The "Dust" Component Of The Interstellar Medium Is Mostly Particles Of About What Size?
The Interstellar Medium
Although infinite is very empty and the stars in the Milky way are very far autonomously, the infinite between the stars contains a very diffuse medium of gas and dust astronomers telephone call the interstellar medium (ISM). This medium consists of neutral hydrogen gas (HI), molecular gas (mostly H2), ionized gas (HII), and grit grains. Although the interstellar medium is, by several orders of magnitude, a meliorate vacuum than any physicists can create in the laboratory there is still near of v-ten billion M 
of gas and grit out there, comprising approximately v% of the mass of visible stars in the Milky way.
Neutral Hydrogen Gas
The Galaxy Milky way is filled with a very diffuse distribution of neutral hydrogen gas which has a typical density of most i atom/cm3 (10-241000/cm3). The interstellar medium is far besides absurd to excite the UV or optical transitions of hydrogen, but there is a feature at 21 cm wavelength in the radio produced past the spins (magnetic fields) of the hydrogen atom'southward nuclear proton and orbiting electron. Considering the proton and electron are spinning distributions of electric charge they create minute magnetic fields which interact, creating a small free energy deviation between the state in which the poles are aligned versus counter-aligned. This energy difference corresponds to the energy of radio waves at 21-centimeters. Every one time in a while (about once per 500 years) hydrogen atoms will collide, exciting an atom into the higher energy spin-aligned configuration. It will take as long as thirty million years for the cantlet to jump back to the lower energy state via a spin-flip, emitting 21 cm radio emission.
The neutral hydrogen is distributed in clumpy fashion with cool, denser regions that astronomers call "clouds" but which are more like filaments. These regions have a typical temperature of about 100K and a density between ten--100 atoms/cmthree. Surrounding the clouds is a warmer lower density medium with nigh 0.1 atom/cm3 and T ~ 1000K.
Molecular Clouds
| Insufficiently dumbo (nH2 > 1000 molecules/cm3), cold (T ~ 10K) clouds of molecular hydrogen and dust, known equally molecular clouds or dark clouds are the birthplaces of stars. Nosotros practice not detect molecular hydrogen straight, but infer its characteristics from other molecules, nearly often CO. Over 50 other molecules have been detected including NH3, CH, OH, CS and molecules as complex as ethyl alcohol (C2H5OH - the stuff in whisky) accept been establish in Milky Way molecular clouds. The Horsehead Nebula (Messier Nebulae, Web Nebulae) to the correct is produced by the incursion of a plumage of dust from a molecular cloud, covering the lower half of the image, into a region of ionized hydrogen. A Giant Molecular Cloud (GMC) may have a mass of ten6M and a diameter of 150 50.y. Within the GMCs are warm dumbo corse of order 2-three l.y. in diameter, with T~100K and densities as high every bit n~10vii-ten9 molecules/cm3. It is in these regions where the star-formation process begins. There are thousands of GMCs in the Milky Way, generally on the Spiral Artillery and concentrated toward the Galactic Middle. The full mass of molecular gas is estimated to be virtually equal to, or maybe somewhat less (~25%) than, the mass of Hello gas. |  |
Ionized Hydrogen Regions
Textile left over from the formation of immature, hot stars represents the about spectacular component of the ISM, the ionized hydrogen or HII regions like the Orion Nebula ( Messier Database, Web Nebulae), shown below in this HST mosaic. Here is the Near Infrared view of the Orion Nebula.
Massive O and B stars, recently formed in molecular clouds (recollect - massive stars live fast & dice immature!) ionize the gas left over from their formation heating it to a temperature, T ~ 10,000K and causing it to fluoresce producing an emission-line spectrum.
Ultraviolet photons from four massive stars called the Trapezium in the nebula have sufficient energy to strip the electrons completely abroad from - ionize - hydrogen atoms. This requires a photon of free energy greater than thirteen.6 eV or wavelength less than 912Å in the ultraviolet. If a hydrogen atom absorbs a photon with wavelength less than 912Å the atom is ionized with the "actress" energy going into the kinetic free energy of the electron. Collisions between electrons "thermalize" this energy heating the nebular gas to a temperature of about 10,000K. Collisions between electrons and ions in the gas excite the ions to higher energy levels producing emission features of O+,O++,N+, Southward+, etc. as shown in the higher up spectrum. Electrons recombining to upper level in hydrogen and helium cascade through many energy levels down to the ground state producing the emission features of H & He.
The Orion Nebula is a chimera on the side of a much larger Behemothic Molecular Deject complex. The GMC contains a lage cluster of newly formed stars
Curlicue downwardly this page to run across a pick of HST images of Orion showing circumstellar disks surrounding pre-master sequence stars of solar mass.
| Principal Constituents of the ISM | ||||
|---|---|---|---|---|
| Total Mass (M ) | "Deject" Mass (M ) | Density (cm-3) | Temperature (1000) | |
| Howdy gas | ~5 x 109 | 0.1-10 | 100-1000 | |
| H2 gas | 1-v x 109 | 105-tensix | 10three-10five | ~10 |
| Dust | ~5 x tenvii | ~forty | ||
| HII gas | 100-1000 | xthree-104 | x,000 | |
Interstellar Dust
About 1% of the mass of the ISM is in the form of tiny grains of dust about the size of particles of cigarette smoke. Nosotros have already described how this dust obscures the plane of the Milky Way from our view. We know something about the characteristics of this dust from the way that it scatters visible and ultraviolet photons. The outcome of dust is to dim the lite from distant objects (interstellar extinction) in the Galaxy and redden the colors (interstellar reddening) because red light is not scattered as efficiently as bluish low-cal.
A graphical exhibition of the furnishings of dust at different wavelengths is shown by the visible and infrared images of the constellation Orion above. Dust scatters and obscures visible wavelengths where stars emit almost of their light (note Betelgeuse the bright red behemothic at Orion'southward left shoulder). Grit is largely transparent in the infrared, just at temperatures of about 40K, emits strongly at wavelengths between 50-100 
one thousand.
We know that grit grains are elongated, perhaps needle shaped, with sizes of well-nigh 1000Å, about the wavelength of lite that the grains scatter nearly efficiently. Dust characteristics vary somewhat from identify to place in the Galaxy, merely a typical grain is believed to be composed of carbon in a graphite-like crystal construction, mixed with silicates (eg MgSi0three like olivine). Virtually all of the elements like Carbon and Silicon in the ISM are tied up in grit. In molecular clouds the grains appear to exist coated with a water-ice shell. Ned Wright at UCLA has developed a fractal model for interstellar dust grains shown in this APOD.
Ned Wright's Fractal Dust Model
Reflection Nebulae
Dust is responsible for the blue haze around the Pleiades star cluster (Messier Database, (Spider web Nebulae); this nebulosity is called a reflection nebula resulting from blue light from the hot B-stars being scattered toward united states of america from dust surrounding the cluster stars.
The Pleiades Star Cluster
Credit & Copyright: D. Malin (AAO), AATB, ROE, UKS Telescope
Why is the sky blue?
Supernova Remnants
Supernova remnants similar the Crab Nebula enrich the ISM with elements heavier than helium as the expand into the ISM with speeds of several thousand km/southward.
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Last updated: 26 April 1999
The "Dust" Component Of The Interstellar Medium Is Mostly Particles Of About What Size?,
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