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Old October 1st 16, 12:37 PM posted to sci.astro.research
Steve Willner
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Default Luminosity of gas/dust in galactic spiral arms?

In article ,
writes:
Question, if I look toward the center of the galaxy, in the direction
of a gas cloud blocking my view (ie, not necessarily from sun/ earth
location) will the gas cloud change the apparent luminosity of the
central galaxy?


I'm not sure exactly what you are asking, but for ultraviolet, visible,
and infrared light, _dust_ attenuates light. For the Milky Way (and
probably most galaxies), the extinction is very patchy. One region of
relatively low extinction is "Baade's Window." The region around
"Sagittarius B2" has very high extinction. The exact center of the
galaxy ("Sgr A" in general or "Sgr A*" in particular) is between.

One good image is at
https://commons.wikimedia.org/wiki/F...Way_Galaxy.jpg
You can find many more if you search the web.

The extinction is strongly wavelength-dependent. Towards Sgr A*, pit's
about 25 magnitudes in visible light, about 2.5 magnitudes at an
infrared wavelength of 2.2 microns, and generally less at longer
wavelengths. Do a web search on "extinction curve" or "reddening curve"
for details of wavelength dependence.

Gas is important at radio wavelengths, but the gas has to be both
ionized and dense to have much effect except at wavelengths of specific
transitions such as the 21 cm hydrogen line.

It seems to me the gas will absorb and then re radiate energy in
all directions,


Substitute "dust" for "gas," and you'll be right. There are actually
two effects: "scattering," in which a photon is redirected, and
"absorption," in which the photon is absorbed, heats the dust grain, and
the grain then reradiates the energy in the infrared. The effects
depend on grain size, shape, and composition and on the wavelength of
incoming light. Try a web search on "Mie theory" if you want to learn
details of how grains interact with light.

Both scattering and absorption contribute to interstellar polarization.

so that, along a continued radial line, the apparent
luminosity will be smaller than if the gas / dust cloud were not
there. The gas cloud will shift power to longer wavelengths,
granted. But won't the total apparent power also be reduced?


Overall energy is conserved, of course, but the light is redirected out
of the line of sight to the observer. Therefore the object looks dimmer
than it otherwise would.

The effects of dust were recognized in the early 20th century and led to
a major revision in interstellar distance estimates.

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