highly ionized helium, nitrogen, carbon, and sometimes oxygen. They are thought to mostly be dying supergiants with their hydrogen layers blown away by stellar winds, thereby directly exposing their hot helium shells. Class W is further divided into subclasses according to the relative strength of nitrogen and carbon emission lines in their spectra (and outer layers). WR spectra range is listed below: WN – spectrum dominated by N III-V and He I-II lines WNE (WN2 to WN5 with some WN6) – hotter or "early" WNL (WN7 to WN9 with some WN6) – cooler or "late" Extended WN classes WN10 and WN11 sometimes used for the Ofpe/WN9 stars h tag used (e.g. WN9h) for WR with hydrogen emission and ha (e.g. WN6ha) for both hydrogen emission and absorption WN/C – WN stars plus strong C IV lines, intermediate between WN and WC stars WC – spectrum with strong C II-IV lines WCE (WC4 to WC6) – hotter or "early" WCL (WC7 to WC9) – cooler or "late" WO (WO1 to WO4) – strong O VI lines, extremely rare, extension of the WCE class into incredibly hot temperatures (up to 200 kK or more) Although the central stars of most planetary nebulae (CSPNe) show O-type spectra, around 10% are hydrogen-deficient and show WR spectra. These are low-mass stars and to distinguish them from the massive Wolf–Rayet stars, their spectra are enclosed in square brackets: e.g. [WC]. Most of these show [WC] spectra, some [WO], and very rarely [WN]. Slash stars The slash stars are O-type stars with WN-like lines in their spectra. The name "slash" comes from their printed spectral type having a slash in it (e.g. "Of/WNL"). There is a secondary group found with these spectra, a cooler, "intermediate" group designated "Ofpe/WN9". These stars have also been referred to as WN10 or WN11, but that has become less popular with the realisation of the evolutionary difference from other Wolf–Rayet stars. Recent discoveries of even rarer stars have extended the range of slash stars as far as O2-3.5If*/WN5-7, which are even hotter than the original "slash" stars. Magnetic O stars They are O stars with strong magnetic fields. Designation is Of?p. Cool red and brown dwarf classes The new spectral types L, T, and Y were created to classify infrared spectra of cool stars. This includes both red dwarfs and brown dwarfs that are very faint in the visible spectrum. Brown dwarfs,