Stellar classification (astrophysics)

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Stars are classed by various properties. A common classification is surface temperature. Beginning with early spectral schema in the 19th century ranking stars from A to P, the spectral classification, called “Morgan-Keenan spectral classification” [1][2] now ranks stars in seven main types: O, B, A, F, G, K, M.[3] Each spectral classification is further divided into tenths. Our sun, Sol is a G2.

History

In 1801,[4] William Hyde Wollaston (1766-1828)[5] noted absorption lines in the solar spectrum. The solar spectrum was not a continuous band of colour and included a series of dark lines. Wollaston perceived this as a possible boundary between colours.

Joseph von Fraunhofer (1787-1826)[6] in 1814, studied this phenomena and concluded that the dark bands represented specific wavelengths.

In 1863, Sir William Huggins (1824-1910)[7] laid the final piece of the puzzle in place and compared these solar spectrums with other stars and terrestrial substances.[8] In this way it was demonstrated that the dark bands, now known as Fraunhofer Lines,[9] are the absorption lines for chemical elements present in the stars. Specific chemicals present in the outer layers of a star absorb specific wavelengths creating the dark lines. This also showed that the stars are made of the same substances here on Earth.

While this work provided for the foundation of the field of spectroscopy it also provided a tool for astrophysics and astronomy. These distinctive spectral lines which provide information about the chemical composition of the stars have become the basis for their classification.

Classification system

The stars are ranked from hottest to coldest:

Class Temperature Conventional vs Apparent color

[10]

Characteristics

[11]

Mass [12] Radius [13] Luminosity [14] Hydrogen lines % of all MSSs[15][16]
O 30,000–60,000 K blue/blue Ionized He & metals;

weak H

60 M☉ 15 R☉ 1,400,000 L☉ Weak ~0.00003%
B 10,000–30,000 K blue white/blue white to white Neutral He, ionized metals, stronger H 18 M☉ 7 R☉ 20,000 L☉ Medium 0.13%
A 7,500–10,000 K white/white Balmer H dominant, singly-ionized metals 3.1 M☉ 2.1 R☉ 80 L☉ Strong 0.6%
F 6,000–7,500 K yellowish white/white H weaker, neutral & singly-ionized metals 1.7 M☉ 1.3 R☉ 6 L☉ Medium 3%
G 5,000–6,000 K yellow/yellow Singly ionized Ca, H weaker, neutral metals 1.1 M☉ 1.1 R☉ 1.2 L☉ Weak 8%
K 3,500–5,000 K orange/yellow orange Neutral Metals, molecular bands begin to appear 0.8 M☉ 0.9 R☉ 0.4 L☉ Very weak 13%
M 2,000–3,500 K red/orange red Ti oxide molecular lines; neutral metals 0.3 M☉ 0.4 R☉ 0.04 L☉ Very weak >78%

[17]

Luminosity classes

Another schema for classifying stars is the Yerkes classification.[18]. This classification scheme measures surface gravities of stars by measuring the shape and nature of certain spectral lines.

Ia Most luminous supergiants
Ib Less luminous super giants
II Luminous giants
III Normal giants
IV Subgiants
V Main sequence stars (dwarf)
VI Main sequence stars (subdwarf)
VII Main sequence stars (white dwarf)

[19]


References

  1. ↑ Note: based on the work of Annie Cannon and other astronomers from the Harvard College Observatory
  2. ↑ Annie Jump Cannon 1883-1941
  3. ↑ A popular mnemonic for this is Oh Be A Fine Girl, Kiss Me Mnemonics for the Harvard Spectral Classification Scheme
  4. ↑ Note: Sources give varying dates between 1801 and 1807
  5. ↑ William Hyde Wollaston Hebrew University of Jerusalem, Institute of Chemistry; William Hyde Wollaston : 1766 – 1828 Dean P. Currier. Australian Centre for the moving image; William Hyde Wollaston Carl Zeiss AG; William Hyde Wollaston Michael W. Davidson, Florida State University; History of Solar Physics - A Time Line of Great Moments 1800-1999 National Center for Atmospheric Research & The UCAR Office Programs; William Hyde Wollaston (1766-1828) High Altitude Observatory, National Center for Atmospheric Research & The UCAR Office Programs
  6. ↑ Joseph von Fraunhofer High Altitude Observatory, National Center for Atmospheric Research & The UCAR Office Programs; Joseph von Fraunhofer New Advent Catholic Encyclopedia; Joseph von Fraunhofer Fraunhofer-Gesellschaft
  7. ↑ William Huggins (February 7, 1824 - May 12, 1910) Messier Catalog, Students for the Exploration and Development of Space
  8. ↑ William Huggins 1904 Bruce Medalist Astronomical Society of the Pacific
  9. ↑ Identificatio of spectral lines-History of Fraunhofer lines University of Ottawa; The Fraunhofer Lines Jet Propulsion Lab, California Institute of Technology, NASA
  10. ↑ The Colour of Stars. Australia Telescope Outreach and Education (December 21 2004). Retrieved on 2007-10-26. — Explains the reason for the difference in color perception.
  11. ↑ The Hertzasprung-Russell diagram (subsection The Harvard spectral sequence). University of Tennessee, Dept of Physics and Astronomy (unknown). Retrieved on 2007-10-26. — Details various aspects of stellar classification.
  12. ↑ NASA glossary Solar Mass. Earth's sun is one solar mass. Equal to about 2 X 10E30 kg or about 333,000 time the mass of the earth
  13. ↑ Solar Radius. Earth's sun is the standard with one solar radius equal to about 6.960 X 10E8 km. This is about twice the distance of the earth from the moon, 110 times the radius of earth, and 0.004652 AU (astronomical unit), the average distance of the earth from the sun. Solar Radius University of Oregon. The Corona Spartan 201. NASA
  14. ↑ Solar Luminosity. The standard is earth's sun equal to one solar luminosity. The luminosity of the sun is about 3.827 X 10E26 watts or 3.827 X 10E33 ergs
  15. ↑ MSS = Main Sequence Star
  16. ↑ LeDrew, G.; The Real Starry Sky, Journal of the Royal Astronomical Society of Canada, Vol. 95, No. 1 (whole No. 686, February 2001), pp. 32–33
  17. ↑ Star classification
  18. ↑ Yerkes Observatory. Also known as MKK using the initials of the authors Morgan, William W., Keenan, Philip C. and Kellman, Edith
  19. ↑ Classification of Stellar spectra Paul Crowther, Dept of Physics and Astronomy, University of Sheffield

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