Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Solar eclipse 1999 4 NR
Photo taken in France during the 1999 solar eclipse

Overview

The stellar atmosphere is divided into several regions of distinct character:

  • The photosphere, which is the atmosphere's lowest and coolest layer, is normally its only visible part.[1] Light escaping from the surface of the star stems from this region and passes through the higher layers. The Sun's photosphere has a temperature in the 5,770 K to 5,780 K range.[2][3] Starspots, cool regions of disrupted magnetic field lie on the photosphere.[3]
  • Above the photosphere lies the chromosphere. This part of the atmosphere first cools down and then starts to heat up to about 10 times the temperature of the photosphere.
  • Above the chromosphere lies the transition region, where the temperature increases rapidly on a distance of only around 100 km.[4]
  • The outermost part of the stellar atmosphere is the corona, a tenuous plasma which has a temperature above one million Kelvin.[5] While all stars on the main sequence feature transition regions and coronae, not all evolved stars do so. It seems that only some giants, and very few supergiants, possess coronae. An unresolved problem in stellar astrophysics is how the corona can be heated to such high temperatures. The answer lies in magnetic fields, but the exact mechanism remains unclear.[6]

During a total solar eclipse, the photosphere of the Sun is obscured, revealing its atmosphere's other layers.[1] Observed during eclipse, the Sun's chromosphere appears (briefly) as a thin pinkish arc,[7] and its corona is seen as a tufted halo. The same phenomenon in eclipsing binaries can make the chromosphere of giant stars visible.[8]

See also

Notes

  1. ^ a b ""Beyond the Blue Horizon" – A Total Solar Eclipse Chase". 1999-08-05. Retrieved 2010-05-21. On ordinary days, the corona is hidden by the blue sky, since it is about a million times fainter than the layer of the sun we see shining every day, the photosphere.
  2. ^ Mariska, J.T. The solar transition region. Cambridge Astrophysics Series. Cambridge University Press. ISBN 978-0-521-38261-8.
  3. ^ a b Lang, K.R. (September 2006). "5.1 MAGNETIC FIELDS IN THE VISIBLE PHOTOSPHERE". Sun, earth, and sky (2nd ed.). Springer. p. 81. ISBN 978-0-387-30456-4. this opaque layer is the photosphere, the level of the Sun from which we get our light and heat
  4. ^ Mariska, J.T. The solar transition region. p. 60. ISBN 978-0-521-38261-8. 100 km suggested by average models
  5. ^ R.C. Altrock (2004). "The Temperature of the Low Corona During Solar Cycles 21–23". Solar Physics. 224: 255. Bibcode:2004SoPh..224..255A. doi:10.1007/s11207-005-6502-4.
  6. ^ "The Sun's Corona – Introduction". NASA. Retrieved 2010-05-21. Now most scientists believe that the heating of the corona is linked to the interaction of the magnetic field lines.
  7. ^ Lewis, J.S. (2004-02-23). Physics and chemistry of the solar system (Second ed.). Elsevier Academic Press. p. 87. ISBN 978-0-12-446744-6. The dominant color is influenced by the Balmer radiation of atomic hydrogen
  8. ^ Griffin, R.E. (2007-08-27). Hartkopft, W.I.; Guinan, E.F. (eds.). Only Binary Stars Can Help Us Actually SEE a Stellar Chromosphere (1 ed.). Cambridge University Press. p. 460. doi:10.1017/S1743921307006163. ISBN 978-0-521-86348-3. Retrieved 2010-05-21.
17 Cygni

17 Cygni is the Flamsteed designation for a binary star system in the northern constellation of Cygnus. It has an apparent visual magnitude of 5.00, so, according to the Bortle scale, it is visible from suburban skies at night. Measurements made with the Hipparcos spacecraft show an annual parallax shift of 0.0478″, which is equivalent to a distance of around 68.2 ly (20.9 pc) from the Sun. It has a relatively high proper motion, traversing the celestial sphere at the rate of 0.451″/year.The stellar classification of the primary star is F7 V, which means it is a main sequence star like the Sun. The star has 1.24 times the mass of the Sun and 0.95–1.1 times the Sun's radius. It is some 2.8 billion years old and shines with 3.66 times the Sun's luminosity. The effective temperature of the stellar atmosphere is 6,455 K, giving it the yellow-white hued glow of an F-type star.At an angular separation of 791.40 arcseconds is a proper motion companion with a classification of M0.4, indicating this is a red dwarf star. At the estimated distance of the pair, this is equal to a projected separation of 16,320 AU. Although the CCDM lists four other companions, these are not associated with the pair.

25 Aquarii

25 Aquarii (abbreviated 25 Aqr) is a single star in the equatorial constellation of Aquarius. 25 Aquarii is the modern Flamsteed designation; in the past it held the designation 6 Pegasi. It also bears the Bayer designation of d Aquarii. It is located near the border with the modern Pegasus constellation. Although faint at an apparent visual magnitude of +5.09, it is bright enough to be viewed from suburban skies. Based upon an annual parallax shift of 0.01440 arcseconds, it is located at a distance of around 226 light-years (69 parsecs) from Earth. The visual magnitude of the star is diminished by 0.09 from extinction caused by intervening gas and dust.The spectrum of this star matches a stellar classification of K0 III, with the luminosity class of III indicating that this is a giant star that has evolved away from the main sequence after exhausting the supply of hydrogen at its core. It belongs to a population known as clump giants and hence is generating energy through the nuclear fusion of helium at the core. The outer envelope has expanded to 11 times the radius of the Sun and it is radiating 54 times the Sun's luminosity. This energy is being emitted from the stellar atmosphere at an effective temperature of 4,721 K, causing it to glow with the orange hue of a K-type star.

2 Aurigae

2 Aurigae is a possible binary star system in the northern constellation of Auriga. This object is visible to the naked eye as a faint, orange-hued star with an apparent visual magnitude of +4.79. It forms an attractive four-star asterism when viewed in a low power eyepiece, together with the nearby HIP 22647 and another very loose visual pair, HIP 22776 and HIP 22744, all above magnitude 8. 2 Aurigae is moving closer to the Earth with a heliocentric radial velocity of −17 km/s.The visible component is an aging giant star with a stellar classification of K3- III Ba0.4. The suffix notation indicates this is a mild barium star, which means the stellar atmosphere is enriched with s-process elements. It is either a member of a close binary system and has previously acquired these elements from a (now) white dwarf companion or else it is on the asymptotic giant branch and is generating the elements itself. 2 Aurigae is 1.80 billion years old with 2.86 times the mass of the Sun and has expanded to 48 times the Sun's radius. It is radiating 599 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,115 K.

33 Pegasi

33 Pegasi is the Flamsteed designation for a visual binary star in the northern constellation of Pegasus. It has an apparent visual magnitude of 6.2, placing it near the limit of naked eye visibility. Measurements made with the Hipparcos spacecraft show an annual parallax shift of 0.02967″, which is equivalent to a distance of roughly 110 ly (34 pc) from the Sun.

The primary component of this system is a main sequence star with a visual magnitude of 6.4 and a stellar classification of F7 V. It is nearly as old as the Sun with an estimated age of 4.1 billion years, but has a lower abundance of elements other than hydrogen and helium. The stellar atmosphere has an effective temperature of 6,169 K, giving it the yellow-white glow of an F-type star.A faint, magnitude 9.3 companion star is located at an angular separation of 0.420 arc seconds along a position angle of 0.0°. The orbital period of this pair is large enough that it has not been computed based on current observations.

46 Ceti

46 Ceti is a single star in the equatorial constellation of Cetus. It is visible to the naked eye with an apparent visual magnitude of 4.9. The distance to this star, as determined from an annual parallax shift of 11.9 mas, is about 273 light years. It is moving closer to the Earth with a heliocentric radial velocity of −23 km/s, and is expected to come as close as 184 light-years in 2.2 million years.At the age of about four billion years, this is an evolved K-type giant star with a stellar classification of K2+ III–IIIb CN0.5. The suffix notation CN0.5 indicates a mild overabundance of cyanogen in the stellar atmosphere. It has 1.38 times the mass of the Sun and has expanded to 19 times the Sun's radius. The star is radiating 132 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,316 K. The projected rotational velocity is too small to be measured.

6 Ceti

6 Ceti is a single star in the equatorial constellation of Cetus. It is visible to the naked eye with an apparent magnitude of 4.89. The annual parallax shift as measured from Earth's orbit is 53.34 mas, which yields a distance estimate of 61.1 light years. The star is moving further from the Sun with a constant radial velocity of +16.70 km/s. It is one of the IAU's standard velocity stars.Gray et al. (2006) assigned this star a stellar classification of F8 V Fe−0.8 CH−0.5, indicating it is an F-type main-sequence star with an underabundance of iron and the CH molecule in its stellar atmosphere. It is about 4.2 billion years old with 1.12 times the mass of the Sun and is spinning with a projected rotational velocity of 4.88 km/s. The star is radiating 3.34 times the Sun's luminosity from its photosphere at an effective temperature of about 6,289 K.

7 Ceti

7 Ceti is a single, variable star in the equatorial constellation of Cetus. It has the variable star designation AE Ceti. The star is visible to the naked eye with a baseline apparent visual magnitude of 4.44. Based upon an annual parallax shift of only 7.3 mas, it is located roughly 450 light years away. It is moving closer to the Sun with a heliocentric radial velocity of −23 km/s. Eggen (1965) listed it as a probable member of the Wolf 630 group of co-moving stars.This is an aging red giant star with a stellar classification of M1 III, currently on the asymptotic giant branch. Samus et al. (2017) has it classed as a slow irregular variable of type LB:, and ranges in magnitude from 4.26 down to 4.46. Tabur et al. (2009) list it as a semiregular variable with four known periods ranging in frequency from 19.2 to 41.7 days. The stellar atmosphere of 7 Ceti has expanded to an estimated 54 times the Sun's radius. It is radiating around 1,019 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 3,800 K.

8 Andromedae

8 Andromedae, abbreviated 8 And, is a probable triple star system in the northern constellation of Andromeda. 8 Andromedae is the Flamsteed designation. It is visible to the naked eye with an apparent visual magnitude of 4.82. Based upon an annual parallax shift of 6.1 mas, it is located 540 light years from the Earth. It is moving closer with a heliocentric radial velocity of −8 km/s.The primary component is an aging red giant star with a stellar classification of M2.5 III Ba0.5. The suffix notation indicates this is a mild barium star, which means the stellar atmosphere is enriched with s-process elements. It is either a member of a close binary system and has previously acquired these elements from a (now) white dwarf companion or else it is on the asymptotic giant branch and is generating the elements itself. This is a periodic variable of unknown type, changing in brightness with an amplitude of 0.0161 magnitude at a frequency of 0.23354 d−1, or once every 4.3 days.The third component is a magnitude 13.0 star at an angular separation of 7.8″ along a position angle of 164°, as of 2015.

Atmosphere

An atmosphere (from Ancient Greek ἀτμός (atmos), meaning 'vapour', and σφαῖρα (sphaira), meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low.

The atmosphere of Earth is composed of nitrogen (about 78%), oxygen (about 21%), argon (about 0.9%) , carbon dioxide (0.04%) and other gases in trace amounts. Oxygen is used by most organisms for respiration; nitrogen is fixed by bacteria and lightning to produce ammonia used in the construction of nucleotides and amino acids; and carbon dioxide is used by plants, algae and cyanobacteria for photosynthesis. The atmosphere helps to protect living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays. The current composition of the Earth's atmosphere is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.

The term stellar atmosphere describes the outer region of a star and typically includes the portion above the opaque photosphere. Stars with sufficiently low temperatures may have outer atmospheres with compound molecules.

Atmosphere (disambiguation)

An atmosphere is a gas layer around a celestial body.

Atmosphere may also refer to:

Atmosphere (unit), a unit of pressure

Atmosphere of Earth

Extraterrestrial atmospheres

Stellar atmosphere

Chandrasekhar polarization

Chandrasekhar Polarization is a partial polarization of emergent radiation at the limb of rapidly rotating early-type stars or binary star system with purely electron-scattering atmosphere, named after the Indian American astrophysicist Subrahmanyan Chandrasekhar, who first predicted its existence theoretically in 1946.Chandrasekhar published series of 26 papers in The Astrophysical Journal titled On the Radiative Equilibrium of a Stellar Atmosphere from 1944 to 1948. In the 10th paper, he predicted that the purely electron stellar atmosphere emits a polarized light using Thomson law of scattering. The theory predicted that 11 percent polarization could be observed at maximum. But when this is applied to a spherical star, the net polarization effect was found to be zero, because of the spherical symmetry. But it took another 20 years to explain under what conditions this polarization can be observed. J. Patrick Harrington and George W. Collins, II showed that this symmetry is broken if we consider a rapidly rotating star (or a binary star system), in which the star is not exactly spherical, but slightly oblate due to extreme rotation (or tidal distortion in the case of binary system). The symmetry is also broken in eclipsing binary star system.

Dimitri Mihalas

Dimitri Manuel Mihalas (March 20, 1939 – November 21, 2013) was a laboratory fellow at the Los Alamos National Laboratory in the field of astronomy, astrophysics, and stellar atmosphere. He was born in Los Angeles, California and was of Greek origin.Mihalas obtained his Bachelor's Degree in Physics, Mathematics, and Astronomy from the University of California, Los Angeles in 1959. In one year, he received his Master's Degree from California Institute of Technology in 1960. He completed his PhD degree in three years in 1963 in Physics and Astronomy, also from California Institute of Technology.At a very early age of 42, he became a member of the National Academy of Science.Besides a large number of scientific papers, mostly related to radiative transfer, Mihalas authored reference books such as "Stellar Atmospheres".Mihalas had bipolar disorder. He wrote a number of essays and books on the problem.Scientific books

D. Mihalas (in collaboration with P.M. Routly) - Galactic Astronomy. (San Francisco: W. H. Freeman & Company), 1968 [182]

D. Mihalas - Stellar Atmospheres. (San Francisco: W. H. Freeman & Company), 1970 [399]

D. Mihalas, B. Pagel, and P. Souffrin Theorie des Atmospheres Stellaires. First Advanced Course of the Swiss Society for Astronomy and Astrophysics. (Geneva: Observatoire de Geneve), 1971 [4]

D. Mihalas - Stellar Atmospheres. 2nd ed. (San Francisco: W. H. Freeman & Company), 1978 [1737]

D. Mihalas and J. Binney - Galactic Astronomy: Structure and Kinematics of Galaxies. (San Francisco: W.H. Freeman & Company), 1981 [948]

D. Mihalas and B. W. Mihalas - Foundations of Radiation Hydrodynamics. (New York: Oxford University Press), 1984. Paperback Edition, (New York: Dover Publications, Inc.), 1999 [984]

R. J. LeVeque, D. Mihalas, E. A. Dorfi, and E. Muller - Computational Methods for Astrophysical Fluid Flow. Saas-Fee Advanced Course 27. Swiss Society for Astronomy and Astrophysics. (Berlin: Springer Verlag), 1998 [1]

Stellar Atmosphere Modeling: Proceedings of an International Workshop Held in Tubingen, Germany, 8–12 April 2002 Stellar Atmosphere Modeling: Proceedings of an International Workshop Held in Tubingen, Germany, 8–12 April 2002 (Astronomical Society of the pacific), 2003 (Editors: D. Mihalas, I. Hubeny, K. Werner)Non-science books

D. Mihalas - Coming Back From The Dead, 1990

D. Mihalas, A. Sawyer, L. Wainwright - Trilogy in a Minor Key. (Trilogy Productions), 1991

D. Mihalas - Cantata for Six Lives And Continuo, 1992

D. Mihalas, C. Pursifull - If I Should Die before I Wake If I Should Die before I Wake. (Hawk Productions), 1993

D. Mihalas - Dream Shadows, 1994

D. Mihalas - Depression and Spiritual Growth. (Pendle Hill Publications), 1996

D. Mihalas - Life Matters: Poems by Dimitri Mihalas, 1995

D. Mihalas - The World Is My Witness, 1997

D. Mihalas - A Distant Summons, 1998

Dredge-up

A dredge-up is a period in the evolution of a star where a surface convection zone extends down to the layers where material has undergone nuclear fusion. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere where they can appear in the spectrum of the star.

The first dredge-up occurs when a main-sequence star enters the red-giant branch. As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: the 12C/13C and C/N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced.

The second dredge-up occurs in stars with 4–8 solar masses. When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle. This second dredge-up results in an increase in the surface abundance of 4He and 14N, whereas the amount of 12C and 16O decreases.The third dredge-up occurs after a star enters the asymptotic giant branch and a flash occurs along a helium-burning shell. This dredge-up causes helium, carbon and the s-process products to be brought to the surface. The result is an increase in the abundance of carbon relative to oxygen, which can create a carbon star.The names of the dredge-ups are set by the evolutionary and structural state of the star in which each occurs, not by the sequence experienced by the star. As a result, lower-mass stars experience the first and third dredge-ups in their evolution but not the second.

HD 193664

HD 193664 is the Henry Draper Catalogue designation for a star in the northern constellation of Draco. With an apparent magnitude of 5.93, according to the Bortle Scale it is visible to the naked eye from suburban skies. Parallax measurements by the Hipparcos spacecraft yield an estimated distance of 57.3 light years. It has a relatively large proper motion of 0.558 arc seconds per year across the sky.This star is considered a solar analog—meaning that it is photometrically analogous to the Sun—and it displays no significant variability. It is a G-type main sequence star with a stellar classification of G3V. Both the mass and radius of HD 193664 differ from those of the Sun by just a few percent, although it has a somewhat lower metallicity. It may be around the same age as the Sun, being an estimated 3.9 billion years old. The effective temperature of the stellar atmosphere is 5,915 K, giving it the yellow-hued glow of a G-type star.HD 193664 has been examined for signs of an infrared excess that could indicate the presence of a circumstellar disk of dust, but none has been found (as of 2012). This is member of the thin disk population of stars that lie near the galactic plane.

HD 39225

HD 39225, also known as HR 2028, is a variable star in the northern constellation Auriga, located around 620 light years away from the Sun. It is visible to the naked eye as a faint, red-hued star with an apparent visual magnitude of around 6.04. This is a suspected runaway star that is moving away from the Sun with a heliocentric radial velocity of 98 km/s.Currently on the asymptotic giant branch, this is an evolved red giant star with a stellar classification of M1+III Fe-1. The suffix notation indicates an underabundance of iron in the stellar atmosphere compared to similar stars of its class. It is suspected of varying in brightness between magnitudes 5.82 and 6.07. Having exhausted the hydrogen at its core, it has expanded to around 43 times the Sun's radius. It shines with a luminosity approximately 398.6 times that of the Sun and has a surface temperature of 3,934 K.

HD 47667

HD 47667 is a single star in the southern constellation of Canis Major. It is visible to the naked eye with an apparent visual magnitude of 4.832. The estimated distance to this star, based upon an annual parallax shift of 3.30±0.35 mas, is roughly 1,000 light years. It is moving further away with a heliocentric radial velocity of +29 km/s. The star made its closest approach to the Sun some 8.7 million years ago at a separation of around 362 ly (111.12 pc).Roughly 40 million years old, this is an evolved K-type giant star with a stellar classification of K2+ IIIa CN0.5 Ca1. The suffix notation indicates overabundances of calcium and the cyanide molecule have been found in the spectrum of the stellar atmosphere. The star has 7.4 times the mass of the Sun and has expanded to 28 times the Sun's radius. It is radiating 2,317 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,200 K.

Kappa–mechanism

The κ–mechanism (Latinised as kappa–mechanism) is the driving mechanism behind the changes in luminosity of many types of pulsating variable stars. The term Eddington valve has been used for this mechanism, but this is increasingly obsolete.Here, the Greek letter kappa (κ) is used to indicate the radiative opacity at any particular depth of the stellar atmosphere. In a normal star, an increase in compression of the atmosphere causes an increase in temperature and density; this produces a decrease in the opacity of the atmosphere, allowing heat energy to escape more rapidly. The result is an equilibrium condition where temperature and pressure are maintained in a balance. However, in cases where the opacity increases with temperature, the atmosphere becomes unstable against pulsations. If a layer of a stellar atmosphere moves inward, it becomes denser and more opaque, causing heat flow to be checked. In return, this heat increase causes a build-up of pressure that pushes the layer back out again. The result is a cyclic process as the layer repeatedly moves inward and then is forced back out again.Stellar non-adiabatic pulsation resulting from the κ–mechanism occurs in regions where hydrogen and helium are partly ionized, or where there are negative hydrogen ions. An example of such a zone is in RR Lyrae variables where the partial second ionization of helium occurs. Hydrogen ionization is most likely the cause of pulsation activity in Mira variables, rapidly oscillating Ap stars (roAp) and ZZ Ceti variables. In Beta Cephei variables, stellar pulsations occur at a depth where the temperature reaches approximately 200,000 K and there is an abundance of iron. The increase in the opacity of iron at this depth is known as the Z bump, where Z is the astronomical symbol for elements other than hydrogen and helium.

Stellar core

A stellar core is the extremely hot, dense region at the center of a star. For an ordinary main sequence star, the core region is the volume where the temperature and pressure conditions allow for energy production through thermonuclear fusion of hydrogen into helium. This energy in turn counterbalances the mass of the star pressing inward; a process that self-maintains the conditions in thermal and hydrostatic equilibrium. The minimum temperature required for stellar hydrogen fusion exceeds 107 K (10 MK), while the density at the core of the Sun is over 100 g cm−3. The core is surrounded by the stellar envelope, which transports energy from the core to the stellar atmosphere where it is radiated away into space.

UX Arietis

UX Arietis is a triple star system located in the northern zodiacal constellation of Aries. Based upon parallax measurements from the Hipparcos satellite, it is roughly 168 light years away. The primary, component Aa, is a variable star of the RS CVn type. The variability of the star is believed due to a combination of cool star spots and warm flares, set against the baseline quiescent temperature of the stellar atmosphere. The variability appears to be cyclical with a period of 8−9 years.

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