#main sequence

Interstellar space is not totally void. In fact, the medium is a complex mix of gasses that are in several phases: ionized, atomic, and molecular. There are giant molecular-cloud complexes and small globules, tenuous hot gasses and wisps of cool atomic gas, and shell-like remnants from ancient explosions of supernovae or winds from massive stars. Molecular clouds are vast collections of gas and dust that can be 10,000 times as dense as the interstellar average, possessing more than 10 million atomic particles per cubic yard. That may sound like a lot, but compared to the Earth's atmosphere, it's basically a hard vacuum. Still, these wispy clouds are dense enough to ensure that their atoms collide more often than not as they are gently buffeted by the density waves that help shape the spiral arms of their galaxy, by gravitational interactions, and by shock waves from supernova explosions. Colliding atoms lead to the formations of molecular hydrogen and more complex molecules, including carbon monoxide, water, formaldehyde, and even amino acid glycine. In the core of one huge molecular cloud in Orion, researches have detected the spectral signatures of more than two dozen different compounds. How do stars - those dense concentrations of matter - coalesce from such insubstantial material? For stars to form, such clouds must collapse under their own gravity to produce extreme densities necessary to trigger nuclear fusion. A cloud's initial collapse depends on many factors, including its density, temperature, mixture of gasses and the effects of nearby magnetic fields. If a cloud is too warm, the movement of its constituents can provide a sort of pressure that resists gravitations contraction. Although the creation of molecules from atoms provides a cooling effect, ultraviolet radiation from outside of a cloud can disrupt this process. If enough dust is present,, the ultraviolet light will be blocked somewhat, and atoms readily link into molecules. Molecules, in turn, radiate thermal energy more effectively than atoms, allowing the gas to cool to the point where gravitational forces can dominate. In addition to thermal pressure, magnetic effects also tend to oppose contraction. However, they can be overcome by outside forces, like the compression caused by a passing supernova shock wave or the sudden removal of a magnetic field. Given the complexity of interactions that trigger the initial collapse, nearly all similar clouds end up with same amount of rotation- typically more than enough to break up a coalescing star. As they contract, they must rotate faster and faster, conserving angular momentum just as spinning ice skaters do when pulling in their arms. Often, a contracting cloud splits apart to form binary or multiple star systems. For an isolated star to survive, the contracting cloud must shed more than 99 percent of its angular momentum. One way is through magnetic braking, in which, energy is transferred to the surrounding environment by magnetic fields. Another way is to transfer the energy to its surrounding disk. Such flattened circumstellar disks, the birthplace of future planets, appear to be common around protostars. As the cloud contracts, but before it reaches the density levels required for nuclear fusion to take place, an observable protostar appears. Protostars can be embedded in large nebulae, like the stellar nurseries in Orion, or in much smaller molecular clouds. In any case, once a collapsing cloud contracts to roughly stellar dimensions - that is, a few astronomical units across - the atoms of gas bump into each other frequently enough to produce substantial outward pressure, so the collapse is slowed. At this point, the object can be known as a pre-main-sequence star. Stars on the main sequence, of course, develop cores so dense that hydrogen fuses into helium in a stable process spanning millions to billions of years, depending on their size. Pre-main-sequence stars also generate heat and maintain hydrostatic equilibrium, but not initially as a result of nuclear fusion. Rather, stability is first achieved through a phenomenon known as "Helmholtz contractions." As molecules fall inward and collide with other particles, temperatures increase to a few thousand degrees, producing a pressure that slows the overall collapse and causes the formative star to emit visible radiation. Temperatures continue to rise during this slow but steady contraction until fusion reactions eventually begin in the star's core. A star's evolution before, during, and after the main sequence depends - like so many things in astronomy - on mass. Low mass stars can take more than 100 million years to evolve into the main sequence, while our Sun gets there in a few tens of millions of years. A five-solar-mass star makes the evolutionary jump in just a million years or so. And a star fifteen times more massive than our Sun reaches the main sequence in just a hundred thousand years. As protostars evolve toward the main sequence, they are typically surrounded by dense clouds of gas and dust that can act like cocoons, blocking visible light from sight. These infant stars are seen primarily through infrared emissions and are thus known as "infrared stars." The obscuring dust eventually dissipates, revealing the central star inside. About the Image: Like a troublesome adolescent, a protostar called Herbig-Haro 32 makes room for itself. Plumes of gas, appearing here in green, contain material being shouldered from the protostar's disk into interstellar space - at speeds of nearly 200 miles a second. This behavior is typical of youthful stars. - Ami Kothari Image Credit: NASA and The Hubble Heritage Team (AURA/STScI). Sources: http://hubblesite.org/newscenter/archive/releases/1999/35/image/a/ http://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve/