Spectral Standard Stars
The revised Yerkes Atlas system (Johnson & Morgan 1953) listed a dense grid of F-type dwarf spectral standard stars, however not all of these have survived to this day as standards. The "anchor points" of the MK spectral classification system among the F-type main sequence dwarf stars, i.e. those standard stars that have remain unchanged over years and can be considered to define the system, are considered to be 78 Ursae Majoris (F2 V) and pi3 Orionis (F6 V). In addition to those two standards, Morgan & Keenan (1973) considered the following stars to be "dagger" standards: HR 1279 (F3 V), HD 27524 (F5 V), HD 27808 (F8 V), HD 27383 (F9 V), and beta Virginis (F9 V). Other primary MK standard stars include HD 23585 (F0 V), HD 26015 (F3 V), and HD 27534 (F5 V). Note that two Hyades members with almost identical HD names (HD 27524 and HD 27534) are both considered strong F5 V standard stars, and indeed they share nearly identical colors and magnitudes. Gray & Garrison (1989) provide a modern table of dwarf standards for the hotter F-type stars. F1 and F7 dwarf standards stars are rarely listed, but have changed slightly over the years among expert classifiers. Often-used standard stars include 37 Ursae Majoris (F1 V) and 107 Piscium (F7 V). No F4 V standard stars have been published. Unfortunately F9 V defines the boundary between the hot stars classified by Morgan, and the cooler stars classified by Keenan, and there are discrepancies in the literature on which stars define the F/G dwarf boundary. Morgan & Keenan (1973) listed beta Virginis & HD 27383 as F9 V standards, but Keenan & McNeil (1989) listed HD 10647 as their F9 V standard. Eta Cassiopeiae A should probably be avoided as a standard star as it was often considered F9 V in Keenan's publications, but G0 V in Morgan's publications.
Read more about this topic: F-type Main-sequence Stars
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