Stellar classification (astrophysics)

Stellar classification is the categorization of stars 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” now ranks stars in seven main types: O, B, A, F, G, K, M. Each spectral classification is further divided into tenths. Our sun, Sol, is a G2.

History

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.

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

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. 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.

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	Characteristics	Mass	Radius	Luminosity	Hydrogen lines	% of all MSSs
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%

Luminosity classes

Another schema for classifying stars is the Yerkes classification, also known as M-K established in 1943. 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)

Stellar evolution

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.

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.

See also

Wolf 359 (star)

Proxima Centauri (star)

Electromagnetic spectrum

Infrared light

Light