Sigma Bootis

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

http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk

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

http://simbad.u-strasbg.fr/simbad/sim-ref?bibcode=2001AJ….121.2148G

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

Robin