An A-type main-sequence star (A) or A dwarf star is a
main-sequence (
hydrogen burning
In astrophysics, stellar nucleosynthesis is the creation of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...
)
star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
of
spectral type A and
luminosity class (five). These stars have
spectra defined by strong
hydrogen Balmer absorption lines. They measure between 1.7 and 2.1
solar mass
The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...
es (), have
surface temperatures between 7,600 and 10,000
K, and live for about a quarter of the lifetime of our Sun. 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 wind
A stellar wind is a flow of gas ejected from the stellar atmosphere, upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spheri ...
s, they lack a means to generate
X-ray emissions.
Spectral standard stars
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 remained unchanged over years and can be considered to define the system, are
Vega (A0 V),
Phecda (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
Zosma (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
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
(IR) radiation beyond what would be expected from the star alone. This IR excess is attributable to dust emission from a
debris disk
A debris disk (American English), or debris disc ( Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris ...
where planets form.
Surveys indicate massive
planets
A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the te ...
commonly form around A-type stars although these planets are difficult to detect using the
Doppler spectroscopy
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in ...
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
Gamma Cephei (γ Cephei, abbreviated Gamma Cep, γ Cep) is a binary star system approximately 45 light-years away in the northern constellation of Cepheus (constellation), Cepheus. The primary (designated Gamma Cephei A, officially name ...
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.
[
]
A-type star systems known to feature planets include
HD 15082,
Beta Pictoris,
HR 8799 and
HD 95086.
[
]
Examples
Within 40 light years:
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
*
Star count, survey of stars
*
B-type main-sequence star
References
{{DEFAULTSORT:A-type main-sequence star
Star types