about 200 km/s. Be and B[e] stars Spectral objects known as "Be stars" are massive yet non-supergiant entities that notably have, or had at some time, 1 or more Balmer lines in emission, with the hydrogen-related electromagnetic radiation series projected out by the stars being of particular scientific interest. Be stars are generally thought to feature unusually strong stellar winds, high surface temperatures, and significant attrition of stellar mass as the objects rotate at a curiously rapid rate, all of this in contrast to many other main-sequence star types. Objects known as B[e] stars are distinct from Be stars in having unusual neutral or low ionization emission lines that are considered to have 'forbidden mechanisms', something denoted by the use of the square brackets. In other words, these particular stars' emissions appear to undergo processes not normally allowed under 1st-order perturbation theory in quantum mechanics. The definition of a B[e] star can include blue giants and blue supergiants. Spectral standard stars The revised Yerkes Atlas system (Johnson & Morgan 1953) listed a dense grid of B-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 B-type main-sequence dwarf stars, i.e. those standard stars that have remain unchanged since at least the 1940s, are upsilon Orionis (B0 V), eta Aurigae (B3 V), eta Ursae Majoris (B3 V). Besides these anchor standards, the seminal review of MK classification by Morgan & Keenan (1973) listed "dagger standards" of Tau Scorpii (B0 V), Omega Scorpii (B1 V), 42 Orionis (B1 V), 22 Scorpii (B2 V), Rho Aurigae (B5 V), and 18 Tauri (B8 V). The Revised MK Spectra Atlas of Morgan, Abt, & Tapscott (1978) further contributed the standards Beta2 Scorpii (B2 V), 29 Persei (B3 V), HD 36936 (B5 V), and HD 21071 (B7 V). Gray & Garrison (1994) contributed two B9 V standards: Omega Fornacis and HR 2328. The only published B4 V standard is 90 Leonis, from Lesh (1968). There has been little agreement in the literature on choice of B6 V standard. Chemical peculiarities Some of the B-type stars of stellar class B0–B3 exhibit unusually strong lines of non-ionized helium. These chemically peculiar stars are termed helium-strong stars. These often have strong magnetic fields in their photosphere. In contrast, there are also helium-weak B-type stars with understrength helium lines and strong hydrogen