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.

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

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

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

In addition to stellar spectra, Huggins looked at selected nebulae

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

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

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

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

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

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

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.

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

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

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

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

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

Ejnar Hertzsprung had made similar plots of the Hyades and Pleiades

We now call them Hertzsprung – Russell Diagrams

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

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

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

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

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

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

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

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!”

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

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

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

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

Alvan Clark Jr. was the first to see Sirius B

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

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.

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

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