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Dense Interstellar Clouds - continued

The Orion Nebula is a star-forming region located in the constellation Orion, the Hunter, about 1500 light years away. The optically visible nebula is excited by one of the young massive stars that formed here about one million years ago together with thousands of lower mass stars. Many of the low mass stars are still surrounded by disks of placental cloud material of gas and dust that formed during the protostellar collapse. Using the Hubble Space Telescope, various of such protoplanetary disks have been detected in silhouette against the nebular emission background. The above mosaic shows several examples. In the bottom left insert the relative size of our own solar system is shown for comparison. The discovery of protoplanetary disks around other stars provides strong evidence for the paradigm of solar system first proposed by Kant and Laplace.

About 1% per mass of the interstellar medium is in the form of solid dust grains, which may be carbon or silicon-based. Dust grains act as catalytic surfaces throughout the interstellar medium, and in general show dimensions on the sub-micron scale. The starlight is absorbed and scattered by dust grains and reaches the observer dimmed, a process referred to as extinction. The extinction-curve of the interstellar medium represents a superposition of the wavelength-dependent extinction properties of different dust particles. Dust particles in diffuse clouds and circumstellar envelopes can be composed of silicates, amorphous carbon (AC), hydrogenated amorphous carbon (HAC), diamonds, organic refractories, and carbonaceous networks such as coal, soot, graphite, quenched-carbonaceous condensates (QCC), and others. The dust size distribution could be inferred from astronomical observations in the UV, VIS, and IR. A three-component model of interstellar dust proposed suggests the coexistence of big grains (silicates with refractory mantles), very small grains (carbonaceous) and polycyclic aromatic hydrocarbons (PAHs).

Dust grains form in the cool expanding circumstellar environment of evolved stars. Stellar winds inject dust into the ambient interstellar medium, which is then distributed by supernovae shock waves over large scales through the ISM. During this period, dust particles cycle several times through dense and diffuse clouds, which allows efficient mixing and processing of interstellar dust. UV irradiation and cosmic rays, together with processes such as grain-grain collisions, sputtering, and grain growth, alter and destroy dust in interstellar and circumstellar regions. Therefore, grains probably retain only traces of their origin, as evidenced by the isotopically anomalous composition of presolar grains found in meteorites. Recent observations suggest that the abundance of carbon in the interstellar medium is only two thirds of its solar value. This poses problems for many recent dust models, because only a limited amount of carbon is available for the dust phase.

Life, as we know it, is based on carbon. In the early Universe only light elements, such as H and He (and traces of other light elements) were formed. The formation of heavier elements had to await the formation of stars. Nucleosynthesis of heavy elements in stars, such as carbon, allowed the formation of organic molecules, which are currently widespread in our Galaxy and beyond. Biogenic elements (H, C, N, O, S, P) and organic matter are today some of the major constituents of the Universe.

The only known life in the Universe resides on a planet orbiting a G-type star. Stars like the Sun are born in dense molecular clouds, and these also provide the initial organic inventory available to protostellar disks for the formation of planets.

The interstellar medium, with its molecules and dust particles, represents the raw material for forming future generations of stars which may develop planetary systems like our own. The discoveries of protoplanetary disks around other stars show that our solar system is no longer the only known example of a planetary system in the Universe. This is supported by the large number of detection exoplanets circling other stars.

Fig. 2: The picture shows the Orion gas nebular which is also a close-by star-forming region.

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