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It is often stated that the spectral subtype indicates the relative position between spectral types, e.g. G5 being halfway from the start of spectral type G to the beginning of spectral type F.

Given that we haven't observed a spectral type cooler than Y, how can subtypes Y0, Y1, Y2 be defined?

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  • $\begingroup$ Spectral class Y is also used on sub-brown dwarfs. Sub-brown dwarfs don't have enough mass to ignite Deuterium. An example for such Y-type body is en.wikipedia.org/wiki/WD_0806%E2%88%92661 $\endgroup$
    – Ioannes
    Jul 21, 2020 at 17:56
  • $\begingroup$ @Greenhorn - this is a question about spectral types, i.e. the classification of spectra, not about how to define "sub-brown dwarfs". $\endgroup$
    – user24157
    Jul 24, 2020 at 18:31

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