The diffuse interstellar medium is composed of matter that fills the space between the stars. Ordinary matter is composed mainly of ionized hydrogen (H+), atomic (H1) and molecular (H2) and dust. Dust is less than 1% of the total mass of the interstellar medium which itself is only 10% of the luminous mass (ordinary matter) of the galaxy. Dust is not a critical component, but the result of stellar explosions that spread recycled atmospheres of stars, in the interstellar medium. Indeed, stars dying produce material as dust which serves as a base material for the formation of new stars.
Interstellar dust or cosmic dust looks like a blend of grains of 0.1 micron maximum diameter (picture opposite). These interstellar dust grains are actually simple assemblies of molecules that become increasingly complex to reach dimensions of 0.1 μ (about 10,000 molecules).
Interplanetary dust grains are larger and can reach sizes of several tens of microns (about one million molecules), moreover they are captured at high altitude in the Earth's stratosphere, with collectors.
Dust also exist in the intergalactic environment and of course even smaller quantity.
Galactic dust grains are much smaller, the size of a few molecules.
The interstellar dust grains recycled in the heart of stars spread back into interstellar space when the star, in later life, extrude layers of material. The material on cooling finds favorable conditions for assembling into molecular clusters. But the dust recycled by stellar machine that we see in the universe or in our solar system is not the same as we know in our homes. Dust in our homes is mostly produced by the erosion of objects in the dust all known elements on Earth, may be present.
While interstellar dust has been necessarily created by a star is even, the residue of its nuclear burning, silicate compounds mainly consisting of carbon, oxygen and silicon. Then the dust released by the star enrich the interstellar medium by mixing with the gas. These grains of solid dust will change depending on the density of the medium. If the cold interstellar medium allows, they are surrounded by a mantle of water ice, in fact, water (H2O) is abundant in the universe because there is a lot of hydrogen (74%) and a lot of oxygen (1%). Everywhere in the universe we find these dust grains that will participate under certain conditions, an interesting chemistry. These grains are able to combine with all the other molecules of the interstellar medium if it is not too shaken by the shock waves. The shock waves caused by supernova explosions are absolutely destructive to these small chemical compounds.
In galaxies, the distribution of dust is obviously not uniform as well we found dust in clouds, distorted by stellar winds, called nebulae. Some nebulae are very bright because they host many nascent stars. Sometimes these huge dust clouds are so dense (100-1000 particles per cubic centimeter) they absorb much of the electromagnetic radiation from stars located behind it are the dark areas of galaxies, dark nebulae. Unlike gas, dust, which is solid, perfectly hides the stars.
nota: Molecular hydrogen (H2) is formed of two hydrogen atoms chemically associated. Molecular clouds of interstellar nebulae whose density allows the formation of H2. The H2 molecule is not easily detectable but scientists have a tracer that allows us to say that there is molecular hydrogen, this tracer is carbon monoxide CO. Indeed, the relationship between the brightness of CO and H2 mass is almost constant.
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Image: This interplanetary dust captured at high altitude in the stratosphere is mainly composed of ice, carbon and micro silica grains. It measures 10 microns wide, or one-tenth the diameter of a human hair. The collection of these particles is done through dust collectors embedded in space satellites specially adapted for this purpose. Dust that walks in the solar system comes mainly from comets and asteroids and date of beginning of the formation of the Sun. Today, under certain conditions we can see a glow produced by the reflection of sunlight on dust particles of the zodiacal disk, this glow is called the zodiacal light. This disk of dust pushed by solar radiation, stretches between Mercury and Mars. The amount of dust particles needed to produce the zodiacal light is extremely low, a few particles per km3.
Interstellar dust is found much too far into space to send our space satellites. Credit NASA.
Seeing all this dust, it's hard to believe that there is only a few tens of atoms per cubic centimeter, in clouds of interstellar space, but space is immense.
The size of the Lagoon Nebula is about 110 light-years, that's a lot of atoms.
The walls of dust are dimly lit by the stars behind as these small solid particles absorb the light perfectly. However telescopes come to break these cosmic wonders by long exposures that reveal the full range of colors, especially the amazing pink hydrogen.
This is William Hershel (1738-1822) who discovered the dark nebulae, clouds of gas and dust without light.
Image: In one picture the splendor of the center of the nebula the lagoon or M8 or NGC 6523. Nebula was discovered in 1747 by Guillaume Le Gentil. Here, interstellar dust spreads over almost the entire image, leaving just see the bright star Herschel 36 that illuminates this region. This vast wall of dust cover many hot young stars barely visible in transparency.