Interstellar absorption

The interstellar absorption indicates the quantity of light absorptive by the interstellar Milieu and the Poussière S along the line of sight of a star given in Astronomie. Absorption is a chromatic effect, since dust diffuses much more the blue light that the red light. For this reason the areas of stellar formation are observed in the field of the infra-red, since in this field absorption is very weak. It is also for the same reason that the sky is blue during the day, and that the Sun is red during the sunsets.

Reddening

When the light of a star is modified by interstellar absorption, it is said that it “is reddened”, and it is this “reddening” which it is necessary to correct to find the intrinsic characteristics of star. Absorption $A_V$ in the visible band (i.e in the neighborhoods of 545 Nm) is obtained in the following way:

$A_V = R \ times E (B-V)$

where “R” is the ratio between total absorption on selective absorption and is called factor of reddening (or very short reddening), and “E (B-V)” is called “the excess of color”. Once “R” is known, the measurement of the excess of color makes it possible to know absorption. Reddening is given by (Olson, 1975, PASP, 87,349-351):

$R = 3.25 + 0.25 (B-V) _0 + 0.05 E (B-V)$

where “B” and “V” indicate the apparent magnitudes in the spectral bands blue and visible, and the index “0” indicates that in fact the intrinsic values (not reddened) must be taken here. These values are tabulées, and known more or less well for all the spectral standard and different the classes from luminosity. The formula above is valid only for $(B-V) _0 < 1.4$ mag and $E (B-V) < 1.5$ mag.

Process of diffusion

Diffusion of Mie
Rayleigh scatter
Polarization

Random links: