Stellar classification

In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. Stellar temperatures can be classified by using Wien's displacement law; but this poses difficulties for distant stars. Stellar spectroscopy offers a way to classify stars according to their absorption lines; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. An early schema (from the 19th century) ranked stars from A to P, which is the origin of the currently used spectral classes.
==Morgan-Keenan system==
Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
Below is the table in which the common classes are seperated and normally listed from hottest to coldest (with mass, radius and luminosity compared to the Sun) and are:
Class	Temperature 	Star colour 	Mass	Radius	Luminosity
O 	30,000 - 60,000 K	Bluish ("blue") 	60 	15 	1,400,000
B 	10,000 - 30,000 K	Bluish-white ("blue-white") 	18 	7 	20,000
A 	7,500 - 10,000 K 	White with bluish tinge ("white")	3.2 	2.5 	80
F 	6,000 - 7,500 K 	White ("yellow-white") 	1.7 	1.3 	6
G 	5,000 - 6,000 K 	Light yellow ("yellow") 	1.1 	1.1 	1.2
K 	3,500 - 5,000 K 	Light orange ("orange") 	0.8 	0.9 	0.4
M 	2,000 - 3,500 K 	Reddish orange ("red") 	0.3 	0.4 	0.04
In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for subgiants, class V for main-sequence stars, class sd (or VI) for subdwarfs, and class D (or VII) for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a surface temperature around 5,800 K.
==Hertzsprung-Russell diagram==
The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature. It should be noted that while these descriptions of stellar colors are traditional in astronomy, they really describe the light after it has been scattered by the atmosphere. The diagram was created independently in 1911 by Ejnar Hertzsprung and by Henry Norris Russell in 1913, and represented a major step towards an understanding of stellar evolution.
Category:Stars
Category:Heliophysics