PHYSICS 20B Lecture Notes - Lecture 13: Henrietta Swan Leavitt, Harlow Shapley
Lecture 13
Discovering the Milky Way II:
The Great Debate
● The Curtis-Shapley Debate
○ Herber Curtis
■ Herber believed that the Milky Way was small
■ Also believed that spiral nebulae were other galaxies
○ Harlow Shapley
■ Contradictory to Herber, Harlow believed the Milky Way is big
● Harlow believed that because of the immense size of the Milky
Way, it was impossible for spirals to be other other galaxies
■ Also believed that the Sun was not at the center of the Milky Way
(correct)
○ These debates posed greater questions about the universe
■ Is the Milky Way the Universe?
■ What are spiral nebulae?
■ Are there other island universes beyond the Milky Way?
● Herber Curtis
○ The reason why Curtis believed that spiral nebulae were other galaxies because
he thought they were moving too quickly to be located in the Milky Way
■ Curtis explained this through the Doppler Shift measurements.
● The high recessional speeds for nebulae meant that they were
objects in our galaxy
○ If the objects were in our galaxy, then it would be moving
much more slowly relative to the Sun
○ Curtis also believed due the the size of the spiral nebulae, matched that of other
galaxies
● Nebulae are redshifted (moving at high speeds)
○ We are able to observe through spectroscopic observations, that most nebulae
are moving away from the Milky Way (an object moving away from the observer)
at very high speeds
● Shapley
○ Shapley was studying “globular clusters:
■ Through his observations and studying, he argued that the Milky Way was
indeed very big and the Sun was not in the center of the Milky Way
○ How did he use globular clusters to argue this?
■ He relied on the Period-Luminosity relation of variable stars
■ Later discovered by Henrietta Swan Leavitt of Shapley’s method
● This method to measure the distances became a huge significant
achievement in the astronomical field
○ Shapley knew that globular clusters are
■ Very dense clusters of stars than contain ~10^5 stars in each cluster
Document Summary
Herber believed that the milky way was small. Also believed that spiral nebulae were other galaxies. Contradictory to herber, harlow believed the milky way is big. Harlow believed that because of the immense size of the milky. Way, it was impossible for spirals to be other other galaxies. Also believed that the sun was not at the center of the milky way (correct) These debates posed greater questions about the universe. The reason why curtis believed that spiral nebulae were other galaxies because he thought they were moving too quickly to be located in the milky way. Curtis explained this through the doppler shift measurements. The high recessional speeds for nebulae meant that they were objects in our galaxy. If the objects were in our galaxy, then it would be moving much more slowly relative to the sun. Curtis also believed due the the size of the spiral nebulae, matched that of other galaxies.
