Finally, due to the increase in the density of individual fragments of the cloud, the gas becomes less transparent.aya yasser
Finally, due to the increase in the density of individual fragments of the cloud, the gas becomes less transparent.
Ultraviolet observations, which began in 1970, from rockets and satellites revealed the main molecule of the interstellar medium – the molecule of hydrogen (H2). And when observing interstellar space with radio telescopes in the centimeter and millimeter ranges, dozens of other molecules were found, sometimes quite complex, containing up to 13 atoms. These include molecules of water, ammonia, formaldehyde, ethyl alcohol and even the amino acid glycerol.
As it turned out, about half of the interstellar gas is contained in molecular clouds. Their density is hundreds of times higher than that of atomic hydrogen clouds, and the temperature is only a few degrees above absolute zero. It is under such conditions that individual seals unstable to gravitational compression appear in a cloud of mass of the order of the Sun’s mass, and star formation becomes possible.
Most molecular clouds are registered only by radio emission. Some, however, have long been known to astronomers, such as the dark Nebula Coal Bag is clearly visible to the naked eye in the southern part of the Milky Way. The diameter of this cloud is 12 pc, but it looks big because it is only 150 pc away from us. Its mass is about 5 thousand solar masses, while in some clouds the mass reaches one million solar masses, and the size of 60 pc. In such giant molecular clouds (there are only a few thousand of them in the Galaxy) are the main centers of star formation.
The closest areas of star formation to us are dark clouds in the constellations Taurus and Serpent Bearer. In the distance is a huge complex of clouds in Orion.
Life of a black cloud
Molecular clouds are much more complex than the familiar water vapor clouds in the Earth’s atmosphere. Externally, the molecular cloud is covered with a thick layer of atomic gas, because the penetrating radiation of stars destroys fragile molecules. But the pollen in the outer layer absorbs radiation, and deeper, in the dark depths of the cloud, the gas is almost entirely composed of molecules.
The structure of clouds is constantly changing under the influence of mutual collisions, heating by stellar radiation, the pressure of interstellar magnetic fields. In different parts of the cloud, the density of the gas differs by a thousand times (the same number of times water is denser than room air). When the density of a cloud (or a separate part of it) becomes so large that gravity overcomes the gas pressure, the cloud begins to collapse uncontrollably.
Its size decreases faster and faster, and the density increases. Small inhomogeneities of density in the process of collapse are amplified, and as a result the cloud is fragmented, ie breaks up into parts, each of which continues to compress itself.
During collapse, the temperature and pressure of the gas increase, which prevents further increase in density. But as long as the cloud remains transparent to radiation, it cools easily and the compression does not stop. Space dust plays an important role in the future. Although by mass it is only 1% of interstellar matter, it is a very important component.
In dark clouds, the dust absorbs the energy of the gas and converts it into infrared radiation, which easily leaves the cloud, removing excess heat. Finally, due to the increase in the density of individual fragments of the cloud, the gas becomes less transparent. Cooling becomes difficult, and the increasing pressure stops the collapse. In the future, a star will form from each fragment, and together they will form a group of young stars in the bowels of the molecular cloud.
The collapse of a dense part of the cloud into a star, and more often into a group of stars, lasts for several million years (relatively quickly https://123helpme.me/narrative-essay-topics/ on a cosmic scale). Newborn stars heat the surrounding gas, and under the action of high pressure, the remnants of the cloud fly away. This is the stage we see in the Orion Nebula. But next to it, the formation of future generations of stars continues. For light, these areas are completely opaque and are observed only with infrared and radio telescopes.
The cloud becomes a star
The birth of the star lasts for millions of years and is hidden from us in the depths of dark clouds, so that this process is virtually inaccessible to direct observation. Astrophysicists try to study it theoretically, using computer modeling. The transformation of a fragment of a cloud into a star is accompanied by a giant change in physical conditions: the temperature of the substance increases approximately 106 times, and the density – 1020 times. Enormous changes in all the characteristics of the emerging star are the main difficulty in the theoretical consideration of its evolution. At the stage of such changes, the original object is no longer a cloud, but not a star. Therefore, it is called a protostar).
In general, the evolution of a protostar can be divided into three stages, or phases. The first stage – the separation of a fragment of the cloud and its compaction – we have already seen. It is followed by a stage of rapid compression. At its inception, the radius of the protostar is about a million times larger than the solar.
It is completely opaque to visible light, but transparent to infrared radiation with a wavelength of more than 10 microns. Radiation carries excess heat released during compression, so that the temperature does not rise and the gas pressure does not prevent collapse. There is a rapid compression, almost free fall of matter to the center of the cloud.
However, as the compression becomes, the protostar becomes less and less transparent, which makes it difficult to emit radiation and leads to an increase in gas temperature. At a certain point, the protostar becomes almost opaque to its own thermal radiation. The temperature, and with it the gas pressure, increases rapidly, the compression slows down.
The increase in temperature causes significant changes in the properties of the substance. At a temperature of several thousand degrees, the molecules break down into individual atoms, and at a temperature of about 10 thousand degrees, the atoms are ionized, that is, their electronic shells are destroyed.
These energy-intensive processes delay the rise in temperature for a while, but then it resumes. The protostar quickly reaches a state when the force of gravity is almost balanced by the internal pressure of the gas. But since the heat is still going out, and the protostar has no other sources of energy than compression, it continues to shrink slowly and the temperature in its bowels increases.
Finally, the temperature in the center of the protostar reaches several million degrees and thermonuclear reactions begin. In this case, the heat released completely compensates for the cooling of the protostar from the surface. Compression stops. The protostar becomes a star.
The “first cry” of the newborn star
Forming stars and very young stars are often surrounded by a gas shell – remnants of matter that have not yet fallen on the star. The shell does not emit light from the inside and completely converts it into infrared radiation. Therefore, the youngest stars usually find themselves only as infrared sources.
At the initial stage of life, the “behavior” of the star depends very much on its mass. The low luminosity of low-mass stars allows them to linger for a long time in the stage of slow compression, “feeding” only on gravitational energy. During this time, the shell has time to partially settle on the star, as well as to form a stellar gas disk. The evolution of a massive star is so rapid that the star lives most of its life, surrounded by the remnants of its protostar shell, often called the gas cocoon.
An example of a cocoon star is the Becklin-Neigebauer object in the Orion Nebula. It is located in the center of a compact and very dense cluster of protostars. Of these, it is the most massive: the star inside the cocoon has a mass of about eight solar. Its luminosity is close to 2,000 solar, and the radiation temperature of the cocoon is about 600 K. Therefore, the Becklin-Neugebauer object was discovered by two astronomers, whose names it bears, in 1966 as a powerful infrared source.
Now we know more than 250 objects of this type. The temperature of their dusty cocoons is 300-600 K. Some of them have almost destroyed the cocoons with their radiation: observations show that their substance is expanding at a speed of 10-15 km / s. A classic example of such a star is the supergiant Kiel at a distance of about 3 kpc from us, immersed in a dense dust nebula Homunculus.
What stars are born
Molecular clouds, these “factories for the production of stars” produce stars of various types. The range of masses of newborn stars extends from a few hundredths of a hundred to 100 masses of the Sun, and small stars are formed much more often than large ones. On average, about a dozen stars are born in the Galaxy each year with a total mass of about five solar masses.
About half of the stars are born single; others form double, triple, and more complex systems. The more components, the less common such systems. Known stars containing up to seven components, more complex have not yet been identified.
The reasons for the appearance of double and multiple stars are quite clear: the initial rotation of the gas cloud does not allow it to shrink into a single compact star. The more the cloud is compressed, the faster it rotates (the known “figure skater effect” which is a consequence of the law of conservation of momentum).
The increasing centrifugal forces during compression first make the cloud flat like a cheesecake, and then pull it into a “melon” and tear it in half. Each of the halves, shrinking further, continues to move in orbit around the common center of mass. If further compression does not break it into pieces, then a double star is formed, and if the distribution continues – a more complex multiple system is born.
Young star bands
Not only individual and multiple young stars, but also their teams are very interested. The young stars are concentrated near the equatorial plane of the Galaxy, which is not surprising: this is where the layer of interstellar gas. On our horizon, young stars of great luminosity and the gas clouds heated by them are located in the strip of the Milky Way.
But if you look closely at the sky on a dark summer night, you can see that in the Milky Way there are some “star clouds”. How real are they and to what extent do they reflect in the evolution of matter? These large groups of young stars are called star complexes. Their characteristic size is several hundred parsecs.
Historically, more compact groups of young stars were first discovered and studied – scattered clusters similar to the Pleiades. These are relatively slitneither groups of several hundred or thousands of stars connected by mutual gravity successfully resist the destructive influence of the galaxy’s gravitational field. Their origin is not controversial: the ancestors of such clusters are dense nuclei of interstellar molecular clouds.