•Don’t forget the second project.
Presentations are next time.
•Exam 3 is Tuesday May 3 at 4:00pm.
Will cover Chapters 8 & 9. Sample questions are posted.

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 in 1862

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

Understanding the Structure of the Milky Way

Hershel had used star counts and brightness to get the shape
John Herschel went down to South Africa and did the same thing, coming up with the same result.

Stephen Alexander suggested it looked like Lord Rosse’s spirals

Others proposed a variety of structures
Richard
Proctor
Wilhelm
Struve
Easton’s Milky Way

J. C. Kepteyn suggest a plan of attack
Basically Herschel’s method but with accuracy. Measure all the stars possible and plot them out.
Assumes the space between stars is empty…bad assumption

Harlow Shapley has a better idea…measure the globular clusters
He uses the 60” at Mount Wilson, the largest telescope in the world at the time

Shapley uses Miss Leavitt’s stars (Cepheid's) to measure the distance
He makes a number of other assumptions about the size of the clusters and the brightest stars in them

In 1918 Shapley announces his results
The Galaxy is 300,000 lightyears across. He doesn’t claim to understand the structure, just how big it is. He is off due to obscuring dust.

By 1900 long exposure photography is showing lots of nebulae

E. E. Barnard publishes a collection of “dark nebulae”

Really long exposures show stuff everywhere
E. E. Barnard discovers lots of glowing gas all around the Orion constellation

Robert Trumpler publishes the definitive study on obscuration in 1930
There is a lot of gas and dust out there. In the disk of the galaxy it cuts the magnitude by one every 5000 lightyears

In 1938 the answer to the structure question is summarized by J. S. Plaskett
By this time
Shapley’s numbers had been corrected for obscuring dust. The galaxy is abut 100,000 lightyears across and the center is in the direction of Sagittarius about 30,000 lightyears away. It probably has a spiral structure

The Spiral Nebulae

Around 1900 most believe the spiral nebulae are in the galaxy
William Huggins suggests they are solar systems in the formation process

Vesto Slipher starts taking spiral nebulae spectra
At first he finds those on one side of the Milky Way are receding while those on the other side are approaching us at hundreds of km/s

Heber Curtis determines that our view of the spiral nebulae is blocked in the plane of the galaxy

Adriaan Maanen claims to have seen movement in the spiral nebulae

The Great Debate
Washington D.C. 1920
Heber Curtis: the spiral nebulae are “island universes” like the Milky Way.
Harlow Shapley: The
Milky Way is enormous beyond imagination and is the entire universe