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
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
- 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
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
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 LimitI 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)