In reply to Veeger's question.
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If stars often form in collapsing hydrogen clouds when sufficient mass and thermal energy are accumulated, why is it beyond understanding to think stars may form in hydrogen clouds collapsing into SgrA's accretion disk?
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Hi Veeger,
The different clusters of young massive, short lived stars are located very close the Milky Way’s black hole. In that region of space, very, very close to a “black hole” the gravitational field from the massive "black hole" creates tidal forces which will pull apart a gas cloud. As noted in the attached quote, for a gas cloud to have sufficient internal gravitational forces to resist the black hole tidal forces, the density of the gas cloud would need to approach that of the density found in the outer atmospheres of stars which does not seem possible. I.e. A massive gas cloud would have needed to have collapse to form a super density gas cloud. The BH tidal force would have rip the gas cloud apart before it could have theoretically collapse. As noted in the comment, there are other known mechanisms that limit the size of a gas cloud that can collapse. (There is another problem/paradox in that the number of stars found concentrated in this region of space is very very high.) The mechanisms that limit how fast a gas cloud can collapse also limit the number of stars that can be formed in one area of space.
Comments:
1) There are other mechanisms (even if there was not a black hole in the immediate vicinity of the stars in question) that limit the size of the gas cloud that can collapses to such high densities. The collapse of the gas cloud is adiabatic, so the temperature of the gas must increase, which limits the maximum density of the collapsing cloud. (The gas can radiate which helps it cool, however the entire massive gas cloud cannot collapse, as the increase in temperature in one part of the gas cloud causes other of the cloud to expand.) Also when a star starts to form, the energy produced by stellar nuclear reactions heats the gas cloud causing it to expand and dissipate.
2)The stars in question do not form in the black hole accretion disc. The accretion discs' temperature is too high for stars to form in it.
http://arxiv.org/abs/0704.1281v1
The Galactic Center by R.Genzel and V. Karas
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If there is indeed a central black hole associated with Sgr A* the presence of so many young stars in its immediate vicinity constitutes a significant puzzle (Allen & Sanders 1986; Morris 1993; Alexander 2005). For gravitational collapse to occur in the presence of the tidal shear from the central mass, gas clouds have to be denser than ~10^9(R/(10′′))^−3 hydrogen atoms per cm^−3. This ‘Roche’ limit exceeds the density of any gas currently observed in the central region. Recent near-diffraction limited AO spectroscopy with both the Keck and VLT shows that almost all of the cusp stars brighter than K~16 mag appear to be normal, main sequence B stars (Ghez et al 2003; Eisenhauer et al 2005a). If these stars formed in situ, the required cloud densities approach the conditions in outer stellar atmospheres.
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