As has been harped on before, the basic scenario for star formation is roughly the same for all mass stars; it is where stars form (and therefore presumably the trigger) and how the star interacts with its environment which are different. For massive protostars

• the core forms very quickly and, in fact, will form before the entire cloud has time to accrete.
• what happens is that the protostar essentially can become a star, generate large amounts of UV radiation and intense stellar winds which may slow down or even stop the accretion of the cloud before it is completed. Eventually, this violent behavior will disrupt the cloud.
  • Note that this also has the effect of keeping the forming star shrouded until it actually turns-on and becomes a star. Massive stars will appear as stars--they do not appear to go through long protostar phases in visible light, as do T Tauri stars
  • How are forming massive stars studied--not in visible light. They are intense emitters of IR and millimeter radiation and these are the most convenient ways to study these regions.
  • How do young massive stars interact with their environments?
  • Sufficiently hot young stars (O and B) stars, emit large amounts of UV radiation capable of ioniziing hydrogen atoms
    • Wien's Law--W(max) = 29,000,000/T(Kelvins) Angstroms ===> for T > 30,000 Kelvin, W(max) < 1,000 Angstroms.
    • Since it takes 13.6 eV (corresponding to a wavelength of 912 Angstroms), stars with T ~ 30,000 Kelvins are easily hot enough to ionize hydrogen. In fact, stars with T as low as 15,000 Kelvin are also able to ionize hydrogen in large amounts (Why?)

      The hot stars form large regions around them of ionized hydrogen (HII) known as HII regions.

      • Why do HII regions glow red?
      • They glow red because they radiate Hydrogen Balmer beta emission lines, which have wavelengths of 6,563 Angstroms. Hmmm. If they are ionized, how do they emit hydrogen lines?

        Well, the UV + H ===> p + e (ionization process). However, what do the protons and electrons do next? Well, if they collide with one another and stick, we say that they recombine, that is, they form a neutral hydrogen atom. Now, usually, the electron will not be in the ground state of the atom, that is, it will be in one of the higher lying energy levels, n >> 1. So, what happens is that because all electrons are lazy, the electron will try to return to the ground state.

        It usually tries to perform this drop in a cascade, a series of steps. The most prominent line produced is the Hydrogen Balmer beta line.

    • Now, note that high T inside of the HII region causes the pressure to be higher inside of the bubble compared to the outside ===> the HII region expands. This coupled with the intense radiation causes the star forming region to eat its way into the surrounding medium.

      This triggers star formation in the neighboring regions and also tends to try and disrupt the molecular cloud.

    • The disruption eventually wins ===> star formation process actually leads to the demise of the Giant Molecular Cloud (GMC). GMC's are thought to be disrupted after a few percent or so of the mass of the cloud is converted into stars.

    Upper Mass Limit

    I noted earlier that the upper mass limit for Main Sequence stars might be due to the dominance of radiation pressure over gas pressure for stars > 50 - 100 M(sun. There, however, is another possibility having to do with the fact that massive stars turn-on before their clouds have had time to accrete completely.

    The more massive protostars if they form from denser regions will form more quickly and are more luminous ===> accretion process can be arrested

    some work ===> this places an upper limit on the mass of a forming star of 50 - 100 or so M(sun)

Information provided by: http://zebu.uoregon.edu