Steward Observatory/NSF’s NOIRLab Joint Colloquium Series: "Three New Things about the Milky Way"

Dr. Bob Benjamin, University of Wisconsin-Whitewater

Abstract:  On December 25, 1951, Bill Morgan of Yerkes Observatory arrived at Case University in Cleveland for a meeting of the American Astronomical Society. That evening he invited Bart and Priscilla Bok to his dorm room where he shared with them the discovery that he was to announce the next day: the first evidence for spiral structure in the Milky Way.  By combining wide-field H-alpha imaging of the Galactic plane (done with Steward Sharpless and Don Osterbrock spectral classification of OB stars (with J.J. Nassau) and photometry/extinction correction (with Al Whitford and Art Code), Morgan presented a map of the two nearest spiral arms: the Orion Arm and the Perseus Arm. After sharing a bit about the history of this field,  I will update you on our understanding about the distribution and ionization state of ionized gas in our Milky Way, including (1) recent revisions in our understanding of the distribution of local HII regions and spiral structure prompted by Gaia, (2) the discovery of several large angular size supernova remnants near the Sun, including one with an angular diameter of 60 degrees, and (3) the discovery of a tilted reservoir of ionized gas in the inner two kiloparsecs of the Milky Way, suggesting that the Milky Way would be classified as a LI(N)ER.  As part of the final point, I provide preliminary evidence of an H-alpha counterpart to the inner Galaxy structure called the Fermi Bubble and how it might relate to other measurements of ionized gas in this region of the Galaxy. 



If you would like to schedule a meeting with Dr. Benjamin, please email Hector Rico ( directly.


Graduate students will meet with the colloquium speaker for lunch on Thursday, April 8th, at 1:00 PM. (MST) via Zoom at:


3:30 p.m. to 4:30 p.m. Thursday