This post continues a discussion (by the Mad Galaxy Man) about extinction in the
Coalsack, which started in the recent Deep Sky imaging forum thread "Widefield Crux and Centaurus".
In astronomy, the descriptor "Extinction" refers, in a very precisely defined way, to the very gradual
dimming of light as it passes through an interstellar cloud of dust and gas. Many of you will be aware that the amount of extinction (for instance, as can be measured in magnitudes) is rather small in many parts of the the
Coalsack, because background stars are easy enough to see in many photographs of this dark nebula.
As explained by DCB Whittet (1992) in his useful monograph "Dust in The Galactic Environment" (an ideal book for a physically oriented non-physicist like me......), it should be understood that
extinction of light by interstellar dust is NOT exactly the same as
absorption of light. The light from a star or other background object is reduced in intensity as it passes through a medium containing small particles, and this process IS known as extinction. But this extinction of light (in popular terms "dimming") is actually composed of two different processes:
- absorption
- scattering
In the physical process of
absorption, a photon hits an interstellar dust particle and the particle is thereby heated, usually converting the energy of the absorbed light into light of another wavelength (normally, the energy is re-emitted in the infrared, which is not accessible to the human eye).
In the process of
scattering, a photon is deflected away from its initial trajectory by an intervening dust particle (as with absorption, this process causes a loss of light intensity from the point of view of a person looking at a background object through an intervening cloud of interstellar dust).
In summary, the process of
extinction is composed of
two processes:
absorption and
scattering. Astronomers usually talk of
interstellar extinction when they talk about stars or galaxies being dimmed by the intervening dust particles.
Visual extinction is the usual jargon that is used by professional astronomers to refer to the amount of extinction, measured in
magnitudes, when we observe light at visible wavelengths (e.g. around 550nm).
Note: The amount of extinction by interstellar dust particles, which are usually 0.2 microns or less in diameter, drastically
decreases with
increased wavelength of observation.
I think that this explanation will do, for now, as I am discussing astronomy rather than basic physics! (Anything to do with the interstellar medium gets
really complex, beyond a "bare bones" explanation)
Anyhow, in the Deep Sky Imaging post that I mentioned, I presented a simple map of how extinction varies at different points over the face of the
Coalsack. This map was reproduced from the pioneering (late 1980s) study of
Coalsack extinction by Seidensticker and Schmidt-Kaler. They found that visual extinction varies between zero (nearly zero) and two magnitudes, at various points over the face of the
Coalsack, though recent workers tend more towards estimating that visual extinction varies between 1 and 3 magnitudes.
Here, I present a more sophisticated map of visual extinction in the
Coalsack, reproduced from L.Cambresy (1999), A&A, Vol.345, p.965.
( Note: this map gives visual extinction in magnitudes, as corresponding to the greyscale at right, but with contours of equal extinction (iso extinction contours) superposed )
This map is representative of a lot of more recent work than that of Seidensticker and Schmidt-Kaler;
with a few small areas of the dark nebula having extinction values of some 6 magnitudes, though with much of the nebula having an extinction value of only 1-2 magnitudes.
cheers,
bad galaxy man
