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. These stars have spectra which are defined by strong hydrogen Balmer absorption lines. They have masses from 1.4 to 2.1 times the mass of the Sun and surface temperatures between 7600 and 10,000 K. Bright and nearby examples are Altair (A7 V), Sirius A (A1 V), and Vega (A0 V). A-type stars don't have a convective zone and thus aren't expected to harbor a magnetic dynamo. As a consequence, because they don't have strong stellar winds they lack a means to generate X-ray emission.
The revised Yerkes Atlas system 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 remain unchanged over years and can be considered to define the system, are Vega (A0 V), Gamma Ursae Majoris (A0 V), and Fomalhaut (A3 V). The seminal review of MK classification by Morgan & Keenan (1973) didn't provide any dagger standards between types A3 V and F2 V. HD 23886 was suggested as an A5 V standard in 1978. Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 1987 and 1989. 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.
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. 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. 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. 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. 
|Vega||A0 Va||Lyra||0.026||2.135||2.362 × 2.818||40.12||25.04|
|Sirius||A0mA1 Va||Canis Major||-1.47||2.063||1.711||25.4||8.60 ± 0.04|
10 Aquarii (abbreviated 10 Aqr) is a A-type main-sequence star in the constellation of Aquarius. 10 Aquarii is the Flamsteed designation. It is approximately 400 ly from Earth, based on parallax.14 Aquilae
14 Aquilae (abbreviated 14 Aql) is a single star in the equatorial constellation of Aquila. 14 Aquilae is the Flamsteed designation though it also bears the Bayer designation g Aquilae. Its apparent magnitude is 5.42 and it is located at a distance of approximately 500 light-years (150 parsecs) from Earth. This is an A-type main sequence star with a stellar classification of A1 V. It is about twice the radius of the Sun and has a high rate of rotation with a projected rotational velocity of 23 km/s.19 Aquarii
19 Aquarii is a star in the constellation of Aquarius. With an apparent magnitude of about 5.7, the star is barely visible to the naked eye (see Bortle scale). Parallax estimates made by the Hipparcos spacecraft put it at a distance of about 300 light years (92 parsecs) away from the Earth.19 Aquarii has a spectral type of A8V, meaning it is an A-type main-sequence star. These types of stars are bluish-white colored, and have effective temperatures between 7100 and 11500 K. It rotates fairly fast, as its projected rotational velocity is about 155 km/s, so it must be rotating at least that fast.22 Aquilae
22 Aquilae, abbreviated 22 Aql, is a star in the equatorial constellation of Aquila. 22 Aquilae is its Flamsteed designation. It is a faint star but visible to the naked eye with an apparent visual magnitude of 5.59. The distance to 22 Aql can be estimated from its annual parallax shift of 4.74 mas, which yields a separation of 690 light years. It is moving closer to the Earth with a heliocentric radial velocity of −23 km/s.Cowley et al. (1969) assigned this star a stellar classification of A3 IV, matching an evolving subgiant star that has exhausted the hydrogen at its core. Houk and Swift (1999) reassigned it as an A-type main-sequence star with a class of A1 V. It has nearly three times the mass of the Sun and is spinning with a projected rotational velocity of 70 km/s. The star is radiating 161 times the Sun's luminosity from its photosphere at an effective temperature of 8,453 K.40 Aurigae
40 Aurigae is a binary star in the constellation Auriga. Its apparent magnitude is 5.345, meaning it can just barely be seen with the naked eye. Based on parallax estimates made by the Hipparcos spacecraft, the system is located some 340 light-years (104 parsecs) away.40 Aurigae is a spectroscopic binary, meaning the two stars are too close to be individually resolved, but periodic Doppler shifts in their spectra indicate there must be orbital motion. In this case, light from both stars can be detected and it is a double-lined spectroscopic binary. The two have an orbital period of 28.28 days and a fairly high eccentricity of 0.56. The primary star is an A-type main-sequence star and shows unusual absorption lines in its spectrum, so it is an Am star with an effective temperature of 7,838 K.51 Aquarii
51 Aquarii (abbreviated 51 Aqr) is a star in the equatorial constellation of Aquarius. 51 Aquarii is its Flamsteed designation. It has an apparent visual magnitude is 5.78 and is located around 410 light-years (130 parsecs) from Earth. This is an A-type main sequence star with a stellar classification of A0 V. It has a high rate of rotation with a projected rotational velocity of 91 km/s.70 Cancri
70 Cancri is a star in the zodiac constellation of Cancer, located around 580 light years from the Sun. It is a challenge to view with the naked eye even under good seeing conditions, having an apparent visual magnitude of 6.7. The star is moving closer to the Earth with a heliocentric radial velocity of -21 km/s, and is expected to come to within 44 light-years in around nine million years. It is an A-type main-sequence star with a stellar classification of A1V. The object has a radius of about 2.7 R☉ and is radiating 76 times the Sun's luminosity from its photosphere at an effective temperature of 8,887 K.Beta Circini
Beta Circini, Latinized from β Circini, is an A-type main sequence star and is the second-brightest star in the constellation of Circinus. It has an apparent visual magnitude of approximately 4.069, which is bright enough to be viewed with the naked eye. Based upon an annual parallax shift of 35.17 mas as seen from the Earth, it is located about 93 light years from the Sun.
With a stellar classification of A3 Va, this is an A-type main-sequence star. It is between 370 and 500 million years old with around 1.3 times the Sun's radius. The star is radiating 19 times the Sun's luminosity from its photosphere at an effective temperature of 8,676 K. It has one known sub-stellar companion.Exocomet
An exocomet, or extrasolar comet, is a comet outside the Solar System, which includes interstellar comets and those that orbit stars other than the Sun. The first exocomets were detected in 1987 around Beta Pictoris, a very young A-type main-sequence star. There are now a total of 11 stars around which exocomets have been observed or suspected.All discovered exocometary systems (Beta Pictoris, HR 10, 51 Ophiuchi, HR 2174, 49 Ceti, 5 Vulpeculae, 2 Andromedae, HD 21620, HD 42111, HD 110411, and more recently HD 172555) are around very young A-type stars.
The exocomets can be detected by spectroscopy as they transit their host stars. The transits of exocomets, like the transits of exoplanets, produce variations in the light received from the star. Changes are observed in the absorption lines of the stellar spectrum: the occultation of the star by the gas cloud coming from the exocomet produces additional absorption features beyond those normally seen in that star, like those observed in the ionized calcium lines. As the comet comes close enough to the star, cometary gas is evolved from the evaporation of volatile ices and dust with it.Observations of comets, and especially exocomets, improve our understanding of planet formation. Indeed, in the standard model of planet formation by accretion, planets are the result of the agglomeration of planetesimals, themselves formed by the coalescence of dust from the protoplanetary disk surrounding the star shortly after its formation. Thus, comets are the residuals of the volatile-rich planetesimals that remained in the planetary system without having been incorporated into the planets. They are considered fossil bodies that have seen the physical and chemical conditions prevailing at the time of planet formation.A gaseous cloud around 49 Ceti has been attributed to the collisions of comets in that planetary system.HD 127304
HD 127304 is a double star in the northern constellation of Boötes. The brighter component is a sixth magnitude A-type main sequence star with a stellar classification of A0Vs It has a faint magnitude 10.62 companion at an angular separation of 25.6″ along a position angle of 256° (as of 2013).HD 130917
HD 130917 is a single star in the northern constellation of Boötes. It is an A-type main sequence star with a stellar classification of A4V. At an apparent magnitude of 5.80, it is visible to the naked eye.HD 153791
HD 153791 is a double star in the southern constellation of Ara. The primary is a sixth magnitude A-type main sequence star. It has a magnitude 12.3 companion at an angular separation of 6.0″ along a position angle of 249°, as of 1999.HD 181342
HD 181342 is a star in the constellation of Sagittarius. With an apparent magnitude of 7.55, it cannot be seen with the naked eye. Parallax measurements made by Hipparcos put the star at a distance of 360 light-years (111 parsecs) away.HD 181342 is a K-type red giant star. It was formerly an A-type main-sequence star, but at an age of 1.56 billion years it has swelled up to a size of 4.55 solar radii. It is currently 1.78 times the mass of the Sun, 16.2 times as luminous, and its surface temperature is 4976 K.HD 187734
HD 187734 is double star in the equatorial constellation of Aquila. The primary is a magnitude 6.6 giant star, while the companion is a magnitude 9.4 A-type main sequence star. As of 2014, the pair had an angular separation of 5″ along a position angle of 99°.HD 205765
HD 205765 is a star in the equatorial constellation of Aquarius. It is an A-type main sequence star with an apparent magnitude of 6.2, which, according to the Bortle scale, makes it faintly visible to the naked eye from dark rural skies. This star is spinning rapidly with a projected rotational velocity of 172 km/s.HD 34557
HD 34557 is a double star in the northern constellation of Auriga. The fainter star has an angular separation of 0.380″ from the primary component. They have a combined apparent magnitude of 5.52, making HD 34557 faintly visible to the naked eye from dark skies. Based upon parallax measurements made with the Hipparcos satellite, this system is roughly 280 light years away. The primary component is spinning rapidly, with a projected rotational velocity of 217 km/s. It has a stellar classification of A3V, making it an A-type main sequence star.HD 34790
HD 34790 is a double-lined spectroscopic binary star system in the northern constellation of Auriga. It has a combined apparent magnitude of 5.66, which means it is faintly visible to the naked eye. Based upon observations by the Hipparcos satellite, it is located around 289 light years away. It has a combined stellar classification of A1Vs, matching that of an A-type main sequence star, and shines with 35 times the luminosity of the Sun.The two stars orbit each other with a period of only 2.15 days and an eccentricity of zero, indicating their orbit is close to circular. They are orbiting sufficiently close to each other that their rotation periods have most likely become tidally locked—meaning they always maintain the same face toward each other.KOI-74
KOI-74 (KIC 6889235) is an eclipsing binary star in the constellation of Cygnus. The primary star is an A-type main-sequence star with a temperature of 9,400 K (9,130 °C; 16,460 °F). It lies in the field of view of the Kepler Mission and was determined to have a companion object in orbit around it which is smaller and hotter than the main star.Omega2 Cancri
ω2 Cancri is a star in the zodiac constellation Cancer, located around 810 light years away from the Sun. It has the Flamsteed designation 4 Cancri; ω2 Cancri is the Bayer designation, which is Latinised to omega2 Cancri and abbreviated to ω2 Cnc or omega2 Cnc. The star is near the lower limit of visibility to the naked eye, having an apparent visual magnitude of 6.32. It is moving closer to the Earth with a heliocentric radial velocity of −8 km/s. The position of this star near the ecliptic means it is subject to lunar occultations.This is an ordinary A-type main-sequence star with a stellar classification of A1 V, which indicates it is generating energy through hydrogen fusion at its core. It has 2.6 times the mass of the Sun and about 2.5 times the Sun's radius. The star is radiating 68 times the Sun's luminosity from its photosphere at an effective temperature of 9354 K.
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