PX282 - B1 - spectral classification

• circa 1802, lines were noticed on the solar spectrum, which were labelled in 1814: $NaI$, $CaII$, $H$, $K$

• in 1859, kirchoff and bunsen found lines in the lab, arising from transitions of electrons between energy states

• initially, stars were ranked by the strength of hydrogen lines by fleming, categorized into the draper system: $A,B,C,D,\dots Q$

• every other category varies irregularly

• cannon devised the harvard classification scheme based on temperature, dropping most of the letters except a few

image: professor dave explains (youtube)

• mnemonic: Oh, Be A Fine Girl, Kiss Me!

• the categories vary as follows:

  • hot & bright $\to$ cold & dim
  • blue & white $\to$ red
  • $\sim 10R_{\odot },>20M_{\odot }\to <0.7R_{\odot },<0.5M_{\odot }$
  • short-lived ($10Myr$) $\to$ long-lived ($>100Gyr$)

• the sun is a G-type star

image: space fm