Trumpler (1930) demonstrated the existence of interstellar absorption by comparing luminosity distances and angular diameter distances of a sample of open clusters.
Trumpler’s main conclusions:
On that basis,
A more appropriate term would be smoke: indeed, the particle sizes are much smaller than in terrestrial dust.
Trumpler's comparison of the luminosity and diameters distances for 100 open clusters. Credit: Trumpler (1930).
Extinction by small particles.
Light absorption along a line of sight the relation between apparent and absolute magnitude is:
where is the extinction.
Exctinction is proportional to the distance travelled by light:
Extinction depends strongly on the direction (up to 30 mag towards Galactic center).
Dust extinction. Extinction is due to dust grains that have diameters about the wavelength of the light.
Gas extinction by scattering, but its scattering efﬁciency per unit mass is much smaller.
Extinction by ISM particles:
Geometrical cross section of spherical particle : .
The extinction cross-section of the particles:
where is the “extinction efﬁciency” factor.
Volume element with length and cross section ; particle density . Number of particles in the volume element: .
Extinction and optical depth:
Total optical depth:
where : mean particle density along the line of sight to the source at distance .
Evidence of interstellar dust comes from interaction with starlight and from emission of light.
Interaction with incident starlight
Radius of dust grains:
Evidence from emitted light
Dust clouds can be revealed by means of light emitted by dust grains.
A complex ISM system with blue reflection nebulae (NGC 6726/6727 & IC 4812) and a dark nebula. Credit: Martin Pugh.
Less direct evidence for dust in the ISM:
Extinction curves for our Galaxy compared with the Magellanic Clouds.
LMC: 2175 Å bump and stronger far-UV extinction in the region associated with the LMC2 supershell.
SMC: no 2175 Å and very strong far-UV extinction in the star forming Bar.
Link with local metallicity.
In starburst galaxies, the dust lacks the 2175 Å bump.
Average extinction curves for MW, LMC, and SMC, based on Gordon et al. (2003). Credit: Wikipedia Commons.
Aigen Li, “Interstellar grains—the 75th anniversary“, J. Phys.: Conf. Ser. 6, 229 (2005)
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