THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian Date: 2458796.16
Julian Date: 2458796.16
2019-2020: LII
"I truly have no idea about reality, to be honest. How fantastic!"
THE DAILY ASTRONOMER
Thursday, November 14, 2019
A Local Group
How remarkable is it that merely a century ago, astronomers were fiercely debating the notion that parts of the Universe exist outside our own Milky Way Galaxy.* Today, we know that not only is our Milky Way not alone, it is merely one of billions, if not trillions, of galaxies in the cosmos. Astronomers know that these galaxies are not scattered indiscriminately throughout the Universe, but are instead arranged in clusters and then in staggeringly huge collections called "Superclusters." We are not inclined to ascend to that level today. Instead, we want to remain comparatively close to home in a vast galactic region called "The Local Group."
The Local Group, a term coined by Edwin Hubble in his 1936 publication "The Realm of the Nebulae" is approximately 10 million light years across, contains material with a mass exceeding two trillion solar masses, and is populated by about 54 known galaxies. Most of these members are dwarf satellites of the Local Group's two largest members, the Andromeda and Milky Way Galaxies. One can think of the Local Group as being dumbbell-shaped, with the Milky Way and Andromeda at opposite ends. Dwarf galaxies cluster around both. One cannot think of this group as being stagnant, however, for even at this level one finds the same restless dynamism one observes throughout the Universe. Despite the cosmological expansion that drives most distant galaxies apart, gravity remains the predominant force impelling most of the constituent galaxies toward each other. The Milky Way and Andromeda Galaxies, for instance, are drawing together at 300,000 miles per hour and will collide within 4 - 6 billion years.
Much remains unknown about the Local Group. Principal among the mysteries is the actual galactic population. Astronomers know of 54 galaxies, but suspect that other remain undiscovered, hidden by the Milky Way Galaxy, itself. As our planet is tucked away inside one of the galaxy's spiral arms, a region directly beyond the galactic disk are largely unobservable due to the material obscuration. Astronomers refer to this region as the "Zone of Avoidance."
We do know that the Local Group, though unfathomably immense by our reckoning, is reduced almost to negligibility at the level of Superclusters which number around 10 million in the observable Universe. We also know that the night sky above reveals a wafer thin layer of this Local Group. And, of course, when we look at the cluster of galaxies within the Local Group that extend far beyond our fathoming, we know that we're looking at our home.
*In April 1920, we'll be celebrating the 100th anniversary of the "Great Debate" between Harlow Shapley and Heber Curtis about the existence of extra galactic objects. Shapley argued that the Milky Way comprised the entire Universe, whereas Curtis insisted that the cosmos was populated by myriad galaxies. We know now who was correct.
To subscribe or unsubscribe from the "Daily Astronomer"
http://lists.maine.edu/cgi/wa?A0=DAILY-ASTRONOMER