Stellar classification is the categorization of stars by various properties. A common classification is surface temperature. Beginning with early spectral schema[1] in the 19th century ranking stars from A to P, the spectral classification, called “Morgan-Keenan spectral classification” [2][3] now ranks stars in seven main types: O, B, A, F, G, K, M.[4] Each spectral classification is further divided into tenths. Our sun, Sol, is a G2.
In 1801, William Hyde Wollaston (1766-1828) 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.[5]
Joseph von Fraunhofer (1787-1826) in 1814, studied this phenomena and concluded that the dark bands represented specific wavelengths.[6]
In 1863, Sir William Huggins (1824-1910) employed the research of Fraunhofer, Gustav Robert Kirchhoff and Robert Wilhelm Eberhard Bunsen to lay the final piece of the puzzle in place and compared these solar spectrums with other stars and terrestrial substances.[7] In this way it was demonstrated that the dark bands, now known as Fraunhofer Lines, 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.[8]
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.
The stars are ranked from hottest to coldest:
Class | Temperature | Conventional vs Apparent color | Characteristics | Mass [11] | Radius [12] | Luminosity [13] | Hydrogen lines | % of all MSSs[14][15] |
---|---|---|---|---|---|---|---|---|
O | 30,000–60,000 K | blue/blue | Ionized He & metals;
weak H |
10-70 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% |
Another schema for classifying stars is the Yerkes classification,[18] also known as M-K established in 1943.[19] 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) |
Depending on their mass, stars will undergo various stages in greatly varying lengths of time. Larger stars are shorter lived than the smallest and their transitions from one stage to another as they burn their fuel can be gradual, such as a red dwarf, or very explosive, such as the largest supergiant. The diagramme below shows the early stage of each class of star and their intermediate and final stages. The length of time roughly corresponds to distance they are spaced on the diagramme.
Early stage | ||||
---|---|---|---|---|
Protostar | Blue supergiant | Supernova (final stage) | ||
Protostar | Blue supergiant | Black hole (final stage) | ||
Protostar | Blue supergiant | Type II Supernova | Black hole (final stage) | |
Protostar | Blue supergiant | Red giant | Type II supernova | Neutron Star (final stage) |
Protostar | Sun-like star | Red giant | Planetary nebula | White dwarf (final stage) |
Protostar | Red dwarf | Red Dwarf | White Dwarf (final stage) | |
Protostar | Brown dwarf | Brown dwarf (final stage) |
Until recently it was believed that blue supergiants would either burn their fuel and collapse into a black hole or undergo a massive explosion and then form a black hole. However, the recent discovery of SN 2006gy shows that at least in this one case, the initial explosion is so complete the largest blue supergiant is completely obliterated.[22]
For stars with a solar mass of about 10-25 (10-25 times the mass of earth's Sun) they will first become red giants. Usually they remain red giants until they explode. For stars with greater masses (more than approximately 20 solar masses) the process of evaporation and radiation pressure may blow out the outer layers and forming a blue giant before it explodes. SN1987A, for example, was apparently a blue giant before it exploded.[23]