Molecular clouds within

The matter that made up the solar nebula from which the solar system was formed already was the product of stellar birth, aging and death, yet the Sun is 4.5 billion years old and will perhaps live to be 8 billion years but the Universe is thought to be 15 billion years old (15 Gyr) suggesting that perhaps we are only in the second cycle of star evolution. It is possible, however, that the massive clouds of H atoms, formed in the close proximity of the early Universe, rapidly formed super-heavy stars that had much shorter lifetimes and entered the supernova phase quickly. Too much speculation becomes worrying but the presence of different elements in stars and the subsequent understanding of stellar evolution is supported by the observations of atomic and molecular spectra within the light coming from the photosphere of stars. 

The general extent of the molecular cloud can be mapped within the sky but the physical conditions and stellar activity lead to different chemical regimes, all of which must be considered if the chemistry of the ISM is to be understood. A... 

Giant molecular clouds the GMCs have a lifetime of order 106—10s years and are the regions of new star formation. The Orion nebula (Orion molecular cloud, OMC) is some 50 ly in diameter and 1500 ly from Earth. The temperature within the cloud is of order 10 K and the atomic density is 106 cm-3. The chemical composition is diverse and contains small diatomic molecules, large polyatomic molecules and dust particles covered with a thick ice mantle. 

Cycle of star formation The collapse of a giant molecular cloud forms a star nuclear synthesis within the star produces more elements the star ages and ultimately dies in a supernova event elements are thrown into the interstellar medium to form a giant molecular cloud. 

Spectral mapping Using a known transition in a molecule, such as the 155 GHz transition in CO, to map the column density or concentration of the molecule within a giant molecular cloud. 

In truth, star formation from molecular clouds is no easy subject to study. This is because the processes involved change the density from 10 g cm to about 1 g cm within a space of only a few tens of millions of years. Only the force of gravity, whose long range plays a key role, is able to produce such staggering compression rates. 

The greater the difference in electronegativity between two atoms, the more polar the bond is that forms between them. Imagine the electrons in the bond as being spread out into a cloud within the molecular orbital. In polar bonds, the cloud is denser in the vicinity of the more electronegative atom. In nonpolar bonds, like those formed between atoms of the Scime element, the cloud is evenly distributed between both atoms. Polar bonds have more ionic chciracter, whereas nonpolar bonds have more covalent character. Here s how to distinguish the chciracter of a bond ... 

Two types of models have been proposed that use this general picture as the basis for understanding volatile depletions in chondrites. Yin (2005) proposed that the volatile element depletions in the chondrites reflect the extent to which these elements were sited in refractory dust in the interstellar medium. Observations show that in the warm interstellar medium, the most refractory elements are almost entirely in the dust, while volatile elements are almost entirely in the gas phase. Moderately volatile elements are partitioned between the two phases. The pattern for the dust is similar to that observed in bulk chondrites. In the Sun s parent molecular cloud, the volatile and moderately volatile elements condensed onto the dust grains in ices. Within the solar system, the ices evaporated putting the volatile elements back into the gas phase, which was separated from the dust. Thus, in Yin s model, the chondrites inherited their compositions from the interstellar medium. A slightly different model proposes that the fractionated compositions were produced in the solar nebula by... 

Collapse of a gas pocket within the omau molecular cloud star ... 

Sequence of events (top to bottom) affecting low-mass stars like our Sun, formed in the vicinity of high-mass stars within molecular clouds. After Hester and Desch (2005). 

Fig. 5. Spectra in the 3200 to 2700 cm-1 range taken in a 5 arcsecond beam at three locations near the ionization ridge in the Orion Nebula. Position 4 is on the ridge between the ionized gas (HII region) and neutral molecular cloud while the positions 10 and 20 arcseconds south are within the molecular cloud. The dotted lines indicate the broad component, the dashed line the presumed continuum. Wavelengths, in microns, of the features are indicated in the top panel (Figure is from Reference [46])...

With reference to absorption spectroscopy, we deal here with photon absorption by electrons distributed within specific orbitals in a population of molecules. Upon absorption, one electron reaches an upper vacant orbital of higher energy. Thus, light absorption would induce the molecule excitation. Transition from ground to excited state is accompanied by a redistribution of an electronic cloud within the molecular orbitals. This condition is implicit for transitions to occur. According to the Franck-Condon principle, electronic transitions are so fast that they occur without any change in nuclei position, that is, nuclei have no time to move during electronic transition. For this reason, electronic transitions are always drawn as vertical lines. 

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