Cecilia Payne determines that the Spectral Classification Sequence is a Temperature Sequence

portrait of Cecilia Payne

Cecilia Payne  (1900-1979), or Cecilia Payne-Gaposchkin 
after her 1934 marriage, took the data from the HD Catalougue 
and discovered its physical significance.

Payne argued that the great variation in stellar absorption lines 
was due to differing amounts of ionization (differing temperatures), 
not different abundances of elements. 

She correctly posited that silicon, carbon, and other common metals 
seen in the sun were found in about the same relative amounts as 
on earth but the helium and particularly hydrogen were vastly 
more abundant (by about a factor of one million in the case of hydrogen).

1924 photo of Payne and two colleagues, Harvia Wilson and Adelaide Ames on Harvard Observatory grounds Most astronomers at the time believed that stars are made of heavy elements, and when she sent a draft of her paper to Dr. Henry Norris Russell, he replied that her result concerning the abundance of hydrogen was "clearly impossible." Pressured to defer to Russell's expertise, Payne added to her thesis the comment that her results were "almost certainly not real." However, within a few short years, most other astronomers had come around to believe Payne's results and Russell published his own paper claiming the Sun was mostly composed of hydrogen. In her autobiography, Payne-Gaposchkin wrote, "On the material side, being a woman has been a great disadvantage. It is a table of low salary, lack of status, slow advancement... It has been a case of survival, not of the fittest, but of the most doggedly persistent."

Example: Hydrogen Lines

photo Cecilia Payne-Gaposchkin in her office (1946),
with Sergei Gaposchkin, and their two children, Katherine and Edward

What lines you see depends primarily on the state of excitation and ionization of the gas.

These are determined primarily by the temperature of the gas.

For example, the Hydrogen absorption lines which appear in the optical region of the spectrum are created when electrons move to higher energy levels from the second excited state.

B Stars (11,000-30,000 K):

Most of H is ionized, so only very weak H lines.

A Stars (7500-11,000 K):

Ideal excitation conditions, strongest H lines.

G Stars (5200-5900 K):

Too cool, little excited H, so only weak H lines.