Stars are grouped into spectral classes by the absorption lines in their spectra. These lines depend mainly on surface temperature. In order of decreasing temperature the classes are O B A F G K M.• O: blue, 25 000-50 000 K; lines of He+, He and H
• B: blue, 11 000-25 000 K; He and H
• A: blue-white, 7500-11 000 K; H (strongest), ionised metals
• F: white, 6000-7500 K; ionised metals
• G: yellow-white, 5000-6000 K; ionised and neutral metals
• K: orange, 3500-5000 K; neutral metals
• M: red, below 3500 K; neutral atoms and molecules such as TiO (titanium oxide)The Sun, at about 5800 K, is a class G star.Hydrogen Balmer lines are absorption lines in the visible spectrum produced when hydrogen atoms in the n = 2 energy level absorb photons and are excited to higher levels. Their strength depends on temperature:
• in cool stars most hydrogen atoms are in the ground state (n = 1), so Balmer absorption is weak
• in very hot stars (class O) most hydrogen is ionised, so again the lines are weak
• at about 10 000 K (class A) many atoms are in n = 2, so Balmer lines are strongestSimilarly, only very hot stars have enough energy to ionise helium, so He+ lines show a class O star, while molecules can only survive in the coolest (class M) stars.
• B: blue, 11 000-25 000 K; He and H
• A: blue-white, 7500-11 000 K; H (strongest), ionised metals
• F: white, 6000-7500 K; ionised metals
• G: yellow-white, 5000-6000 K; ionised and neutral metals
• K: orange, 3500-5000 K; neutral metals
• M: red, below 3500 K; neutral atoms and molecules such as TiO (titanium oxide)The Sun, at about 5800 K, is a class G star.Hydrogen Balmer lines are absorption lines in the visible spectrum produced when hydrogen atoms in the n = 2 energy level absorb photons and are excited to higher levels. Their strength depends on temperature:
• in cool stars most hydrogen atoms are in the ground state (n = 1), so Balmer absorption is weak
• in very hot stars (class O) most hydrogen is ionised, so again the lines are weak
• at about 10 000 K (class A) many atoms are in n = 2, so Balmer lines are strongestSimilarly, only very hot stars have enough energy to ionise helium, so He+ lines show a class O star, while molecules can only survive in the coolest (class M) stars.