Extinction is a term used in astronomy to describe the absorption and scattering of light emitted by astronomical objects by matter (dust and gas) between the emitting object and the observer. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from s are significant physical entities, associations or structures which current Science has confirmed to exist in Space. Observation is either an activity of a living being (such as a Human) which senses and assimilates the Knowledge of a Phenomenon, or the recording of data For Earth-bound observers, extinction arises both from the interstellar medium (ISM) and the Earth's atmosphere; it may also arise from circumstellar dust around an observed object. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Circumstellar Dust is astronomical dust around a Star. It can be in the form of a spherical shell or a disk e The strong atmospheric extinction in some wavelength regions (for example X-ray, ultraviolet, and infrared) requires the use of Space-based observatories. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of Since blue light is much more strongly attenuated than red light in the optical wavelength regions, resulting in an object which is redder than expected, interstellar extinction is often termed reddening. Blue is a Colour, the Perception of which is evoked by Red is any of a number of similar Colors evoked by light consisting predominantly of the longest wavelengths of Light discernible by the human eye in the wavelength In Astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of Electromagnetic radiation

General Characteristics

Broadly speaking, interstellar extinction varies with wavelength such that the shorter the wavelength the stronger the extinction. Superimposed on this general trend are absorption features, which have various origins and can give clues as to the composition of dust grains. Known discrete absorptions features include (but not limited to) the 2175 Å bump, the diffuse interstellar bands, the 3. An ångström or angstrom (symbol Å) (ˈɔːŋstrəm Swedish: ˈɔ̀ŋstrœm is an internationally recognized non- SI unit of length equal Diffuse interstellar bands (DIBs are absorption features seen in the spectra of Astronomical objects in our Galaxy. 1 μm water ice feature, and the 10 and 18 μm silicate features. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI

The general shape of the ultraviolet through near-infared (0. 125 to 3. 5 μm) extinction curve in our own Galaxy, the Milky Way, is fairly well characterized by the single parameter R(V),^[1][2] but there are known deviations from this single parameter characterization. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[3] The R(V) parameter equals A(V)/E(B-V) and is a measurement of the total, A(V), to selective, E(B-V) = A(B)-A(V), extinction. R(V) is known to be correlated with the average dust grain size. For our own Galaxy, the Milky Way, the typical value for R(V) is 3. 1. ^[4] The relationship between the total extinction, A(V), and the amount of hydrogen, N_H = number of hydrogen atoms in a 1 cm² column, gives how the gas and dust in the interstellar medium are related. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 From studies using ultraviolet spectroscopy of reddened stars and X-ray scattering halos in the Milky Way, the relationship

has been determined. ^[5][6][7]

Measuring extinction towards an object

To measure the extinction curve for a star, the star's spectrum is compared to the observed spectrum of a similar star known not to be affected by extinction (unreddened). A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth ^[8] It is also possible to use a theoretical spectrum instead of the observed spectrum for the comparison, but this is less common. 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. An emission nebula is a Cloud of Ionized Gas ( ie a plasma) emitting light of various colors 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 Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different For example, the ratio of hydrogen alpha to hydrogen beta emission is always around 2. In Physics and Astronomy, H-alpha, also written Hα, is a specific Emission line created by Hydrogen at 6562 In Physics and Astronomy, H-alpha, also written Hα, is a specific Emission line created by Hydrogen at 6562 85 under a wide range of conditions prevailing in nebulae. A ratio other than 2. 85 must therefore be due to extinction, and the amount of extinction can thus be calculated.

The 2175 Å feature

One prominent feature in measured extinction curves of many objects within the Milky Way is a broad 'bump' at about 2175 Å, well into the ultraviolet region of the electromagnetic spectrum. An ångström or angstrom (symbol Å) (ˈɔːŋstrəm Swedish: ˈɔ̀ŋstrœm is an internationally recognized non- SI unit of length equal Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies This feature was first observed in the 1960s^[9][10] 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. The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. Polycyclic aromatic hydrocarbons ( PAH s are Chemical compounds that consist of fused Aromatic rings and do not contain Heteroatoms or Investigations of interstellar grains embedded in interplanetary dust particles (IDP) observed this feature and identified the carrier with organic carbon and amorphous silicates present in the grains. ^[11]

Extinction curves of other galaxies

Plot showing the average extinction curves for the MW, LMC2, LMC, and SMC Bar. ^[12] The curves are plotted versus 1/wavelength to emphasize the UV.

The form of the standard extinction curve depends on the composition of the ISM, which varies from galaxy to galaxy. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter In the Local Group, the best-determined extinction curves are those of the Milky Way, the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). The Local Group is the group of galaxies that includes our galaxy the Milky Way. The Small Magellanic Cloud (SMC is a Dwarf galaxy. It contains several hundred million stars The Large Magellanic Cloud (LMC is a nearby Satellite galaxy of our own galaxy the Milky Way. In the LMC, there is significant variation in the characertistics of the ultraviolet extinction with a weaker 2175 Å bump and stronger far-UV extinction in the region associated with the LMC2 supershell (near the 30 Doradus starbursting region) than seen elsewhere in the LMC and in the Milky Way. ^[13][14] In the SMC, more extreme variation is seen with no 2175 Å and very strong far-UV extinction in the star forming Bar and fairly normal ultraviolet extinction seen in the more quiescent Wing. ^[15][16][17] This gives clues as to the composition of the ISM in the various galaxies. Previously, the different average extinction curves in the Milky Way, LMC, and SMC were thought to be the result of the different metallicities of the three galaxies: the LMC's metallicity is about 40% of that of the Milky Way, while the SMC's is about 10%. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply Finding extinction curves in both the LMC and SMC which are similar to those found in the Milky Way^[12] and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC^[18] and in the SMC Bar^[19] has given rise to a new interpretation. The variations in the curves seen in the Magellanic Clouds and Milky Way may instead be caused by processing of the dust grains by nearby star formation. This interpretation is supported by work in starburst galaxies (which are undergoing intense star formation episodes) that their dust lacks the 2175 Å bump. ^[20][21]

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. Nevertheless, the atmosphere is completely opaque to many wavelengths requiring the use of satellites to make observations. This article is about artificial satellites For natural satellites also known as moons see Natural satellite.

Atmospheric extinction has three main components: Rayleigh scattering by air molecules, scattering by aerosols, and molecular absorption. Rayleigh scattering (named after Lord Rayleigh) is the elastic Scattering of Light or other electromagnetic radiation by particles much smaller Particulates, alternatively referred to as particulate matter (PM or fine particles, are tiny particles of solid or liquid suspended in a gas Molecular absorption is often referred to as 'telluric absorption', as it is caused by the Earth ("telluric" is a synonym of "terrestrial"). Telluric contamination is contamination of the astronomical spectra by the Earth's atmosphere. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 This article deals with the general meaning of the term "synonym" The most important sources of telluric absorption are molecular oxygen and ozone, which absorb strongly in the near-ultraviolet, and water, which absorbs strongly in the infrared. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the OZONE is an object oriented Operating system written in the C programming language. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays Water is a common Chemical substance that is essential for the survival of all known forms of Life. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of

The amount of atmospheric extinction depends on the altitude of an object, being lowest at the zenith and at a maximum near the horizon. Altitude is the Elevation of a point or object from a known level or datum (plural data In broad terms the zenith is the direction pointing directly above a particular location ( Perpendicular, Orthogonal) The horizon ( Ancient Greek ὁ ὁρίζων, /ho horídzôn/ from ὁρίζειν, "to limit" is the apparent line that separates It is calculated by multiplying the standard atmospheric extinction curve by the mean airmass calculated over the duration of the observation. For air mass in Meteorology, see Air mass. In Astronomy, airmass is the optical path length through Earth's

References

General References for Article

1. Binney, J. and Merrifield, M. , 1998, Galactic Astronomy, Princeton University Press
2. Howarth I. D. (1983), LMC and galactic extinction, Royal Astronomical Society, Monthly Notices, vol. 203, Apr. 1983, p. 301-304.
3. King D. L. (1985), Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma, RGO/La Palma technical note 31
4. 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