field in the form of a weakly stretched sheet. Here we launch a small ping-pong ball on this sheet (this is our gas and dust cloud): under the ball, the sheet is bent more strongly, a hole appears - the lines of force bend. Other balls (clouds) begin to roll into the pit, making it deeper and deeper. This phenomenon is called Rayleigh-Jeans instability. That is, any initial heterogeneity in a magnetic field, for example, a cloud flying into this inhomogeneity, for example, is enough - and it’s ready: a bag with the collected gas — a gas-dust complex — hangs high above (or below) the plane of the galaxy.
Stage Four - Gravity
So, hydrogen (and even not only it) is now in abundance. Next, the mechanisms described by the theory of star formation come into effect. Its foundations were laid by Sir Isaac Newton, and the theory was further developed by the works of the Japanese astrophysicist Hayashi. If we have a homogeneous gas, then condensations inevitably begin to form in it: places in which there is more gas than in others. But this is not thermal instability, as is the case with interstellar gas, but gravitational. Under the influence of gravity, more and more new portions of gas rush to these initial clumps. Each bunch is a future star. A greatly enlarged bunch takes the shape of a ball, the most stable geometric shape. The gas layers are mixed and compacted, in the center of the ball pressure begins to increase. The ball gradually heats up, constantly increasing its mass, receiving and receiving new building material. At this stage, the protostar is still invisible; it is obscured by the gathered around and strongly condensed clouds. By the way, it became possible to make out such objects only with the advent of telescopes operating in the infrared ranges. But in addition to the forces of gravity, other forces are now beginning to appear - gas pressure forces that tend to pull the ball apart. This eternal struggle of centrifugal forces with centripetal forces accompanies the star throughout its entire existence. If the first win in the end, the star will explode, and we will see a flash of the Supernova. If the latter (gravitational forces) - the star collapses into itself: a mysterious object such as a black hole appears.
The fifth stage - the beginning of the thermonuclear reaction
Why does the star glow? The fact is that a star is, in fact, a thermonuclear reactor in which the energy released to the star’s radiation and which keeps it from turning into a black hole, from gravitational collapse is released.
But to start a thermonuclear reaction, a very high temperature is needed - 10 million degrees. And only after the protostar switches to thermonuclear fuel, it can be called a young star. What sources to take energy from for such a colossal warm-up? After all, we are talking about a gigantic mass of gas, several tens of times more than the mass of our Sun!
At the very beginning of protostar life, the entire mass of its substance is involved in movement from the center to the surface and vice versa, and its temperature does not exceed another four thousand degrees. After several hundred thousand years of compression (sometimes less), convection flows weaken, do not fill the entire inside of the protostar, but flow more closely to the surface. Due to this, the temperature of the central region begins to grow faster and approximately one million years after the start of compression, it reaches a level sufficient for light thermonuclear reactions (conversion of lithium nuclei to beryllium), and then for the main proton-proton cycle. And this is a real young star. (By the way, the star’s birth time depends on its initial mass - massive protostars go through stages faster.)
In a dust cloud, of course, more than one single asterisk is born. The cloud is huge, and the initial thickenings in it usually appear several dozen at once. Therefore, a beautiful object arises in the sky from dozens of closely spaced stars shining with bright and young blue light. The most remarkable example of such a star cluster is the Pleiades, a small island, the "kindergarten of the stars" in the constellation Taurus. In large telescopes and now around these stars are visible the remains of unused dust. An example of a gas-dust complex in which stars are at the final stage of birth is the Orion nebula in the eponymous constellation. By the way, the brightest stars of the constellation Orion came from one dust cloud, but due to the rotation of our Galaxy, they began to scatter and are now separated from each other by several light years. In the Ophiuchus nebula, stars only appear. They are hidden from us by huge dust clouds, cocoons, in the center of which the protostar is compressed into a star. Of course, many more questions remain in the processes of birth of stars, the answers to which should be given by the next generation of researchers. I hope these answers will be received before the stars shining now in the night sky go out.The article was published in the journal Popular Mechanics (No. 5, May 2004). Do you like the article?
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