Spectral classification

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Robin Leadbeater

Hi Kate,

Spectral classification is a rather inexact science as many stars show anomalies which don’t fit neatly into the simple MK classification system. As a result  you get different opinions for the classification even for non variable stars (Variable stars can change their classification with time for example due to temperature changes caused by pulsations)

A good source for spectral classifications is Brian Skiff’s huge catalogue which has all the published classifications with the references for currently approaching  a million stars


Here we see for sigma Boo a range of classifications dating from 1897 to 2001

F4V kF2 mF1 is the latest one and comes from a paper by Richard Gray who is famous in stellar classification circles and for example co-authored the current “bible” on the subject “Stellar Spectral Classification” by Gray and Corbally

The paper referenced is here


There he notes that sigma Boo is metal weak and looking in the footnotes to  table I (page 2155)  he explains the multiple classification nomenclature he has used.  So for sigma Boo we have a metal weak star with the traditional classification F4V, presumably based on the Balmer lines  but  based on the strength of the Ca II K lines it looks like an F2 and based on the metal line spectrum it looks like an F1