A-type main-sequence star

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An artist's impression of Sirius A and Sirius B, a binary star system. Sirius A, an A-type main-sequence star, is the larger of the two. Sirius A and B artwork.jpg
An artist's impression of Sirius A and Sirius B, a binary star system. Sirius A, an A-type main-sequence star, is the larger of the two.

An A-type main-sequence star (AV) or A dwarf star is a main-sequence (hydrogen burning) star of spectral type A and luminosity class V (five). These stars have spectra defined by strong hydrogen Balmer absorption lines. [1] They measure between 1.4 and 2.1  solar masses (M☉) and have surface temperatures between 7,600 and 10,000  K. Bright and nearby examples are Altair (A7), Sirius A (A1), and Vega (A0). A-type stars do not have convective zones and thus are not expected to harbor magnetic dynamos. As a consequence, because they do not have strong stellar winds, they lack a means to generate X-ray emissions.

Contents

Spectral standard stars

Properties of typical A-type main-sequence stars [2] [3] [4] [5] [6]
Spectral
type 		Mass (M☉) Radius (R☉) Luminosity (L☉) Effective
temperature
(K)		 Color
index
(B − V)
A0V2.182.19338.029,7000.00
A1V2.052.13630.909,3000.04
A2V1.982.11723.998,8000.07
A3V1.931.86116.988,6000.10
A4V1.881.79413.498,2500.14
A5V1.861.78512.308,1000.16
A6V1.831.77511.227,9100.19
A7V1.811.75010.007,7600.21
A8V1.771.7489.127,5900.25
A9V1.751.7478.327,4000.27

The revised Yerkes Atlas system [7] listed a dense grid of A-type dwarf spectral standard stars, but not all of these have survived to this day as standards. The "anchor points" and "dagger standards" of the MK spectral classification system among the A-type main-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years and can be considered to define the system, are Vega (A0 V), Gamma Ursae Majoris (A0 V), and Fomalhaut (A3 V). [8] [9] The seminal review of MK classification by Morgan & Keenan (1973) [9] didn't provide any dagger standards between types A3 V and F2 V. HD 23886 was suggested as an A5 V standard in 1978. [10]

Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 1987 [11] and 1989. [12] They list an assortment of fast- and slow-rotating A-type dwarf spectral standards, including HD 45320 (A1 V), HD 88955 (A2 V), 2 Hydri (A7 V), 21 Leonis Minoris (A7 V), and 44 Ceti (A9 V). Besides the MK standards provided in Morgan's papers and the Gray & Garrison papers, one also occasionally sees Delta Leonis (A4 V) listed as a standard. There are no published A6 V and A8 V standard stars.

The Morgan-Keenan spectral classification Morgan-Keenan spectral classification.svg
The Morgan-Keenan spectral classification


Planets

A-type stars are young (typically few hundred million years old) and many emit infrared (IR) radiation beyond what would be expected from the star alone. This IR excess is attributable to dust emission from a debris disk where planets form. [13] Surveys indicate massive planets commonly form around A-type stars although these planets are difficult to detect using the Doppler spectroscopy method. This is because A-type stars typically rotate very quickly, which makes it difficult to measure the small Doppler shifts induced by orbiting planets since the spectral lines are very broad. [14] However, this type of massive star eventually evolves into a cooler red giant which rotates more slowly and thus can be measured using the radial velocity method. [14] As of early 2011 about 30 Jupiter class planets have been found around evolved K-giant stars including Pollux, Gamma Cephei and Iota Draconis. Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun are orbited by one or more Jupiter-sized planets, compared to about 1 in 16 for Sun-like stars. [15]

A-type star systems known to feature planets include HD 15082, Beta Pictoris, HR 8799 and HD 95086. [16]

Examples

Within 40 light years:

Name Spectral
type 		Constellation vis Mag Mass
(M☉)		 Radius
(R☉)		 Luminosity
(L☉)		 Distance
(ly)
Sirius A0mA1 Va Canis Major −1.472.0631.71125.48.60 ± 0.04
Altair A7 V Aquila 0.761.791.63–2.0310.616.73
Vega A0 Va Lyra 0.0262.1352.362 × 2.81840.1225.04
Fomalhaut A3 V Piscis Austrinus 1.171.911.841625.1
Denebola A3 V Leo 2.141.781.731535.8
Delta Capricorni A5 IV Capricornus 2.832.01.911138.6

Delta Capricorni is likely a subgiant or giant star, and Altair is a disputed subgiant. In addition, Sirius is the brightest star in the night sky.

See also

Related Research Articles

<span class="mw-page-title-main">Stellar classification</span> Classification of stars based on spectral properties

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.

<span class="mw-page-title-main">Red dwarf</span> Dim, low mass stars on the main sequence

A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one star that fits the stricter definitions of a red dwarf is visible to the naked eye. Proxima Centauri, the star nearest to the Sun, is a red dwarf, as are fifty of the sixty nearest stars. According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way.

<span class="mw-page-title-main">G-type main-sequence star</span> Stellar classification

A G-type main-sequence star, also often, and imprecisely, called a yellow dwarf, or G star, is a main-sequence star of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective temperature between about 5,300 and 6,000 K. Like other main-sequence stars, a G-type main-sequence star converts the element hydrogen to helium in its core by means of nuclear fusion, but can also fuse helium when hydrogen runs out. The Sun, the star in the center of the Solar System to which the Earth is gravitationally bound, is an example of a G-type main-sequence star. Each second, the Sun fuses approximately 600 million tons of hydrogen into helium in a process known as the proton–proton chain, converting about 4 million tons of matter to energy. Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri, Tau Ceti, and 51 Pegasi.

<span class="mw-page-title-main">Sigma Draconis</span> Star in the constellation Draco

Sigma Draconis is a single star in the northern constellation of Draco. It has the proper name Alsafi, while Sigma Draconis, which is latinised from σ Draconis and abbreviated Sig Dra or σ Dra, is the Bayer designation. It has an apparent visual magnitude of 4.7, which is bright enough to be faintly visible to the naked eye. Based on parallax measurements, this star is located at a distance of 18.8 light years from the Sun. It is receding from the Sun with a radial velocity of 26.6 km/s.

<span class="mw-page-title-main">Subgiant</span> Type of star larger than main-sequence but smaller than a giant

A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star.

<span class="mw-page-title-main">K-type main-sequence star</span> Stellar classification

A K-type main-sequence star, also referred to as a K-type dwarf, or orange dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars and yellow/white G-type main-sequence stars. They have masses between 0.6 and 0.9 times the mass of the Sun and surface temperatures between 3,900 and 5,300 K. These stars are of particular interest in the search for extraterrestrial life due to their stability and long lifespan. Well-known examples include Alpha Centauri B and Epsilon Indi.

<span class="mw-page-title-main">F-type main-sequence star</span> Stellar classification

An F-type main-sequence star is a main-sequence, hydrogen-fusing star of spectral type F and luminosity class V. These stars have from 1.0 to 1.4 times the mass of the Sun and surface temperatures between 6,000 and 7,600 K.Tables VII and VIII. This temperature range gives the F-type stars a whitish hue when observed by the atmosphere. Because a main-sequence star is referred to as a dwarf star, this class of star may also be termed a yellow-white dwarf. Notable examples include Procyon A, Gamma Virginis A and B, and KIC 8462852.

<span class="mw-page-title-main">B-type main-sequence star</span> Stellar classification distinguished by bright blue luminosity

A B-type main-sequence star is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars are extremely luminous and blue. Their spectra have strong neutral helium absorption lines, which are most prominent at the B2 subclass, and moderately strong hydrogen lines. Examples include Regulus and Algol A.

HD 38529 is a binary star approximately 138 light-years away in the constellation of Orion.

109 Piscium is a yellow hued G-type main-sequence star located about 108 light-years away in the zodiac constellation of Pisces. It is near the lower limit of visibility to the naked eye with an apparent visual magnitude of 6.27. The star is moving closer to the Earth with a heliocentric radial velocity of −45.5 km/s. It has one known exoplanet.

<span class="mw-page-title-main">Tau Centauri</span> White-hued star in the constellation Centaurus

Tau Centauri, Latinized from τ Centauri, is a solitary star in the southern constellation of Centaurus. It is visible to the naked eye with an apparent visual magnitude of +3.86. The distance to this star, based upon an annual parallax shift of 24.85 mas, is 131 light years. There is a 98% chance that it is a co-moving companion of Gamma Centauri; the two stars have an estimated separation of 1.7 ly (0.53 pc).

HD 129685 is a single star in the southern constellation of Centaurus. It is also known by its Bayer designation c2 Centauri, while HD 129685 is the star's identifier in the Henry Draper catalogue. This object has a white hue and is faintly visible to the naked eye with an apparent visual magnitude of +4.92. It is located at a distance of approximately 231 light years from the Sun based on parallax, and it has an absolute magnitude of 0.83. The star is drifting closer with a radial velocity of around −5 km/s.

HD 111968, also known by the Bayer designation n Centauri, is a single star in the southern constellation of Centaurus. It is a white-hued star that is faintly visible to the naked eye with an apparent visual magnitude of +4.25. The star is located at a distance of approximately 149 light years from the Sun based on parallax. The radial velocity of the star is poorly constrained, with an estimated value of 2.5 km/s.

HD 195019 is a binary star system in the northern constellation of Delphinus. The brighter star has a close orbiting exoplanet companion. This system is located at a distance of 122 light years from the Sun based on parallax measurements, but it is drifting closer with a radial velocity of −91.3 km/s. Although it has an absolute magnitude of 4.01, at that distance the system is considered too faint to be viewed with the naked eye, having a combined apparent visual magnitude of 6.87. However, it should be readily visible with a pair of binoculars or a small telescope.

<span class="mw-page-title-main">O-type main-sequence star</span> Main-sequence star of spectral type O

An O-type main-sequence star is a main-sequence star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 50,000 K. They are between 40,000 and 1,000,000 times as luminous as the Sun.

HD 72659 is a star in the equatorial constellation of Hydra. With an apparent visual magnitude of 7.46, his yellow-hued star is too faint to be viewed with the naked eye. Parallax measurements provide a distance estimate of 169.4 light years from the Sun, and it has an absolute magnitude of 3.98. The star is drifting closer with a radial velocity of −18.3 km/s.

HD 126614 is a trinary star system in the equatorial constellation of Virgo. The primary member, designated component A, is host to an exoplanetary companion. With an apparent visual magnitude of 8.81, it is too faint to be seen with the naked eye. The system is located at a distance of 239 light years from the Sun based on parallax measurements, but is drifting closer with a radial velocity of −33 km/s.

<span class="mw-page-title-main">HD 114613</span> Star in the constellation Centaurus

HD 114613 is a fifth magnitude yellow subgiant that lies 66.7 light-years away in the constellation of Centaurus. The star may be host to a long-period giant planet.

HD 212771, also named Lionrock, is a solitary star in the southern zodiac constellation Aquarius. It has an apparent magnitude of 7.60, making it readily visible with binoculars but not the naked eye. Parallax measurements place the object at a distance of 364 light years, and is currently receding with a radial velocity of 15 km/s.

<span class="mw-page-title-main">34 Leonis Minoris</span> Star in the constellation Leo Minor

34 Leonis Minoris, also known as HD 91365 or 11 H. Leonis Minoris is a solitary star located in the northern constellation Leo Minor. It is faintly visible to the naked eye as a white-hued point of light with an apparent magnitude of 5.58. Gaia DR3 parallax measurements imply a distance of 510 light-years, and it is currently receding with a poorly constrained heliocentric radial velocity of 7 km/s. At its current distance, 34 LMi's brightness is diminished by interstellar extinction of 0.16 magnitudes and it has an absolute magnitude of −1.02.

References

  1. ↑ "Stellar Spectral Types". hyperphysics.phy-astr.gsu.edu. Retrieved June 19, 2007.
  2. ↑ Pecaut, Mark J.; Mamajek, Eric E. (1 September 2013). "Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-main-sequence Stars". The Astrophysical Journal Supplement Series. 208 (1): 9. arXiv: 1307.2657 . Bibcode:2013ApJS..208....9P. doi:10.1088/0067-0049/208/1/9. ISSN   0067-0049. S2CID   119308564.
  3. ↑ Mamajek, Eric (2 March 2021). "A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence". University of Rochester, Department of Physics and Astronomy. Retrieved 5 July 2021.
  4. ↑ Dale A. Ostlie; Bradley W. Carroll (2007). An Introduction to Modern Stellar Astrophysics. Pearson Addison-Wesley. ISBN   978-0-8053-0348-3.
  5. ↑ Habets, G. M. H. J.; Heintze, J. R. W. (1981). "Empirical bolometric corrections for the main-sequence". Astronomy and Astrophysics Supplement Series. 46: 193. Bibcode:1981A&AS...46..193H.Tables VII, VIII
  6. ↑ Schröder, C.; Schmitt, J. H. M. M. (November 2007). "X-ray emission from A-type stars". Astronomy and Astrophysics. 475 (2): 677–684. Bibcode:2007A&A...475..677S. doi: 10.1051/0004-6361:20077429 .
  7. ↑ Johnson, H. L.; Morgan, W. W. (1953). "Fundamental stellar photometry for standards of spectral type on the Revised System of the Yerkes Spectral Atlas". The Astrophysical Journal. 117: 313. Bibcode:1953ApJ...117..313J. doi:10.1086/145697.
  8. ↑ Robert F. Garrison. "MK ANCHOR POINTS". Archived from the original on 2019-06-25. Retrieved 2022-01-05.
  9. 1 2 Morgan, W. W.; Keenan, P. C. (1973). "Spectral Classification". Annual Review of Astronomy and Astrophysics. 11: 29. Bibcode:1973ARA&A..11...29M. doi:10.1146/annurev.aa.11.090173.000333.
  10. ↑ Morgan, W. W.; Abt, Helmut A.; Tapscott, J. W. (1978). Revised MK Spectral Atlas for stars earlier than the sun. Bibcode:1978rmsa.book.....M.
  11. ↑ Gray, R. O.; Garrison, R. F. (1987). "The Early A-Type Stars: Refined MK Classification, Confrontation with Stroemgren Photometry, and the Effects of Rotation". The Astrophysical Journal Supplement Series. 65: 581. Bibcode:1987ApJS...65..581G. doi: 10.1086/191237 .
  12. ↑ Gray, R. O.; Garrison, R. F. (1989). "The Late A-Type Stars: Refined MK Classification, Confrontation with Stroemgren Photometry, and the Effects of Rotation". The Astrophysical Journal Supplement Series. 70: 623. Bibcode:1989ApJS...70..623G. doi: 10.1086/191349 .
  13. ↑ Song, Inseok; et al. (2002). "M-Type Vega-like Stars". The Astronomical Journal. 124 (1): 514–518. arXiv: astro-ph/0204255 . Bibcode:2002AJ....124..514S. doi:10.1086/341164. S2CID   3450920.
  14. 1 2 Johnson, John Asher; Fischer, Debra A.; Marcy, Geoffrey W.; Wright, Jason T.; Driscoll, Peter; Butler, R. Paul; Hekker, Saskia; Reffert, Sabine; Vogt, Steven S. (2007). "Retired a Stars and Their Companions: Exoplanets Orbiting Three Intermediate‐Mass Subgiants". The Astrophysical Journal. 665: 785–793. arXiv: 0704.2455 . doi:10.1086/519677. S2CID   15076579.
  15. ↑ Johnson, J. A. (2011). "The Stars that Host Planets". Sky & Telescope (April): 22–27.
  16. ↑ Smalley, J. B. (2014). "Eclipsing Am binary systems in the SuperWASP survey". Astronomy and Astrophysics (April): 20.
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