1. Announcements
Exam 3 is Monday May 5 at the usual class time,. Will cover Chapters 8 & 9. Sample questions are posted.
Observing nights this week Tuesday, Wednesday & Thursday. Forecast for Tuesday & Wednesday looks good so those will be the only two we do. I am expecting a crowd so I need all the help I can get.
Don’t forget the second project over something of historical importance to astronomy. Presentations will be Wednesday April 30.

2. The first stellar spectrum was made by Henry Draper in 1872
Draper put a prism between the photographic plate and the telescope. It recorded Vega and other nearby stars. The image was black & white and uncalibrated.
Henry Draper was John William Draper’s son and much of his equipment was developed by his father

3. Starting in 1863, Angelo Secchi examines hundreds of stellar spectra and devised a scheme to classify them

4. Secchi classified stars into four types based on the complexity of the spectrum
Karl Friedrich Zollner suggested the differences were due to temperature

5. In addition to stellar spectra, Huggins looked at selected nebulae

6. Huggins spectrum of the Cat’s Eye nebula proved its true nebulosity
He could identify the hydrogen lines but didn’t know what the other lines were

7. After examining the spectra of dozens of nebulae he deduced different types
Some spectra were continuous with absorption lines. We now know they are galaxies
Emission nebulae were similar to planetary nebulae

8. Christian Doppler thought his Doppler Effect would apply to starlight, changing the color of stars

9. The Doppler effect isn’t large enough to change a stars color
The change in wavelength of a typical star due to the Doppler Effect is small enough to be influenced by temperature changes in the camera over the course of a night and loading differences as the orientation of the camera changes during an exposure

10. Hermann Vogel and Julius Scheiner were the first to successfully measure stellar Doppler shift

11. The Doppler Effect did explain the splitting of lines in binary systems
Edward Pickering and Antonia Maury at the Harvard Observatory had observed a double line spectrum in the star Mizar. The Doppler Effect proved it was actually two stars orbiting a common center of mass. They became known as spectroscopic binaries

12. The sweat-shop of Harvard Observatory
In the late 1800’s the Harvard Observatory was one of the only places that allowed women to work as astronomers.

13. Stellar classification became assembly line at the Harvard Observatory in the 1890’s

14. Annie Jump Cannon led the way in coming up with a new stellar classification scheme
The original scheme of Secchi had been expanded but it was still alphabetical by complexity

15. Cannon’s classification system is still used: OBAFGKM
Cannon realized the temperature dependence of the spectra and re-organized the Secchi scheme

16. The nature of stars
Annie Jump Cannon had developed a stellar classification system based on temperature and the women of Harvard Observatory had classified hundreds of thousands of stars but the lifecycle, energy source and evolution of stars was still not understood

17. With enough data on the distance to nearby stars, Henry Norris Russell made a plot of absolute magnitude versus spectral type in 1910

18. Ejnar Hertzsprung had made similar plots of the Hyades and Pleiades

19. We now call them Hertzsprung – Russell Diagrams

20. At the time they only identified two “bands”: the Main Sequence and the Giant Branch

21. W. S. Adams and Arnold Kohlschutter figured out how to tell main sequence from giant in the spectra

22. W.W. Morgan developed the technique of Spectroscopic Parallax
Determine the luminosity class and spectral class from the spectrum, plot it on the H-R diagram and then read off the luminosity

23. Advances in atomic physics led to a better understanding of stellar spectra
Max Planck, Ernest Rutherford and Niels Bohr pioneered the new theory of quantum mechanics which explained existing lines seen in spectra which had not been identified

24. Meghnad Saha used the new quantum theory to explain the differences in main sequence and giant spectra

25. Cecilia Payne used QM to determine that hydrogen and helium were the most abundant elements is stars

26. As the understanding of stars improved, Arthur Eddington was able to deduce the mass-luminosity relationship of main sequence stars

27. Eddington reopens the debate on the source of energy for stars
Although he doesn’t know the details of how to fuse four hydrogen into a helium, he finds the abundance of hydrogen and helium ample evidence that it occurs. “If not in stars… go and find a hotter place!”

28. Einstein’s special theory of relativity had provided a means of getting the energy out: mass-energy
Atomic mass of 4 hydrogen = 4 amu
Atomic mass of one helium = 3.97 amu
Difference = 0.03 amu

29. Hans Bethe was the individual that eventually worked out the details of the proton-proton cycle

30. Unusual Stars
White dwarf stars had been know of since the mid 1800’s but they couldn’t be believed. Most though there was something wrong or missing in the measurements

31. In 1844 F. W. Bessell announced that Sirius has an invisible companion based on proper motion studies

32. Alvan Clark Jr. was the first to see Sirius B

33. Subramanian Chandrasekhar used QM to work out the details of degenerate stars and their mass

34. Theories of stellar evolution are starting to form
Norman Lockyear proposed a theory of stars forming from meteoric materials. When heated enough elements in the rocks decomposed into protoelements.

35. Hertzsprung and Russell both still held to the contraction energy idea so had stars starting hot and cooling off

36. It wasn’t until Bethe’s hydrogen fusion theory that a truer evolutionary picture emerged