‘Formation of the star, probably the most familiar astronomical topic amount the public, was once though to be a consequential event; however, this process involves many important but insignificant processes and balances, that is essential to the formation of the stars.’

‘When I was manufactured in the Innovation Cor., my chipset is preloaded with infomation about the formation of stars. Originally, I can't think of any posible usage of this infomation, but now, the chance of application has finally come.'

‘Then congratulations! Within cosmology, the formation of stars and planets, which will be discussed later, are relatively well-understanded and predominantly studied area. Because of the high abundance of stars just within the Milky Way, studying formation of stars is quite convenient.’

EVOLUTION OF THE UNIVERSE>FORMATION OF THE STARS>MOLECULAR CLOUD

The raw material for star formation is interstellar gas. The Milky Way galaxy contains about 2 billion solar masses of diffuse gas, some 5 percent of its stellar mass. The density of a typical interstellar gas cloud would be similar to that of a perfect vacuum on the earth, perhaps with a hundred atoms in every cubic centimeter. Individual clouds extend for many light-years, however; cumulatively, they give a detectable signal in different wavelength bands. Interstellar gas was first discovered in the optical part of the spectrum, when binary stars were found to have absorption lines that resulted from intervening sodium and calcium atoms and whose strength did not vary with the binary period. Evidently the absorption was not circumstellar but interstellar, or produced in intervening gas clouds. Hydrogen was discovered to pervade inter-stellar space in great quantity as a consequence of its 21-centimeter radiation emitted because of the spin-flip transition; the electron and the proton can have their spin axes either parallel or antiparallel in the hydrogen atom, and there is a tiny energy difference corresponding to a photon of wavelength 21 centimeters between these two states. Hydrogen atoms form in both states, but prefer to end up in the state of lowest energy, the transition to which requires emission of a photon of 21-centimeter wavelength. This transition occurs so infrequently that it was measured for the first time in interstellar space, where in a cloud containing some thousands of solar masses of hydrogen, enough hydrogen atoms radiated at 21 centimeters to give a strong signal. The main constituent of these gas clouds turned out to be hydrogen, which was first detected in the 1950s in interstellar clouds at 21 centimeters. The 1970s heralded a new discovery: about half of the interstellar gas was found to be in the form of molecular hydrogen. High-resolution mapping of interstellar clouds at microwave frequencies during the subsequent decade led to the discovery of many complex molecular species that coexisted with the molecular hydrogen. Such molecules as carbon monoxide, formaldehyde, water, and ammonia are abundant; indeed, there is enough ethyl alcohol in a single molecular cloud to serve a party of enough human beings to populate the entire galaxy. We know that the interstellar medium is a complex mix of gas in several phases: ionized, atomic, and molecular. There are giant molecular cloud complexes and small globules, tenuous very hot gas and wisps of cool atomic gas, and shell-like remnants from ancient explosions of supernovae or winds from massive stars. Molecular clouds are of especial interest because they are the sites of ongoing star formation. Molecular clouds are denser than clouds of atomic hydrogen; the molecules form as a consequence of this high density. About 1 percent of the mass of an interstellar cloud is in the form of small, solid grains of dust. The concentration of these grains in a cloud is dense enough to block out light from background stars and be visible as a black cloud silhouetted against the Milky Way. Many dark clouds, often globular shaped and containing some ten or hundred solar masses of gas have been mapped. These regions are especially rich in molecular species: only when ultraviolet radiation is blocked can molecules form in great abundance. The molecules permit the gas to cool effectively because energy is radiated in microwave photons by excitation of molecular states as well as by the dust grains. Loss of thermal energy means that pressure forces cannot play much of a role in cloud support, and the molecular clouds are destined to collapse and form stars.