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Encyclopedia > Extinction (astronomy)

Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. Extinction arises both from the interstellar medium and the atmosphere. In both cases, blue light is much more strongly absorbed than red light in a process known as reddenning. Lunar astronomy: the large crater is Daedalus, photographed by the crew of Apollo 11 as they circled the Moon in 1969. ... Absorption has a number of meanings: In physics, absorption is a process in which particles of some sort encounter another material and are taken up by or even disappear in it. ... See also Lists of astronomical objects Category: ... Possible meanings: In general, an observer is any system which receives information from an object. ... The distribution of ionized hydrogen (known by astronomers as H II (aitch two) from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earths northern hemisphere (from the Wisconsin H-Alpha Mapper Survey) In astronomy, the interstellar medium (or ISM) is the matter and... Layers of Atmosphere (NOAA) Earths atmosphere is a layer of gases surrounding the planet Earth and retained by the Earths gravity. ... For other uses, see Blue (disambiguation) Blue is one of the three primary additive colors; blue light has the shortest wavelength range (about 420–490 nanometers) of the three additive primary colors. ... Red is a color at the lowest frequencies of light discernible by the human eye. ... In astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which was emitted. ...

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Interstellar extinction

Broadly speaking, interstellar extinction varies with wavelength in a way which is generally surprisingly uniform along different lines of sight within our galaxy, and can be characterised by a standard extinction curve. However, the amount of extinction varies greatly, from almost no absorption at all to many cases where objects are entirely invisible at optical wavelengths and can only be seen in infrared or radio wavelengths. Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of microwave radiation. ...


Superimposed on the standard extinction curve are many small absorption features, which have various origins and can give clues as to the composition of the interstellar medium. One of the most important types of absorption features are the diffuse interstellar bands, about 100 of which are seen in typical stellar spectra. The origin of these bands has been a hotly disputed topic for many years, but current ideas suggest that polycyclic aromatic hydrocarbons may be responsible for most or all of them. The distribution of ionized hydrogen (known by astronomers as H II (aitch two) from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earths northern hemisphere (from the Wisconsin H-Alpha Mapper Survey) In astronomy, the interstellar medium (or ISM) is the matter and... Relative strengths of observed diffuse interstellar bands Diffuse interstellar bands (DIBs) are absorption features seen in the spectra of astronomical objects in the Galaxy. ... Polycyclic aromatic hydrocarbons (PAHs) are chemical compounds that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. ...


Calculating a standard extinction curve

Several methods can be used to calculate a standard extinction curve. One way is by comparing the spectra of stars thought to be very similar, but at different distances. The difference between the shape of the spectra will then be due only to extinction. Another way is by calculating a theoretical unreddened spectrum, and comparing it to the observed spectrum.


The 2175Å feature

One prominent feature in derived standard extinction curves in our galaxy is a broad 'bump' at about 2175Å, well into the ultraviolet region of the electromagnetic spectrum. This feature was first observed in the 1960s, but its origin is still not well understood. Several models have been presented to account for this bump which include graphitic grains with a mixture of PAH molecules. Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. ... Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultrahigh frequency VHF = Very high frequency HF = High frequency...


Measuring extinction towards an object

Once a standard extinction curve has been obtained, the amount of extinction towards an individual object can be determined. With stars, theoretical spectra can be compared to the observed spectra to determine the amount of redenning. In the case of emission nebulae, it is common to look at the ratio of two emission lines which should not be affected by the temperature and density in the nebula. For example, the ratio of hydrogen alpha to hydrogen beta emission is always around 2.85 under a wide range of conditions prevailing in nebulae. Therefore, a ratio other than 2.85 must be due to extinction, and the amount of extinction can thus be calculated. The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... An emission nebula is a cloud of ionized gas (i. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... In physics and astronomy, H-alpha, also written Hα, is a particular emission line created by hydrogen. ...


Extinction curves of other galaxies

The form of the standard extinction curve depends on the composition of the interstellar medium, which varies between galaxies. In the Local Group, the best-determined extinction curves are those of our Galaxy, the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). In the LMC, the 2175Šfeature is less prominent than it is in our Galaxy, while ultraviolet extinction is stronger. In the SMC, ultraviolet extinction is intermediate but the 2175Šfeature is absent. This gives clues as to the composition of the ISM in the various galaxies, and implies that whatever is responsible for the 2175Šfeature is related to the metallicity of the galaxy: the LMC's metallicity is about 40% of that of the Milky Way, while the SMC's is about 10%. The distribution of ionized hydrogen (known by astronomers as H II (aitch two) from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earths northern hemisphere (from the Wisconsin H-Alpha Mapper Survey) In astronomy, the interstellar medium (or ISM) is the matter and... Map of the local group The Local Group is the group of galaxies that includes our galaxy, the Milky Way. ... The Small Magellanic Cloud (SMC) —also known as NGC 292— is an irregular galaxy, sometimes classified as a dwarf galaxy, in orbit around the Milky Way Galaxy. ... The Large Magellanic Cloud (also known as LMC) is a dwarf galaxy that is in orbit around our own Milky Way galaxy. ... In astronomy, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. ... A NASA artists conception of what the Milky Way would look like if seen off-axis. ...


Atmospheric extinction

Atmospheric extinction varies with location and altitude. Astronomical observatories generally are able to characterise the local extinction curve very accurately, to allow observations to be corrected for the effect.


Atmospheric extinction has three main components: Rayleigh scattering by air molecules, scattering by aerosols, and molecular absorption. Molecular absorption is often referred to as 'telluric absorption', as it is caused by the Earth (telluric is a synonym of terrestrial). The most important source of telluric absorption is ozone, which absorbs strongly in the near-infrared. Rayleigh scattering causing a reddened sky at sunset Rayleigh scattering (named after Lord Rayleigh) is the scattering of light by particles much smaller than the wavelength of the light. ... http://visibleearth. ... Look up Synonym in Wiktionary, the free dictionary Synonyms (in ancient Greek syn συν = plus and onoma όνομα = name) are different words with similar or identical meanings and are interchangable. ... For the Moldavian pop group see O-Zone Ozone (O3) is an allotrope of oxygen, the molecule consisting of three oxygen atoms instead of the more stable diatomic O2. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of microwave radiation. ...


The amount of atmospheric extinction depends on the altitude of an object, being lowest at the zenith and at a maximum near the horizon. It is calculated by multiplying the standard atmospheric extinction curve by the mean airmass calculated over the duration of the observation. hi mom ... The zenith, in astronomy, is the point in the sky which appears directly above the observer. ... In meteorology, an airmass or air mass is a large volume of air having fairly uniform characteristics of temperature, atmospheric pressure, and water vapor content. ...


References

  1. Howarth I.D. (1983), LMC and galactic extinction, Royal Astronomical Society, Monthly Notices, vol. 203, Apr. 1983, p. 301-304.
  2. King D.L. (1985), Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma, RGO/La Palma technical note 31
  3. Rouleau F., Henning T., Stognienko R. (1997), Constraints on the properties of the 2175Å interstellar feature carrier, Astronomy and Astrophysics, v.322, p.633-645

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Evolution: Extinction: A Modern Mass Extinction? (461 words)
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