Editing Stellar classification (section)

{{Short description|Classification of stars based on spectral properties}}
{{Redirect|Star classification|the ranking system, e.g. "four out of five stars"|Star (classification)}}
{{Use dmy dates|date=December 2017}}
[[File:Stellar Classification Chart.png|thumb|upright=1.75|A simple chart for classifying the main star types using Harvard classification]]
In [[astronomy]], '''stellar classification''' is the classification of [[star]]s based on their [[stellar spectrum|spectral]] characteristics. [[Electromagnetic radiation]] from the star is analyzed by splitting it with a [[Prism (optics)|prism]] or [[diffraction grating]] into a [[spectrum]] exhibiting the [[Continuum (spectrum)|rainbow]] of colors interspersed with [[spectral line]]s. Each line indicates a particular [[chemical element]] or [[molecule]], with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the [[photosphere]], although in some cases there are true abundance differences. The ''spectral class'' of a star is a short code primarily summarizing the [[ionization]] state, giving an objective measure of the photosphere's temperature.

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 three classes for other stars that do not fit in the classical system: ''[[W-type star|W]]'', ''[[S-type star|S]]'' and ''[[C-type star|C]]''. Some [[Compact object|stellar remnants]] or objects of deviating mass have also been assigned letters: ''[[Stellar classification#White dwarf classifications|D]]'' for [[white dwarf]]s and ''[[L dwarf|L]]'', ''[[T dwarf|T]]'' and ''[[Y Dwarf|Y]]'' for [[brown dwarfs]] (and [[exoplanet]]s).

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&nbsp;''0'' or ''Ia+'' is used for ''[[hypergiant]]s'', class&nbsp;''I'' for ''[[supergiant]]s'', class&nbsp;''II'' for bright ''[[Giant star|giants]]'', class&nbsp;''III'' for regular ''giants'', class&nbsp;''IV'' for ''[[subgiant]]s'', class&nbsp;''V'' for ''[[main-sequence]] stars'', class&nbsp;''sd'' (or ''VI'') for ''[[subdwarf]]s'', and class&nbsp;''D'' (or ''VII'') for ''[[white dwarf]]s''. The full spectral class for the [[Sun]] is then G2V, indicating a main-sequence star with a surface temperature around 5,800&nbsp;K.