B-type main-sequence star

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Artist's impression of a B-type star

A B-type main-sequence star (B V) is a

Algol A and Acrux[2]

History

This class of stars was introduced with the Harvard sequence of stellar spectra and published in the

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.[3][4]

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.[5] 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.[4]

Properties

Properties of typical B-type main-sequence stars[6][7]
Spectral
type
 Mass (M☉)
Radius (R☉
) 		Luminosity (L☉) Effective
temperature
(K) 		Color
index
(B − V)
B0V 17.70 7.16 44,668 31,400 -0.301
B1V 11.00 5.71 13,490 26,000 -0.278
B2V 7.30 4.06 2,692 20,600 -0.215
B3V 5.40 3.61 977 17,000 -0.178
B4V 5.10 3.46 776 16,400 -0.165
B5V 4.70 3.36 589 15,700 -0.156
B6V 4.30 3.27 372 14,500 -0.140
B7V 3.92 2.94 302 14,000 -0.128
B8V 3.38 2.86 155 12,300 -0.109
B9V 2.75 2.49 72 10,700 -0.070

Type-B stars do not have a

thermonuclear fusion. 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.[9] Many B-type stars have a rapid rate of rotation, with an equatorial rotation velocity of about 200 km/s.[10]

Be and B[e] stars

Main articles: Be star and B[e] star

Spectral objects known as "Be stars" are massive yet non-

Balmer lines in emission, with the hydrogen-related electromagnetic radiation 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 as the objects rotate at a curiously rapid rate, all of this in contrast to many other main-sequence star types.[11]

Objects known as

emission lines that are considered to have 'forbidden mechanisms', something denoted by the use of the square brackets. In other words, these particular stars' emissions appear to undergo processes not normally allowed under 1st-order perturbation theory in quantum mechanics. The definition of a B[e] star can include blue giants and blue supergiants
.

Spectral standard stars

The secondary component of the double star Albireo is a B8 main sequence star, the blue contrasting with the cooler yellow giant primary.

The revised Yerkes Atlas system (Johnson & Morgan 1953)

Eta Ursae Majoris (B3 V).[13][14]
Besides these anchor standards, the seminal review of MK classification by Morgan & Keenan (1973)[14] listed "dagger standards" of Tau Scorpii (B0 V), Omega Scorpii (B1 V), 42 Orionis (B1 V), 22 Scorpii (B3 V), Rho Aurigae (B5 V), and 18 Tauri (B8 V). The Revised MK Spectra Atlas of Morgan, Abt, & Tapscott (1978)[15] further contributed the standards Beta2 Scorpii (B2 V), 29 Persei (B3 V), HD 36936 (B5 V), and HD 21071 (B7 V). Gray & Garrison (1994)[16] contributed two B9 V standards:
90 Leonis
, from Lesh (1968).[17] 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 the mercury-manganese stars
with spectral types B7-B9.

Planets

B-type stars known to have planets include the main-sequence B-type HIP 78530 and HD 129116.

See also

References

  1. Bibcode:1981A&AS...46..193H
    ., Tables VII and VIII.
  2. ^ SIMBAD, entries on Regulus, Algol A and Acrux accessed on June 19, 2007.
  3. Bibcode:1908AnHar..50....1P
    . Retrieved 2009-09-21.
  4. ^
    ISBN 978-0691125114
    .
  5. Bibcode:1943assw.book.....M
    .
  6. S2CID 119308564
    .
  7. ^ 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.
  8. ISBN 0387949283
    .
  9. ISBN 0521348714
    .
  10. Bibcode:1965Obs....85..166M
    .
  11. doi:10.1086/132234
    .
  12. ^ Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313
  13. ^ MK ANCHOR POINTS Archived 2019-06-25 at the Wayback Machine, Robert F. Garrison
  14. ^ a b Spectral Classification, W.W. Morgan & P.C. Keenan, 1973, Annual Review of Astronomy and Astrophysics, vol. 11, p.29
  15. ^ 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
  16. ^ The late B-type stars: Refined MK classification, confrontation with stromgren photometry, and the effects of rotation, R.F. Gray & R.O. Garrison, 1994, The Astronomical Journal, vol. 107, no. 4, p. 1556-1564
  17. ^ The Kinematics of the Gould Belt: an Expanding Group? J.R. Lesh, 1968, Astrophysical Journal Supplement, vol. 17, p.371 (Table 1)
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