A pre-main sequence star (PMS star, or PMS object) is a star in the stage when it has not yet reached the main sequence. It can be a T Tauri star or FU Orionis star (<2 solar mass) or an Herbig Ae/Be stars (2-8 solar mass).
The energy source of these objects is gravitational contraction (as opposed to hydrogen burning in main sequence stars). On the HR diagram, the pre-main sequence stage of stars with masses>0.5 Solar mass translates into a move along Hayashi tracks (almost vertically down) and later along Henyey tracks (almost horizontally to the left, towards the main sequence).
PMS stars can be differentiated from dwarf stars by using stellar spectra to measure the correlation between gravity and temperature. A PMS star will be "puffier" than a main sequence star.
Until the surrounding matter is falling onto the central condensation, it is considered to be a protostar. When the surrounding gas/dust envelope disperses and accretion process stops, the star is considered as pre-main sequence star. Pre-main sequence stars become optically visible after stellar birthline. Pre-main sequence stage will last less than 1% of a star's life (in contrast, the star will spend about 80% of its life on the main sequence).
It is believed that during this stage all stars have dense circumstellar disks, probable sites of planet formation.
Star FormationLast Updated on 2007-12-13 00:00:00
The formation paradigm for low and intermediate mass (less than roughly 8 times the mass of our own sun) has been assembled over many years through the combination of observational and theoretical work. In isolated star formation, which takes place in the nearest molecular clouds such as Taurus and Ophiuchus, a cold clump of gas and dust collapses under self-gravity and forms a central star, still surrounded by its birth cloud. Due to conservation of angular momentum, some of the material collapsing onto the star forms a flattened disk, while other material falls directly onto the star from a surrounding envelope. At this stage, it is commonly accepted that a high velocity jet forms, which ejects only a small mass of material, but serves to rid of the star of much of its initial angular momentum and in the case of energetic outflows, injects energy back into the cloud. After roughly 1... More »
Sun: FormationLast Updated on 2007-11-26 00:00:00
About 4.6 billion years ago, an anonymous interstellar cloud began to collapse, forming many dense cores of dust and gas hidden under a thick, obscuring haze. The cores, perhaps a light year across, continued their slow gravitational implosion. As they spun faster and faster, their shapes changed from roundish globules to flattened pancakes and disks, with most of their mass falling into a central dense ball of contracting gas. Within a million years, the central core temperatures climbed above 10 million degrees and hydrogen atoms began to fuse into deuterium and helium at an accelerated pace. The enormous outward pressure provided by thermonuclear fusion quickly halted the further contraction of the fetal sun, and our sun became a full-fledged star for the first time. But the activity did not cease. As vestigial gas and dust continued to fall into the central core of the orbiting... More »
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