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A B-type main-sequence star (B V) is a main-sequence (
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
-burning)
star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...
of spectral type B and luminosity class V. These stars have from 2 to 16 times the
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different element ...
of the Sun and
surface temperature Surface temperature is the temperature at a surface. Specifically, it may refer to: * Surface air temperature, the temperature of the air near the surface of the earth * Sea surface temperature, the temperature of water close to the ocean's sur ...
s between 10,000 and 30,000 K. B-type stars are extremely luminous and blue. Their spectra have neutral
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, which are most prominent at the B2 subclass, and moderate hydrogen lines. Examples include Regulus and
Algol A ALGOL (; short for "Algorithmic Language") is a family of imperative programming, imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm d ...
. This class of stars was introduced with the Harvard sequence of stellar spectra and published in the ''Revised Harvard photometry'' catalogue. The definition of type B-type stars was the presence of non- ionized helium lines with the absence of singly ionized helium in the blue-violet portion of the spectrum. All of the spectral classes, including the B type, were subdivided with a numerical suffix that indicated the degree to which they approached the next classification. Thus B2 is 1/5 of the way from type B (or B0) to type A. Later, however, more refined spectra showed lines of ionized helium for stars of type B0. Likewise, A0 stars also show weak lines of non-ionized helium. Subsequent catalogues of stellar spectra classified the stars based on the strengths of absorption lines at specific frequencies, or by comparing the strengths of different lines. Thus, in the MK Classification system, the spectral class B0 has the line at wavelength 439 nm being stronger than the line at 420 nm. The Balmer series of hydrogen lines grows stronger through the B class, then peak at type A2. The lines of ionized silicon are used to determine the sub-class of the B-type stars, while magnesium lines are used to distinguish between the temperature classes. Type-B stars don't have a corona and lack a convection zone in their outer atmosphere. They have a higher mass loss rate than smaller stars such as the Sun, and their stellar wind has velocities of about 3,000 km/s. The energy generation in main-sequence B-type stars comes from the CNO cycle of
thermonuclear fusion Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the re ...
. Because the CNO cycle is very temperature sensitive, the energy generation is heavily concentrated at the center of the star, which results in a convection zone about the core. This results in a steady mixing of the hydrogen fuel with the helium byproduct of the nuclear fusion. Many B-type stars have a rapid rate of
rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
, with an equatorial rotation velocity of about 200 km/s.


Be and B(e) stars

Spectral objects known as "Be stars" are massive yet non-
supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars s ...
entities that notably have, or had at some time, 1 or more Balmer lines in emission, with the
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
-related
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible ...
series projected out by the stars being of particular scientific interest. Be stars are generally thought to feature unusually strong stellar winds, high surface temperatures, and significant attrition of
stellar mass Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (). Hence, the bright star Sirius has around . A star's mass will vary over i ...
as the objects
rotate Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
at a curiously rapid rate, all of this in contrast to many other main-sequence star types. Though the related terminologies are confusingly ambiguous, spectral objects known as "B(e)" or "B stars" are distinct from Be stars since said B(e) entities are in possession of distinctive neutral or low ionization emission lines that are considered to have ' forbidden mechanisms', something denoted by the use of brackets or parenthesis. In other words, these particular stars' emissions appear to undergo processes not normally allowed under 1st-order perturbation theory in
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, q ...
. The definition of a "B(e) star" can include objects that are large enough to be in Blue giant and Blue supergiant territory, beyond the size of standard main-sequence stars.


Spectral Standard Stars

The revised Yerkes Atlas system (Johnson & Morgan 1953) listed a dense grid of B-type dwarf spectral standard stars, however not all of these have survived to this day as standards. The "anchor points" of the MK spectral classification system among the B-type main-sequence dwarf stars, i.e. those standard stars that have remain unchanged since at least the 1940s, are upsilon Orionis (B0 V),
eta Aurigae Eta Aurigae (η Aurigae, abbreviated Eta Aur, η Aur), officially named Haedus , is a star in the northern constellation of Auriga. With an apparent visual magnitude of 3.18, it is visible to the naked eye. Based upon parallax measuremen ...
(B3 V), eta Ursae Majoris (B3 V).MK ANCHOR POINTS
Robert F. Garrison
Spectral Classification
W.W. Morgan & P.C. Keenan, 1973, Annual Review of Astronomy and Astrophysics, vol. 11, p.29
Besides these anchor standards, the seminal review of MK classification by Morgan & Keenan (1973) listed "dagger standards" of Tau Scorpii (B0 V), Omega Scorpii (B1 V),
42 Orionis 42 Orionis is a class B1V (blue main-sequence) star in the constellation Orion. Its apparent magnitude is 4.59 and it is approximately 900 light years away based on parallax. The primary star, Aa, has one spectroscopic companion Ab of m ...
(B1 V),
22 Scorpii 22 Scorpii (i Scorpii) is a single star in the southern zodiac constellation of Scorpius, about one degree from Antares. It is faintly visible to the naked eye with an apparent visual magnitude of 4.78. The distance to this star is estimate ...
(B2 V), Rho Aurigae (B5 V), and
18 Tauri 18 Tauri is a single star in the zodiac constellation of Taurus, located 444 light years away from the Sun. It is visible to the naked eye as a faint, blue-white hued star with an apparent visual magnitude of 5.66. The star is moving ...
(B8 V). The Revised MK Spectra Atlas of Morgan, Abt, & Tapscott (1978)Revised MK Spectral Atlas for stars earlier than the sun
W.W. Morgan, W. W., H.A. Abt, J.W. Tapscott, 1978, Williams Bay: Yerkes Observatory, and Tucson: Kitt Peak National Observatory
further contributed the standards Beta2 Scorpii (B2 V),
29 Persei 29 Persei is a single star in the northern constellation of Perseus, located approximately 640 light years away from the Sun based on parallax. It is visible to the naked eye as faint, blue-white hued star with an apparent visual magnit ...
(B3 V), HD 36936 (B5 V), and
HD 21071 HD 21071 is a blue-white hued variable star in the northern constellation of Perseus. Also known as V576 Persei, it is a sixth magnitude star that is dimly visible to the naked eye under good viewing conditions, having an apparent visual ...
(B7 V). Gray & Garrison (1994) contributed two B9 V standards:
Omega Fornacis Omega Fornacis, which is Latinized from ω Fornacis, is a wide binary star system in the southern constellation of Fornax. It has a blue-white hue and is faintly visible to the naked eye as a fifth-magnitude star. The system lies at a d ...
and
HR 2328 HR, Hr or hr may refer to: Arts and media Film and television * '' H.R. Pufnstuf'', a children's television series from 1969 * ''HR'', a 2013 television drama starring Alicia Silverstone * HR, a criminal organisation in the American TV series ''P ...
. The only published B4 V standard is 90 Leonis, from Lesh (1968). There has been little agreement in the literature on choice of B6 V standard.


Chemical peculiarities

Some of the B-type stars of stellar class B0–B3 exhibit unusually strong lines of non-ionized helium. These chemically peculiar stars are termed helium-strong stars. These often have strong magnetic fields in their photosphere. In contrast, there are also helium-weak B-type stars with understrength helium lines and strong hydrogen spectra. Other chemically peculiar B-types stars are mercury–manganese stars with spectral types B7-B9. Finally, the aforementioned Be stars show a prominent emission spectrum of hydrogen.


Planets

B-type stars known to have planets include the main-sequence B-types HIP 78530, the subgiants Kappa Andromedae and a few (19 are now known) B-type subdwarfs.


See also

* Herbig Ae/Be star * Stellar classification, Class B *
Star count Star counts are bookkeeping surveys of stars and the statistical and geometrical methods used to correct the survey data for bias. The surveys are most often made of nearby stars in the Milky Way galaxy. One of the interests of astronomy is to de ...


References

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