Extinction (astronomy)
Extinction, sometimes called reddening, is variously known as galactic extinction, intersellar extinction, and atmospheric extinction when it happens within Earth's atmosphere. It usually happens when dust - either in space or in the earth's atmosphere - dims light, mainly by absorbing and scattering (short) blue light waves, allowing (longer) red light waves through, making stars appear redder than they are, like when dust particles in the atmosphere of earth contribute the red effect of sunset.[1]
Atmospheric extinction
Atmospheric extinction is the reduction in brightness of steller objects as their light passes through Earth's atmosphere. The longer the path length through the atmosphere, the more the starlight is dimmed, hence a star will appear brighter when high in the sky than when it is close to the horizon. The path length through the atmosphere is known as 'air mass'. If you look straight up, that is the shortest air mass. There are mathematical calculations that can be made with regard to atmospheric extinction.[2][3] There are three main factors that cause atmospheric extinction. Molecular absorption, mainly due to atmospheric ozone and water, accounts for about 0.02 magnitudes per air mass. Molecular scattering, known as Rayleigh scattering, by air molecules accounts for up to 0.14 magnitudes per air mass. Rayleigh scattering is the cause of the blue sky - the average colour of scattered sunlight is the blue of the sky.[4] Aerosol scattering (by dust, water and manmade pollutants) adds about 0.12 magnitudes per air mass. There is less extinction in winter than in summer, believed to be due to less water in the atmosphere. Extinction scatters blue light more than red, and so causes a reddening effect with stellar objects, the lower in the sky a stellar object is, the more extinction, causing the light to appear redder.
Galactic extinction
The concept of galactic extinction is believed to have originated from Åke Anders Edvard Wallenquist.[5] Starlight is dimmed and reddened by galactic dust (absorbing clouds).[6]) The Galactic extinction causes a non uniform selection effect across the sky.[7]
Absorbing clouds
The composition of absorbing clouds is deduced using dust spectroscopy.[8] The absorbing clouds and their extinction effect are subject to continuing study.[9]
Effect of extinction on redshift
Various claims are made regarding the effect of extinction on redshift. Extinction was shown to have an effect on redshift in The Optical Redshift Survey II.[10] Redshift is believed to be intrinsic[11] and distance related. A fringe theory by Lin Jung Wan suggests that extinction is largely responsible for redshift.[12] Mainstream science holds that the effect of extinction on redshift is small.[13]
References
- ^ http://www.astro.virginia.edu/class/skrutskie/astr121/notes/redden.html
- ^ http://www.asterism.org/tutorials/tut28-1.htm
- ^ http://star-www.rl.ac.uk/star/docs/sc6.htx/node15.html
- ^ http://mintaka.sdsu.edu/GF/explain/optics/scatter.html
- ^ http://adsabs.harvard.edu/cgi-bin/bib_query?1930LicOB.420..154T Attribution to Wallenquist on Page 165
- ^ http://adsabs.harvard.edu/full/1937AnHar.105..359G
- ^ http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.80 Page 18
- ^ http://ned.ipac.caltech.edu/level5/Sept07/Li2/Li3.html
- ^ http://iopscience.iop.org/1538-3881/125/1/98/pdf/1538-3881_125_1_98.pdf
- ^ http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.80
- ^ http://iopscience.iop.org/0004-637X/518/1/103/pdf/0004-637X_518_1_103.pdf Page 104
- ^ http://www.utc.edu/Faculty/LingJun-Wang/RedshiftEssay.pdf
- ^ http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2010.16464.x/abstract
Further reading
- Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 0691004021.
{{cite book}}: Unknown parameter|lastauthoramp=ignored (|name-list-style=suggested) (help) - Howarth, I. D. (1983). "LMC and galactic extinction". Royal Astronomical Society Monthly Notices. 203: 301–304. Bibcode:1983MNRAS.203..301H.
- King, D. L. (1985). "Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma". RGO/La Palma technical note. 31.
- Rouleau, F.; Henning, T.; Stognienko, R. (1997). "Constraints on the properties of the 2175Å interstellar feature carrier". Astronomy and Astrophysics. 322: 633–645. arXiv:astro-ph/9611203. Bibcode:1997A&A...322..633R.