by Chris Peterson » Tue Jun 12, 2012 2:27 pm
flash wrote:I presume that the physical meaning of "dissipated by their hostile environment" is that the solids in the dust are being vaporized by the intense ultraviolet radiation, that the resultant gas molecules are then accelerated by the radiation away from the bulk of the globules, and that the globules are visible because they are opaque to the radiation due to their composition (solid dust particles), but that the resultant gas is transparent. Is this the case?
No. Nothing is being vaporized. The dust remains dust, and the gas remains gas. The energy from the nearby stars is just dissipating the dust. Where it's dense, we don't see through it; where it's thin, we do.
I'm confused by this because It seems to me that we see evidence of gas out there all the time: some of it blocks radiation when it becomes ionized and absorbs energy, and some of it glows when it gives this energy back as it becomes de-ionized. So the notion that solids block radiation and gasses transmit it seems too simple. What, exactly, is meant by "dissipated" in such descriptions, and what, exactly, is going on at the surfaces of these globules?
Gases certainly absorb some radiation, especially in narrow bands that the atoms are "tuned" for. But they do not do this efficiently; most radiation will pass unaffected through many light years of gas clouds. Dense dust clouds, however, efficiently absorb continuum radiation over wide portions of the spectrum.
What is happening at the surface of the globules is similar to evaporation in a fluid (indeed, the dust regions act as fluids in many respects). Because the clouds are opaque, the incident energy is absorbed at the surface and does not have a significant effect on the interior. The energized outer layer is rapidly ejected by simple dissipation (the particles absorb energy which increases their momentum; those with natural velocity vectors away from the cloud are lost, since the particles are moving faster than the escape velocity of the gravitationally bound dust cloud).
[quote="flash"]I presume that the physical meaning of "dissipated by their hostile environment" is that the solids in the dust are being vaporized by the intense ultraviolet radiation, that the resultant gas molecules are then accelerated by the radiation away from the bulk of the globules, and that the globules are visible because they are opaque to the radiation due to their composition (solid dust particles), but that the resultant gas is transparent. Is this the case?[/quote]
No. Nothing is being vaporized. The dust remains dust, and the gas remains gas. The energy from the nearby stars is just dissipating the dust. Where it's dense, we don't see through it; where it's thin, we do.
[quote]I'm confused by this because It seems to me that we see evidence of gas out there all the time: some of it blocks radiation when it becomes ionized and absorbs energy, and some of it glows when it gives this energy back as it becomes de-ionized. So the notion that solids block radiation and gasses transmit it seems too simple. What, exactly, is meant by "dissipated" in such descriptions, and what, exactly, is going on at the surfaces of these globules?[/quote]
Gases certainly absorb some radiation, especially in narrow bands that the atoms are "tuned" for. But they do not do this efficiently; most radiation will pass unaffected through many light years of gas clouds. Dense dust clouds, however, efficiently absorb continuum radiation over wide portions of the spectrum.
What is happening at the surface of the globules is similar to evaporation in a fluid (indeed, the dust regions act as fluids in many respects). Because the clouds are opaque, the incident energy is absorbed at the surface and does not have a significant effect on the interior. The energized outer layer is rapidly ejected by simple dissipation (the particles absorb energy which increases their momentum; those with natural velocity vectors away from the cloud are lost, since the particles are moving faster than the escape velocity of the gravitationally bound dust cloud).