Changes

In the late nineteenth century, astronomers at the [[Harvard University]] observatory developed the first classification scheme for stellar spectra that would become known as the '''Harvard spectral classification'''. In 1924, Annie Jump Cannon<ref name=Cannon>"[http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/LC_main_p8.html Life Cycles of Stars]." ''Goddard Space Flight Center'', November 21, 2002. Accessed April 22, 2008.</ref> refined the classification from the original A-Q gamut to the familiar "OBAFGKM" gamut. Astronomers have since added classes to this range at the high end and the low.<ref name=Swinburne>"[http://astronomy.swin.edu.au/cosmos/H/Harvard+Spectral+Classification Harvard Spectral Classification]." ''Study Astronomy Online at Swinburne University''. Accessed April 22, 2008.</ref><ref name=Seattle>Irizarry, David. "[http://www.seattleastro.org/webfoot/feb00/pg2.htm The Secrets of the Harvard Classification Revealed]." ''The Webfooted Astronomer'', Seattle Astronomical Society, February 2000. Accessed April 22, 2008.</ref>

In the late nineteenth century, astronomers at the [[Harvard University]] observatory developed the first classification scheme for stellar spectra that would become known as the '''Harvard spectral classification'''. In 1924, Annie Jump Cannon<ref name=Cannon>"[http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/LC_main_p8.html Life Cycles of Stars]." ''Goddard Space Flight Center'', November 21, 2002. Accessed April 22, 2008.</ref> refined the classification from the original A-Q gamut to the familiar "OBAFGKM" gamut. Astronomers have since added classes to this range at the high end and the low.<ref name=Swinburne>"[http://astronomy.swin.edu.au/cosmos/H/Harvard+Spectral+Classification Harvard Spectral Classification]." ''Study Astronomy Online at Swinburne University''. Accessed April 22, 2008.</ref><ref name=Seattle>Irizarry, David. "[http://www.seattleastro.org/webfoot/feb00/pg2.htm The Secrets of the Harvard Classification Revealed]." ''The Webfooted Astronomer'', Seattle Astronomical Society, February 2000. Accessed April 22, 2008.</ref>

The classic Harvard spectral classes are O, B, A, F, G, K, and M. Each of these has ten subclasses, varying from 0 to 9 in order of decreasing stellar temperature. Thus, for example, the next class after an F9 star is a G0 star. Recently astronomers recognized one class of stars hotter than the O stars (the very hot Wolf-Rayet stars) and three classes of stars (the N, R, and S stars) cooler than the M stars. (Some astronomers include the N and R stars in one class, the C stars, for the carbon compounds that their spectra exhibit).  There is an additional spectral class for the smallest and dimmest stars (Class L), that still stellar fusion, although warmer [[brown dwarf|brown dwarfs]] also fall into this class (but referred to as L dwarfs instead of L stars). Cooler still methane dwarfs are classified as [[Brown_dwarf#Spectral_class_T|T dwarfs]].<ref>http://adsabs.harvard.edu/abs/2007arXiv0704.1522K</ref>  A proposed spectral class Y has been suggested for the coolest brown dwarfs, which also have a different spectra from T class dwarfs.<ref>http://xxx.lanl.gov/abs/astro-ph/0607305</ref>

The classic Harvard spectral classes are O, B, A, F, G, K, and M. Each of these has ten subclasses, varying from 0 to 9 in order of decreasing stellar temperature. Thus, for example, the next class after an F9 star is a G0 star. Recently astronomers recognized one class of stars hotter than the O stars (the very hot Wolf-Rayet stars) and three classes of stars (the N, R, and S stars) cooler than the M stars. (Some astronomers include the N and R stars in one class, the C stars, for the carbon compounds that their spectra exhibit).  There is an additional spectral class for the smallest and dimmest stars (Class L), that still fuse hydrogen, although warmer [[brown dwarf|brown dwarfs]] also fall into this class (but referred to as L dwarfs instead of L stars). Cooler still methane dwarfs are classified as [[Brown_dwarf#Spectral_class_T|T dwarfs]].<ref>http://adsabs.harvard.edu/abs/2007arXiv0704.1522K</ref>  A proposed spectral class Y has been suggested for the coolest brown dwarfs, which also have a different spectra from T class dwarfs.<ref>http://xxx.lanl.gov/abs/astro-ph/0607305</ref>

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