Classification of Stellar Spectra Late 1800s: first high-quality spectral measurements of stars What are the main features – and how to classify them?

Spectral Lines Balmer absorption lines occur when an incoming photon causes an electron in the n = 2 level in hydrogen to jump to a higher level. The hotter a star, the more likely that hydrogen electrons will be in the n = 2 level But for very hot stars, hydrogen will lose its electrons completely Balmer lines thus reach their maximum “depth” in the spectra of stars with T=9250 K, so let’s call those stars class ‘A’ Balmer emission series

Dependence of Spectral Lines vs. Temperature

Stellar Spectral Lines Why do spectral lines depend upon temperature? –Populations of various atomic states depends upon temperature Degeneracy of levels –Stage of ionization Depends on Pressure and density… Depends somewhat on composition of star as well

The Spectral Sequence In 1890, Edward Pickering and his assistant at Harvard classified thousands of stellar spectra at Harvard. Named them ‘A’ through ‘Q’ based on Balmer depth. Approx. 20 years later, blackbody theory was developed. A. Cannon ‘improved’ the scheme and re-ordered it by temperature: O,B,A,F,G,K,M Subdivided each into 0 through 9 (AO: hot – A9:cooler) Later on, L and T were added. E. Pickering and his housekeeper W. Fleming A. J. Cannon classifying one of 200,000 spectra by eye for 25¢ an hour ($6 today)

Spectral Type Classification System O B A F G K M Oh Be A Fine Girl/Guy, Kiss Me (Long Time)! 50,000 K 1,000 K Temperature (L T) Only Brilliant Astronomers Find Glorious Knowledge Manning Large Telescopes Only Boring Astronomers Find Gratification Knowing Mnemonics Like This…

The Spectral Sequence and Temperature 7000 K 5500 K 4500 K 3000 K 2000 K < 1300 K 9000 K K K “Brown dwarfs” (no H fusion) “Stars” Our sun: G2  Cecilia Payne- Gaposchkin

Stellar Luminosity Classes In 1930s, W. Morgan and P. Keenan noticed that stars with the same temperature could have different Balmer absorption depths. Called the narrowest ones I and the deepest ones VI A0 I A0 II A0 V Spectra of three A0 stars of different luminosity class

Origin of Luminosity Classes At higher pressure, the gas particles in a stellar atmosphere are closer together and can interact more frequently. The energy levels of the atoms are perturbed, so that a wider range of photon frequencies can be absorbed. Pressure broadening of a CO 2 absorption line Narrow line, low density pressure

Morgan-Keenan Luminosity classes Most common type (includes our Sun) Betelgeuse Arcturus, Capella Sirius B

Hertzsprung-Russell diagram In early 1900s, H. Russell and E. Hertzsprung independently plotted absolute V magnitude against spectral class Most stars fall along a band (main sequence) Some are very luminous compared to main sequence stars of their spectral class (implied large radius)  giants Some are very underluminous for their class  white dwarfs

Star with Hipparcos distance measurements Note multiple axis labels

Hertzsprung-Russell diagram Stefan-Boltzmann law implies lines of constant radius:

Note the increasing mass and shorter lifetimes as you climb the main sequence Stars leave the main sequence toward the end of their lives

Morgan-Keenan Luminosity classes Recall that luminosity class varies with surface gravity, which varies as M / R 2 Leads to luminosity class regions on the H-R diagram

Spectroscopic Parallax Can use H-R diagram to estimate absolute magnitude of star given its spectral type and luminosity class Use apparent magnitude and distance modulus formula: Scatter of +/- 1 magnitude results in a factor of 1.6 uncertainty in distance