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An extreme example of visible light extinction, caused by a dark nebula

In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler.[1][2] However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve,[3] and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust. For stars lying near the plane of the Milky Way which are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies (photometric system) is roughly 1.8 magnitudes per kiloparsec.[4]

For Earth-bound observers, extinction arises both from the interstellar medium and the Earth's atmosphere; it may also arise from circumstellar dust around an observed object. Strong extinction in Earth's atmosphere of some wavelength regions (such as X-ray, ultraviolet, and infrared) is overcome by the use of space-based observatories. Since blue light is much more strongly attenuated than red light, extinction causes objects to appear redder than expected; this phenomenon is called interstellar reddening.[5]

Interstellar reddening

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Interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which the object originally emitted. Reddening occurs due to the light scattering off dust and other matter in the interstellar medium. Interstellar reddening is a different phenomenon from redshift, which is the proportional frequency shifts of spectra without distortion. Reddening preferentially removes shorter wavelength photons from a radiated spectrum while leaving behind the longer wavelength photons, leaving the spectroscopic lines unchanged.

In most photometric systems, filters (passbands) are used from which readings of magnitude of light may take account of latitude and humidity among terrestrial factors. Interstellar reddening equates to the "color excess", defined as the difference between an object's observed color index and its intrinsic color index (sometimes referred to as its normal color index). The latter is the theoretical value which it would have if unaffected by extinction. In the first system, the UBV photometric system devised in the 1950s and its most closely related successors, the object's color excess is related to the object's B−V color (calibrated blue minus calibrated visible) by:

For an A0-type main sequence star (these have median wavelength and heat among the main sequence) the color indices are calibrated at 0 based on an intrinsic reading of such a star (± exactly 0.02 depending on which spectral point, i.e. precise passband within the abbreviated color name is in question, see color index). At least two and up to five measured passbands in magnitude are then compared by subtraction: U, B, V, I, or R during which the color excess from extinction is calculated and deducted. The name of the four sub-indices (R minus I etc.) and order of the subtraction of recalibrated magnitudes is from right to immediate left within this sequence.

General characteristics

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Interstellar reddening occurs because interstellar dust absorbs and scatters blue light waves more than red light waves, making stars appear redder than they are. This is similar to the effect seen when dust particles in the atmosphere of Earth contribute to red sunsets.[6]

Broadly speaking, interstellar extinction is strongest at short wavelengths, generally observed by using techniques from spectroscopy. Extinction results in a change in the shape of an observed spectrum. Superimposed on this general shape are absorption features (wavelength bands where the intensity is lowered) that have a variety of origins and can give clues as to the chemical composition of the interstellar material, e.g. dust grains. Known absorption features include the 2175 Å bump, the diffuse interstellar bands, the 3.1 μm water ice feature, and the 10 and 18 μm silicate features.

In the solar neighborhood, the rate of interstellar extinction in the Johnson–Cousins V-band (visual filter) averaged at a wavelength of 540 nm is usually taken to be 0.7–1.0 mag/kpc−simply an average due to the clumpiness of interstellar dust.[7][8][9] In general, however, this means that a star will have its brightness reduced by about a factor of 2 in the V-band viewed from a good night sky vantage point on earth for every kiloparsec (3,260 light years) it is farther away from us.

The amount of extinction can be significantly higher than this in specific directions. For example, some regions of the Galactic Center are awash with obvious intervening dark dust from our spiral arm (and perhaps others) and themselves in a bulge of dense matter, causing as much as more than 30 magnitudes of extinction in the optical, meaning that less than 1 optical photon in 1012 passes through.[10] This results in the zone of avoidance, where our view of the extra-galactic sky is severely hampered, and background galaxies, such as Dwingeloo 1, were only discovered recently through observations in radio and infrared.

The general shape of the ultraviolet through near-infrared (0.125 to 3.5 μm) extinction curve (plotting extinction in magnitude against wavelength, often inverted) looking from our vantage point at other objects in the Milky Way, is fairly well characterized by the stand-alone parameter of relative visibility (of such visible light) R(V) (which is different along different lines of sight),[11][12] but there are known deviations from this characterization.[13] Extending the extinction law into the mid-infrared wavelength range is difficult due to the lack of suitable targets and various contributions by absorption features.[14]

R(V) compares aggregate and particular extinctions. It is

Restated, it is the total extinction, A(V) divided by the selective total extinction (A(B)−A(V)) of those two wavelengths (bands). A(B) and A(V) are the total extinction at the B and V filter bands. Another measure used in the literature is the absolute extinction A(λ)/A(V) at wavelength λ, comparing the total extinction at that wavelength to that at the V band.

R(V) is known to be correlated with the average size of the dust grains causing the extinction. For the Milky Way Galaxy, the typical value for R(V) is 3.1,[15] but is found to vary considerably across different lines of sight.[16] As a result, when computing cosmic distances it can be advantageous to move to star data from the near-infrared (of which the filter or passband Ks is quite standard) where the variations and amount of extinction are significantly less, and similar ratios as to R(Ks):[17] 0.49±0.02 and 0.528±0.015 were found respectively by independent groups.[16][18] Those two more modern findings differ substantially relative to the commonly referenced historical value ≈0.7.[11]

The relationship between the total extinction, A(V) (measured in magnitudes), and the column density of neutral hydrogen atoms column, NH (usually measured in cm−2), shows how the gas and dust in the interstellar medium are related. From studies using ultraviolet spectroscopy of reddened stars and X-ray scattering halos in the Milky Way, Predehl and Schmitt[19] found the relationship between NH and A(V) to be approximately:

(see also:[20][21][22]).

Astronomers have determined the three-dimensional distribution of extinction in the "solar circle" (our region of our galaxy), using visible and near-infrared stellar observations and a model of distribution of stars.[23][24] The dust causing extinction mainly lies along the spiral arms, as observed in other spiral galaxies.

Measuring extinction towards an object

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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).[25] 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. 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. A ratio other than 2.85 must therefore be due to extinction, and the amount of extinction can thus be calculated.

The 2175-angstrom feature

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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. This feature was first observed in the 1960s,[26][27] 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. 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.[28]

Extinction curves of other galaxies

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Plot showing the average extinction curves for the MW, LMC2, LMC, and SMC Bar.[29] 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. 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).

In the LMC, there is significant variation in the characteristics 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.[30][31] In the SMC, more extreme variation is seen with no 2175 Å bump and very strong far-UV extinction in the star forming Bar and fairly normal ultraviolet extinction seen in the more quiescent Wing.[32][33][34]

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%. Finding extinction curves in both the LMC and SMC which are similar to those found in the Milky Way[29] and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC[35] and in the SMC Bar[36] 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) which shows that their dust lacks the 2175 Å bump.[37][38]

Atmospheric extinction

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Atmospheric extinction gives the rising or setting Sun an orange hue and 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 extinction has three main components: Rayleigh scattering by air molecules, scattering by particulates, and molecular absorption. Molecular absorption is often referred to as telluric absorption, as it is caused by the Earth (telluric is a synonym for terrestrial). The most important sources of telluric absorption are molecular oxygen and ozone, which strongly absorb radiation near ultraviolet, and water, which strongly absorbs infrared.

The amount of such extinction is lowest at the observer's zenith and highest near the horizon. A given star, preferably at solar opposition, reaches its greatest celestial altitude and optimal time for observation when the star is near the local meridian around solar midnight and if the star has a favorable declination (i.e., similar to the observer's latitude); thus, the seasonal time due to axial tilt is key. Extinction is approximated by multiplying the standard atmospheric extinction curve (plotted against each wavelength) by the mean air mass calculated over the duration of the observation. A dry atmosphere reduces infrared extinction significantly.

References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Extinction (astronomy)

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In astronomy, extinction refers to the absorption and of from distant celestial objects by interstellar and gas, resulting in a reduction in the observed brightness and a wavelength-dependent reddening of the light, particularly affecting shorter wavelengths more severely than longer ones. This phenomenon, primarily caused by microscopic grains in the () with sizes on the order of 0.01 to 1 micrometer, accounts for approximately 30-50% of the total stellar light absorption in the , despite comprising only about 0.1% of the Galaxy's baryonic mass. The gas-to- mass ratio in the Galaxy is roughly 100:1 by mass, with re-emitting absorbed energy as far-infrared radiation. The effects of extinction include not only overall dimming but also selective reddening, where blue light is more strongly attenuated than red, mimicking the appearance of cooler stars or altering color-magnitude diagrams; this is quantified by the color excess E(B-V), the difference between observed and intrinsic B-V colors. Extinction also produces reflection nebulae, polarizes starlight due to aligned grains, and features absorption bands from silicates at 9.7 μm and 18 μm or ices. The wavelength dependence follows an empirical law approximately proportional to λ⁻¹ from ultraviolet to near-infrared wavelengths, with notable deviations such as the prominent 2175 Å ultraviolet bump attributed to carbonaceous grains and variations in the far-infrared. A key parameter is R_V = A_V / E(B-V), the ratio of total visual extinction A_V to selective extinction, which typically averages 3.1 for the diffuse ISM but ranges from 2 to 6 depending on the environment, influencing the slope of the extinction curve. Astronomers measure through comparisons of observed fluxes with intrinsic stellar properties or via statistical methods on star fields, enabling maps of distribution and structure; corrections are essential for accurate distance estimates, photometry, and of distant objects. Influential models, such as the parametric derived by Cardelli, Clayton, and Mathis (1989), parameterize the curve across to wavelengths using R_V as the primary variable, applicable to both diffuse and denser regions and widely used in observational corrections. Variations in curves reveal insights into composition—likely a mix of silicates, graphites, and polycyclic aromatic hydrocarbons—and grain size distributions, which evolve with conditions like density and radiation field.

Historical Development

The concept of interstellar extinction was first noted in the 18th century by , who observed that stars appeared dimmer than expected based on their positions. In the early , Trumpler (1930) provided definitive evidence through studies of open clusters, showing that interstellar material causes both absorption and reddening, challenging the then-prevailing view of a transparent . Subsequent work in the mid-20th century, including measurements by Stebbins and Whitford, refined the extinction law. The 2175 Å feature was identified in the from satellite observations, and modern parametric models like Cardelli et al. (1989) built on earlier empirical fits by Savage and Mathis (1979). Recent advances, including (JWST) observations as of 2023, continue to probe variations in extinction laws across the .

Introduction

Definition and Importance

In astronomy, interstellar extinction refers to the attenuation of electromagnetic radiation from distant sources, such as , due to the absorption and of photons by interstellar dust grains and gas along the . This process results in an apparent dimming of the source's , quantified as the extinction magnitude AλA_\lambda at λ\lambda, where shorter wavelengths are more strongly affected, leading to a wavelength-dependent color shift known as reddening. Absorption represents true removal of photons from the beam, converting their into within the grains, while redirects light out of the without destroying the photons, though both contribute to the overall dimming effect. The total extinction at a given wavelength is commonly expressed by the relation Aλ=AV×k(λ)RVA_\lambda = A_V \times \frac{k(\lambda)}{R_V}, where AVA_V is the extinction in the visual V-band, k(λ)k(\lambda) describes the shape of the extinction curve normalized to the color excess, and RVR_V is the total-to-selective extinction ratio, with a typical value of 3.1 for diffuse regions in the . Interstellar reddening, a key manifestation of this differential extinction, makes objects appear redder by preferentially attenuating blue light. Extinction plays a crucial role in by influencing the accuracy of distance estimates to celestial objects, as uncorrected dimming leads to overestimation of distances through standard magnitude-based methods like magnitude-reddening relations. It is essential for studies of stellar populations, where failure to account for distorts color-magnitude diagrams used to infer ages, metallicities, and evolutionary stages. Furthermore, in the context of galaxy evolution, alters observed properties of stars and gas, affecting models of and dust processing, particularly in dense environments like molecular clouds where corrections are vital for interpreting embedded sources.

Historical Development

In the 19th century, astronomers began documenting irregularities in star fields and nebulae that suggested the dimming of starlight by intervening material, with early suggestions by F.G.W. Struve in 1847, who inferred absorption from star counts indicating a decline in stellar density with distance. These observations laid preliminary groundwork for recognizing interstellar absorption, though the phenomenon was often attributed to observational limitations rather than a pervasive medium. In 1904, Johannes Hartmann provided the first direct evidence of interstellar material by detecting stationary absorption lines from calcium in the spectrum of the Delta Orionis. A pivotal breakthrough came in 1930 when Robert Trumpler analyzed open star clusters and demonstrated that interstellar caused both dimming and reddening of starlight, with greater effects toward the explaining the "" where fewer extragalactic objects were visible. This work quantified extinction as a distance-dependent process, shifting the paradigm from a transparent to one filled with absorbing grains. In the , Hendrik van de Hulst advanced theoretical models of grains, predicting their and absorption properties based on compositions like silicates and carbon, which aligned with emerging observational data on . During the mid-20th century, William Hiltner and collaborators developed empirical extinction laws in the 1950s, deriving relationships between reddening and total absorption from multi-wavelength photometry of stars, which standardized measurements across the . Concurrently, polarization studies by Hiltner and J.S. Hall in 1949 confirmed the presence of aligned grains, as starlight polarization correlated with extinction and pointed to alignment mechanisms proposed by L. Davis and J.L. Greenstein in 1951. In the , ultraviolet spectroscopy revealed the prominent 2175 extinction feature, a broad absorption bump attributed to graphitic or carbonaceous grains, marking a key spectroscopic signature of interstellar . The modern era saw infrared missions transform dust mapping, with the in 1983 providing all-sky surveys that traced cool dust emission and revealed its clumpy distribution throughout the Galaxy. The , operational from 2003 to 2020, extended this with higher-resolution mid-infrared imaging, enabling detailed studies of dust heating by stars and variations in grain properties across environments. Launched in 2013, the mission has since delivered precise parallaxes and estimates for over a billion stars through its Data Release 3 in 2022, combining with to map three-dimensional dust structures with unprecedented accuracy. Recent advances since 2020, powered by the (JWST), have uncovered extinction effects in distant galaxies, including obscured quasars at redshifts z > 6 where dust in early hosts attenuates light from accreting black holes, as detected in surveys like SHELLQs through 2025. These observations highlight dust formation and evolution in the first billion years after the , bridging laws to cosmic scales.

Fundamentals of Interstellar Extinction

General Characteristics

Interstellar extinction arises primarily from the interaction of starlight with dust and gas in the (ISM), where dust grains account for most of the opacity despite comprising only about 1% of the ISM mass. The ISM consists of diffuse atomic and molecular gas, including (H and H₂) and trace metals, distributed across various environments such as the diffuse ISM, dense molecular clouds, and ionized H II regions. Dust grains, typically ranging in size from 0.01 to 1 μm, are composed mainly of amorphous silicates and carbonaceous materials like , with smaller polycyclic aromatic hydrocarbons (PAHs) contributing to the population. The primary mechanisms of extinction are absorption and scattering of photons by these grains. In absorption, photons are captured by the grain, heating it and leading to re-emission at longer infrared wavelengths, while scattering redirects photons without absorption, with the process depending on grain size relative to the wavelength. For grains much smaller than the wavelength, Rayleigh scattering dominates, whereas Mie scattering applies to larger grains comparable in size, resulting in stronger extinction at ultraviolet and blue wavelengths that diminishes toward the infrared. Dust distribution in the is highly inhomogeneous and clumpy, forming dense clouds along lines of sight that cause variable , with concentrations highest in the due to the disk's structure. On average, the visual extinction A_V is approximately 1 mag per kpc in the 's disk, though this varies regionally. This clumpy nature leads to broadband dimming of stellar spectra across all wavelengths, reducing observed fluxes without producing strong emission lines directly from the process itself, unlike in regions with active . Dust grains exhibit specific physical properties that influence , including their composition of amorphous silicates for the bulk and PAHs for smaller components, which can coat larger grains in icy mantles under certain conditions. Grains often align with the local due to radiative torques or paramagnetic interactions, leading to of transmitted by up to 10% in the optical range in aligned regions. This differential by also causes interstellar reddening, a color shift that makes distant stars appear redder.

Interstellar Reddening

Interstellar reddening arises from the preferential extinction of shorter-wavelength (blue) light compared to longer-wavelength (red) light by interstellar dust grains, causing distant stars to appear redder than their intrinsic colors. This effect occurs because dust grains scatter and absorb blue photons more efficiently than red ones, altering the observed spectral energy distribution of background sources. The phenomenon is a key observable signature of interstellar dust and is quantified through color excesses, such as E(B-V), which measures the difference between observed and intrinsic colors in the B and V bands: E(B-V) = (B - V)observed - (B - V)intrinsic. Observationally, interstellar reddening manifests in color-magnitude diagrams (CMDs) of star clusters, where the is displaced redward from its expected position. For instance, in the open cluster, CMDs reveal a systematic shift in the stellar sequence due to foreground , requiring dereddening corrections to align with theoretical isochrones. Similarly, in Hertzsprung-Russell (H-R) diagrams, reddening introduces characteristic vectors that shift stars horizontally toward redder colors without significantly altering their luminosities, distorting the apparent evolutionary tracks of stellar populations. The selective nature of reddening is further quantified by the color excess at a given relative to the V band, defined as E(λ - V) = Aλ - AV, where Aλ is the at λ. This relates to the total visual AV through the parameter RV = AV / E(B-V), which averages approximately 3.1 in the diffuse and reflects the overall shape of the extinction law. Reddening measurements enable mapping of (ISM) structure, as demonstrated by all-sky dust maps that infer column densities from color excesses toward stars. For example, the Schlegel et al. (1998) maps use infrared emission to trace reddening and reveal large-scale ISM features like the Local Bubble and high-velocity clouds. In extragalactic contexts, reddening affects the observed light curves of Type Ia supernovae, necessitating corrections to standardize their luminosities for cosmological distance estimates; failure to account for it can bias Hubble constant measurements by several percent. Variations in reddening occur along different sightlines, with anomalous values in dense clouds where RV > 4 indicate larger grain sizes due to , leading to less selective compared to diffuse regions. These deviations highlight environmental differences in the , such as enhanced in molecular clouds.

Extinction Laws and Measurements

Extinction Curves

In astronomy, the curve quantifies the wavelength-dependent absorption and of light by interstellar , typically normalized as k(λ)=Aλ/E(B−V)k(\lambda) = A_\lambda / E(B-V), where AλA_\lambda is the extinction magnitude at wavelength λ\lambda and E(B−V)E(B-V) is the color excess in the B-V bands, providing a standardized description from the ultraviolet (UV) to the infrared (IR). This normalization allows comparison across different lines of sight by factoring out the overall strength, with E(B−V)E(B-V) serving as a measure of total reddening integrated along the curve. The standard extinction curve for the Milky Way exhibits a steep rise in the UV regime, approximately proportional to λ−1\lambda^{-1}, transitioning to a relatively flat profile in the optical and near-infrared (NIR), followed by a subtle upturn in the far-IR due to larger grain contributions. These curves are commonly parametrized using empirical models such as the Fitzpatrick & Massa (1990) formalism, which employs piecewise polynomials and Lorentzians to fit UV-to-optical data, or the Cardelli, Clayton, & Mathis (1989) model, an analytic representation that extends from the IR through the UV as a function of a single parameter RVR_V. Key variations in the curves are captured by RV=AV/E(B−V)R_V = A_V / E(B-V), with an average value of 3.1 for diffuse interstellar medium sightlines and a range from about 2.1 to 5.5 depending on dust properties and environment. The far-UV (FUV) rise, prominent below 1700 Å, arises primarily from efficient scattering and absorption by small grains under 0.1 μm in size. Theoretically, extinction curves are derived from dust grain models assuming a power-law size distribution, such as the Mathis, Rumpl, & Nordsieck (1977) law where the number density n(a)∝a−3.5n(a) \propto a^{-3.5} for radii aa from 0.005 to 0.25 μm, comprising silicate and carbonaceous materials. Optical properties are calculated using Mie theory for spherical grains, which solves to yield extinction efficiencies Qext(λ,a)Q_{\rm ext}(\lambda, a) that, when integrated over the size distribution, reproduce observed curve shapes. Empirical data underpinning these models come from UV of reddened hot stars, primarily using satellites like OAO-2 for early surveys and the International Ultraviolet Explorer (IUE) for high-resolution spectra of over 200 sightlines.

Methods for Measuring Extinction

One primary technique for measuring interstellar involves the pair method, which compares the observed or photometry of a reddened target to that of an unreddened analog of similar spectral type and luminosity class. By matching these pairs, astronomers derive the at a specific AλA_\lambda from the ratio of their fluxes, assuming the intrinsic fluxes are equal; this difference quantifies the absorption and along the . This method is particularly effective for field s where direct estimates are challenging, as it isolates the effect without requiring absolute distances. The color excess method estimates extinction by measuring the deviation of a star's observed color from its intrinsic value, typically using broadband photometry in filters like Johnson B and V. The color excess E(B−V)E(B-V) is calculated as the difference between the observed (B−V)(B-V) and the intrinsic (B−V)0(B-V)_0, often determined from spectral type calibrations or model atmospheres; this excess is then scaled by the extinction curve to obtain total extinction AV≈3.1E(B−V)A_V \approx 3.1 E(B-V). For star clusters, this approach is refined by fitting the zero-age (ZAMS) to the color-magnitude diagram, where the horizontal shift in colors due to reddening provides E(B−V)E(B-V) for the entire cluster, assuming uniform foreground . These measurements can be extrapolated across wavelengths using established extinction curves to infer AλA_\lambda at other bands. Spectroscopic methods provide independent extinction estimates by analyzing line ratios or strengths in stellar or nebular spectra. In H II regions and star-forming nebulae, the Balmer decrement—specifically the observed flux ratio of Hα\alpha/Hβ\beta compared to the intrinsic Case B value of approximately 2.86 (for electron temperatures around 10,000 K and low densities)—reveals differential extinction, as shorter-wavelength lines like Hβ\beta suffer more absorption; the ratio deviation yields E(B−V)E(B-V) or AVA_V via the extinction law. For absorption-line spectra of stars, changes in equivalent widths of lines like the Ca II K line or , relative to unreddened standards, can indicate continuum extinction, though this is sensitive to and velocity broadening. Modern techniques leverage large-scale surveys for precise, three-dimensional mapping of extinction. The mission's Data Release 3 (DR3, 2022) combines high-precision parallaxes with multi-band photometry (G, , RP) to infer distances and fit stellar parameters, enabling extinction estimates via isochrone or spectral fitting for over 470 million sources; when integrated with all-sky surveys like Pan-STARRS1, , and WISE, this produces 3D dust maps such as Bayestar, which models reddening as a function of position and distance using on millions of stars. Multi-wavelength fitting across UV-optical-IR spectra further refines these by simultaneously solving for intrinsic stellar properties and extinction parameters. Uncertainties in extinction measurements arise from intrinsic stellar variability, poor analogs in pair comparisons, foreground contamination in clusters, and assumptions about the extinction law's uniformity. Spectroscopic methods can be affected by non-LTE effects or density variations in nebulae, while photometric approaches suffer from calibration errors. With Gaia DR3, precisions have improved to approximately 0.01 mag in AGA_G (Gaia band's extinction) for bright, nearby sources, reducing overall uncertainties to levels that enable detailed 3D dust structure resolution.

Specific Features and Variations

The 2175 Å Feature

The 2175 Å feature, a prominent broad absorption bump in the interstellar curve, was first detected in the through spectra obtained via flights and subsequently confirmed using data from the (OAO) satellites. This spectral signature peaks at approximately 2175 Å, equivalent to a of 4.6 μm^{-1}, and exhibits a typical (FWHM) of about 1000 Å, making it the strongest discrete absorption component in the observed . The feature contributes significantly to the overall , enhancing the steep rise in opacity shortward of 2000 Å characteristic of . In the diffuse (), the 2175 Å bump is particularly conspicuous, with an excess Δτ of roughly 1–2 at its peak, reflecting efficient absorption by small dust grains along typical sightlines. However, its prominence diminishes in denser clouds, where the bump becomes weaker due to and altered size distributions that reduce the population of small carriers. The strength of the feature also correlates with the total-to-selective extinction parameter R_V, reaching maximum intensity for R_V ≈ 3.1, which corresponds to the standard diffuse conditions, while it weakens in environments with higher R_V indicative of larger grains. The origin of the 2175 Å bump is widely attributed to π → π* electronic transitions in carbonaceous materials, particularly small grains or polycyclic aromatic hydrocarbons (PAHs), which provide the necessary aromatic ring structures for such absorptions. Seminal simulations of these carbon-based compounds, including UV absorption spectra of PAH ions and nanoparticles, have replicated the bump's central , width, and , strongly supporting their role as the dominant carriers over alternative proposals like silicates or carbides. High-resolution observations from the International Ultraviolet Explorer (IUE) and the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS) have provided detailed profiles of the feature across diverse sightlines, highlighting its variability in shape and intensity. Notably, in certain anomalous sightlines—such as those toward HD 204827 or regions with evidence of dust processing—the bump is notably weak or entirely absent, pointing to grain evolution through mechanisms like in dense regions or destruction by shocks and radiation. Post-2020 studies have further illuminated the feature's properties in the local through refined analyses of archival UV data and theoretical modeling, confirming its persistence and variability in unprocessed diffuse environments. Observations with the (JWST), particularly using the Near Infrared Spectrograph (NIRSpec) to detect the redshifted analog at high redshifts (z > 7), affirm the rapid formation of similar carbon dust in early galaxies, with implications for analogous evolutionary processes shaping the 2175 Å carriers in the over .

Extinction in Other Galaxies

Extinction properties in external galaxies often mirror those of the but exhibit notable variations influenced by metallicity, dust composition, and galactic structure. In the (LMC) and (SMC), which serve as nearby analogs to Milky Way-like environments, extinction curves show similarities in the optical regime but differ in the (UV). The LMC's average curve has an R_V value of approximately 3.1, comparable to the Milky Way's 3.1, while the SMC's is lower at around 2.7, indicating a relatively steeper optical-to-UV transition. The 2175 Å feature, prominent in the Milky Way, is weaker in the LMC and often absent or significantly reduced in the SMC's average curve, particularly along sightlines through dense regions like the SMC Bar, though some individual lines of sight display a detectable bump correlated with emission. These maps have been extensively detailed through surveys such as the Optical Gravitational Lensing Experiment (OGLE-IV) and the VISTA Magellanic Clouds (VMC) survey, which utilize stars to resolve reddening variations across the clouds with high spatial resolution. In spiral galaxies like M31 (Andromeda) and M33, extinction curves in the optical are broadly comparable to the 's, with R_V values near 3.1, but they tend to steepen in the UV, especially along spiral arms where is active. This steeper UV rise suggests a higher proportion of small dust grains, akin to those in the LMC, and is evident in observations of OB associations and supergiants. Dust lanes in these galaxies create patchy extinction, with visual extinctions (A_V) reaching up to several magnitudes in dense regions— for instance, A_V values exceeding 1 mag are common in M31's arms, and localized peaks up to approximately 3 mag occur in obscured regions—leading to heterogeneous reddening that complicates integrated light measurements. Such variations highlight how spiral structure enhances differential extinction compared to the more uniform disk. Irregular and dwarf galaxies, characterized by lower metallicities (often 10-40% solar), display shallower curves overall, with reduced UV opacity and a diminished or absent 2175 bump, as seen in examples like . This shallowing arises from fewer heavy elements available for formation, resulting in smaller dust-to-gas ratios and larger average grain sizes that scatter rather than absorb UV light efficiently. In , a metal-poor irregular with active , the low facilitates greater escape of UV photons from young stars, influencing estimates of rates and feedback processes; observations indicate A_V typically below 0.5 mag, allowing unobscured views into starburst regions that reveal efficient massive despite limited . At high redshifts (z > 1), extinction curves appear to evolve, becoming flatter in the early due to the prevalence of larger grains produced in , as probed by (HST) and (JWST) observations of distant galaxies. These flatter profiles, with reduced UV steepness relative to local curves, are inferred from (GRB) afterglow spectra, where power-law continua enable precise fitting of host-galaxy ; for instance, GRB 070802 at z ≈ 2.45 shows an LMC-like curve with a detectable 2175 Å bump shifted to observed wavelengths. Similarly, Type Ia (SN Ia) at high-z exhibit host extinction consistent with evolving laws, with analyses of over 100 events indicating a transition to SMC-type curves at z > 1, aiding cosmological distance measurements by accounting for -induced dimming. For integrated light from entire galaxies, particularly starbursts, attenuation laws differ from line-of-sight extinction due to mixed geometries of stars and dust. The Calzetti law, derived from UV-selected starbursts, provides an effective attenuation curve that is grayer (less steep) in the UV than typical extinction, reflecting forward-scattering and patchy distributions that reduce net UV absorption relative to optical wavelengths. This law, with an effective R_V ≈ 4.05, is widely applied to correct rate indicators in extragalactic studies, emphasizing the role of in global dust effects beyond simple extinction.

Atmospheric Extinction

Principles and Effects

Atmospheric extinction refers to the attenuation of from celestial sources as it passes through Earth's atmosphere, primarily affecting ground-based astronomical observations. This phenomenon arises from interactions between incoming light and atmospheric constituents, leading to both absorption and of photons. Unlike interstellar extinction, which operates on galactic scales with as the dominant medium, atmospheric extinction is localized to Earth's and is dominated by molecular and particulate effects, resulting in much shorter path lengths and thus lower overall magnitudes of dimming—typically on the order of 0.1 to 0.3 magnitudes at visible wavelengths under standard conditions. The primary mechanisms of atmospheric extinction include by air molecules, such as and oxygen, which is inversely proportional to the of the (∝λ−4\propto \lambda^{-4}), making it particularly pronounced in the and regions of the . particles, including , , and droplets, contribute through both absorption and that are relatively wavelength-independent in the visible range, while selective molecular absorption occurs at specific bands, such as those from oxygen (O₂) near 760 nm and (H₂O) in the near-infrared. These processes collectively reduce the flux of incoming light, with accounting for the majority of extinction at shorter wavelengths. Wavelength dependence is a key characteristic, with extinction being strongest at short wavelengths—for instance, approximately 0.3 magnitudes per at 400 nm—due to the dominance of , and diminishing to negligible levels in the beyond 2-3 μm. At the , where the is minimal (X=1), typical visual-band (V-band, centered around 550 nm) extinction ranges from 0.1 to 0.2 magnitudes at mid-latitude sites with good seeing conditions. This differential extinction across the spectrum causes reddening of , as shorter wavelengths are scattered more efficiently than longer ones. Variations in extinction arise from observational site characteristics and environmental factors. is higher at sea-level locations due to greater along the , often reaching 0.3 magnitudes per in the V-band, compared to about 0.1 magnitudes at high-altitude observatories like (4,200 m elevation), where thinner atmosphere reduces the column density of scattering particles. Seasonal changes, such as increased during humid periods, and anthropogenic can elevate contributions, leading to higher and more variable ; for example, volcanic eruptions have historically increased global loading and thus by factors of 2-3 in affected regions. Observationally, atmospheric extinction results in the dimming of point sources like , reducing their measured magnitudes, and can introduce blurring or halo effects around images due to scattered light, particularly under high conditions (e.g., near the horizon). Photometric measurements are more severely impacted than , as the former rely on absolute , while the latter can sometimes resolve absorption features; unresolved extinction leads to systematic underestimation of source brightness, especially for faint objects. Historically, early astronomical catalogs, such as those from the , often incorporated uncorrected atmospheric effects, resulting in systematic errors in magnitude scales and color indices that propagated into subsequent analyses until standardized site-specific measurements became routine in the mid-20th century.

Corrections and Applications

Atmospheric corrections are essential for obtaining accurate photometric measurements from ground-based telescopes, primarily through the application of Bouguer's law, which relates the observed magnitude mobservedm_{\text{observed}} to the corrected magnitude via the extinction coefficient kk and airmass XX: mcorrected=mobserved−kXm_{\text{corrected}} = m_{\text{observed}} - k X This empirical relation accounts for the linear increase in with atmospheric path length, where kk is determined nightly by observing standard stars at varying airmasses to fit the data and solve for the coefficient, typically in magnitudes per airmass. Photometric standards, such as those in Landolt fields, provide the reference stars needed to compute kk, enabling precise by comparing observed fluxes against known magnitudes across a range of colors and airmasses. Real-time monitoring with all-sky cameras further refines these corrections by delivering continuous maps of and coarse extinction estimates, allowing observers to adjust for rapid changes in sky transparency. Site-specific extinction tables, like those developed for ESO's , offer pre-calibrated coefficients tailored to local conditions, reducing errors in absolute flux measurements for spectroscopic and photometric programs. Advanced techniques mitigate extinction effects beyond basic corrections; adaptive optics systems use deformable mirrors and wavefront sensors to counteract scattering from atmospheric turbulence, sharpening images and indirectly reducing differential extinction impacts in high-resolution observations. Space-based telescopes such as the (HST) and (JWST) entirely avoid atmospheric by operating above the Earth's atmosphere, enabling unobstructed and photometry critical for faint or dusty targets. For ground-based relative photometry, differential methods compare target and nearby reference stars observed simultaneously, minimizing common-mode errors when absolute is unnecessary or infeasible. These corrections underpin applications in large-scale surveys, where the Legacy Survey of Space and Time (LSST) demands sub-0.01 magnitude accuracy to map billions of galaxies and detect transients without systematic biases from variable extinction. In transit observations, precise extinction removal is vital to distinguish planetary signals from , achieving the millimagnitude precision needed for . Similarly, monitoring variable stars requires extinction corrections to track intrinsic brightness changes accurately, avoiding false variability induced by airmass shifts during long-term campaigns. Post-2020 developments include models for forecasting atmospheric conditions, such as seeing and transparency proxies related to , using weather data to optimize (ELT) scheduling and preemptively adjust observation plans for maximal efficiency.

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