The revised Yerkes Atlas system 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 remained unchanged over years and can be used to define the system, are considered to be 78 Ursae Majoris and pi3 Orionis. In addition to those two standards, Morgan & Keenan considered the following stars to be dagger standards: HR 1279, HD 27524, HD 27808, HD 27383, and Beta Virginis. Other primary MK standard stars include HD 23585, HD 26015, and HD 27534. Note that two Hyades members with almost identical HD names are both considered strong F5 V standard stars, and indeed they share nearly identical colors and magnitudes. Gray & Garrison 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 and Iota Piscium. 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 listed Beta Virginis and HD 27383 as F9 V standards, but Keenan & McNeil listed HD 10647 as their F9 V standard. Eta Cassiopeiae A should probably be avoided as a standard star because it was often considered F9 V in Keenan's publications, but G0 V in Morgan's publications.
Some studies show that there is a possibility that life could also develop on planets that orbit an F-type star. It is estimated that the habitable zone of a relatively hot F0 star would extend from about 2.0 AU to 3.7 AU and between 1.1 and 2.2 AU for a relatively cool F8 star. However, relative to a G-type star the main problems for a hypothetical lifeform in this particular scenario would be the more intense light and the shorter stellar lifespan of the home star. F-type stars are known to emit much higher energy forms of light, such as UV radiation, which in the long term can have a profoundly negative effect on DNA molecules. Studies have shown that, for a hypothetical planet positioned at an equivalent habitable distance from an F-type star as the Earth is from the Sun, and with a similar atmosphere, life on its surface would receive about 2.5 to 7.1 times more damage from UV light compared to that on Earth. Thus, for its native lifeforms to survive, the hypothetical planet would need to have sufficient atmospheric shielding, such as an ozone layer in the upper atmosphere. Without a robust ozone layer, life could theoretically develop on the planet's surface, but it would most likely be confined to underwater or underground regions.
