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According to current astronomical theories, galaxies had their origin in large conglomerates of gas and cosmic dust that turned slowly, fragmenting into turbulent vortices and condensing into stars.
In some regions where star formation was very active, almost all dust and gas went to one star or another. Little or none of this material remained in the intermediate space.
This is true for globular clusters, elliptical galaxies and the central core of spiral galaxies.
This process was much less effective on the outskirts of spiral galaxies. The stars formed in much smaller numbers and there was a lot of dust and a lot of gas. We, the inhabitants of the Earth, find ourselves in the spiral arms of our galaxy and see the dark spots that project the clouds of dust against the glow of the Milky Way. The center of our own galaxy is completely obscured by such clouds.
The material from which the universe is formed consists mostly of hydrogen and helium. Helium atoms have no tendency to join with each other. The hydrogen ones do, but only in pairs, forming hydrogen molecules (H2). It is meant that most of the material that floats between the stars consists of small atoms of helium or small atoms and molecules of hydrogen. All this constitutes interstellar gas, which forms most of the matter between the stars.
Interstellar dust (or cosmic dust) that is present in much smaller quantities, consists of tiny particles, but much larger than atoms or molecules, and therefore must contain atoms that are neither hydrogen nor helium.
The most common type of atom in the universe, after hydrogen and helium, is oxygen. Oxygen can be combined with hydrogen to form oxhydryl groups (OH) and water molecules (H2O), which have a marked tendency to join other groups and molecules of the same type that they find along the way. Thus, little by little, very small particles consisting of millions and millions of such molecules are formed. Oxhydryl groups and water molecules can be part of the cosmic dust.
In 1965, oxhydryl groups were first detected in space and their distribution began to be studied. Since then, the existence of more complex molecules, which contain carbon atoms, as well as hydrogen and oxygen, has also been reported. The cosmic dust must also contain atomic clusters formed by atoms even less common than those of hydrogen, oxygen and carbon. In the interstellar space, calcium, sodium, potassium and iron atoms have been detected, observing the light that these atoms absorb.
Within our solar system there is a similar material, perhaps contributed by comets. It is possible that outside the visible limits of the solar system there is a layer with a large number of comets, and some of them precipitate towards the Sun (perhaps due to the gravitational effects of nearby stars). Comets are loose conglomerates of tiny solid fragments of metal and rock, joined by a mixture of frozen ice, methane and ammonia and other similar materials.
Every time a comet approaches the Sun, part of its matter evaporates, releasing tiny solid particles that spread through space in the form of a long tail. Ultimately the comet disintegrates completely.
Throughout the history of the solar system innumerable comets have disintegrated and the interior space of the system has been filled with dust. Earth collects billions of these dust particles every day. Space scientists are interested in them for various reasons; one of them is that larger micrometeoroids could pose a danger to future astronauts and colonizers of the Moon.
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