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An A-type main-sequence star (A V) 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][2] They measure between 1.4 and 2.1 solar masses (M_☉) and have surface temperatures between 7,600 and 10,000 K.[3] Bright and nearby examples are Altair (A7 V), Sirius A (A1 V), and Vega (A0 V).[4] 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.[5]

In July 2019, astronomers reported finding an A-type star, S5-HVS1, traveling 1,755 km/s (3,930,000 mph), faster than any other star detected so far. The star is in the Grus (or Crane) constellation in the southern sky, about 29,000 light-years from Earth, and may have been ejected out of the Milky Way after interacting with Sagittarius A*, the supermassive black hole at the center of the galaxy.[6][7][8][9][10]

Spectral standard stars

Properties of typical A-type main-sequence stars[11][12]

Spectral type	Mass (M☉)	Radius (R☉)	Luminosity (L☉)	Effective temperature (K)	Color index (B − V)
A0V	2.18	2.193	38.02	9,700	0.00
A1V	2.05	2.136	30.90	9,300	0.04
A2V	1.98	2.117	23.99	8,800	0.07
A3V	1.93	1.861	16.98	8,600	0.10
A4V	1.88	1.794	13.49	8,250	0.14
A5V	1.86	1.785	12.30	8,100	0.16
A6V	1.83	1.775	11.22	7,910	0.19
A7V	1.81	1.750	10.00	7,760	0.21
A8V	1.77	1.747	9.12	7,590	0.25
A9V	1.75	1.747	8.32	7,400	0.27

The revised Yerkes Atlas system[13] 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).[14][15] The seminal review of MK classification by Morgan & Keenan (1973)[15] didn't provide any dagger standards between types A3 V and F2 V. HD 23886 was suggested as an A5 V standard in 1978.[16] Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 1987[17] and 1989.[18] 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.

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.[19] 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.[20] 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.[20] 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. [21]

A-type star systems known to feature planets include Fomalhaut, HD 15082, Beta Pictoris, and HD 95086.[22]

Examples

Within 40 light years:

Name	Spectral type	Constellation	vis Mag	Mass (M☉)	Radius (R☉)	Luminosity (L☉)	Distance (ly)
Sirius	A0mA1 Va	Canis Major	−1.47	2.063	1.711	25.4	8.60 ± 0.04
Altair	A7 V	Aquila	0.76	1.79	1.63–2.03	10.6	16.73
Vega	A0 Va	Lyra	0.026	2.135	2.362 × 2.818	40.12	25.04
Fomalhaut	A3 V	Piscis Austrinus	1.17	1.91	1.84	16	25.1
Denebola	A3 V	Leo	2.14	1.78	1.73	15	35.8
Delta Capricorni	A5 IV	Capricornus	2.83	2.0	1.91	11	38.6

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

See also

• Star count, survey of stars
• B-type main-sequence star

References

На других языках

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- [en] A-type main-sequence star

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[es] Estrella blanca de la secuencia principal

En astronomía, una estrella blanca de la secuencia principal (denominada también estrella de tipo-A de la secuencia principal o enana blanca A) es una estrella de tipo espectral A y clase de luminosidad V que se encuentra en la fase evolutiva -al igual que en el Sol- de donde su energía proviene de la fusión del hidrógeno en helio. Estas estrellas tienen espectros que están definidos por fuertes líneas de absorción del hidrógeno de Balmer así como líneas de metales ionizados. Tienen masas de 1.4 a 2.1 veces la masa del Sol y temperaturas superficiales entre 7.600 y 10.000 K. Ejemplos de estrellas blancas brillantes de la secuencia principal y cercanas al Sistema Solar son: Altair (A7 V), Sirio A (A1 V) y Vega (A0 V).

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[ru] Звёзды главной последовательности спектрального класса A

Звёзды главной последовательности спектрального класса A (A V) — это карликовые звёзды главной последовательности, использующие водород в качестве топлива, спектрального класса A и класса светимости V. Эти звёзды имеют спектры, которые определяются сильными линиями бальмеровского поглощения водорода[1][2]. Эти звёзды имеют массу в 1,4-2,1 раз больше массы Солнца и температуры поверхности от 7 000 до 11 500 К[3]. Этот температурный диапазон придаёт звёздам A-типа бело-жёлтый оттенок. Поскольку звезда главной последовательности называются карликовыми звёздами, то этот класс звёзд можно также назвать бело-жёлтыми карликами. Яркими и близкими примерами являются Альтаир (A7 V), Сириус A (A1 V) и Вега (A0 V)[4].

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