 Stars are given a single @-@ letter classification according to their spectra , ranging from type O , which are very hot , to M , which are so cool that molecules may form in their atmospheres . The main classifications in order of decreasing surface temperature are : O , B , A , F , G , K , and M. A variety of rare spectral types are given special classifications . The most common of these are types L and T , which classify the coldest low @-@ mass stars and brown dwarfs . Each letter has 10 sub @-@ divisions , numbered from 0 to 9 , in order of decreasing temperature . However , this system breaks down at extreme high temperatures as classes O0 and O1 may not exist . 
 Stars are given a single @-@ letter classification according to their spectra , ranging from type O , which <a href='drop_aux' title='1'><mark>are</mark></a>very hot , to M , which <a href='drop_aux' title='2'><mark>are</mark></a>so cool that molecules may form in their atmospheres . The main classifications in order of decreasing surface temperature are : O , B , A , F , G , K , and M. A variety of rare spectral types are given special classifications . The most common of these <a href='drop_aux' title='3'><mark>are</mark></a>types L and T , which classify the coldest low @-@ mass stars and brown dwarfs . Each letter <a href='got' title='4'><mark>has</mark></a> 10 sub @-@ divisions , numbered from 0 to 9 , in order of decreasing temperature . However , this system <a href='uninflect' title='5'><mark>breaks</mark></a> down at extreme high temperatures as classes O0 and O1 may not exist . 
Stars are given a single @-@ letter classification according to their spectra , ranging from type O , which very hot , to M , which soo coo that molecules may form in their atmospheres . da main classifications in order of decreasing surface temperature r : O , B , A , F , G , K , and M. A variety of rare spectral types arriven special classifications . Tdamost common of dem types L and T , which classify the coldest low @-@ mass stars and brown dwarfs . Each letter got 10 sub @-@ divisions , numbered from 0 to 9 , in order of decreasing temperature . However , this system break down at extreme high temperatures as classes O0 and O1 may not exist .