Looking Up: The coming collision
"The Great Andromeda Galaxy," M31, and our own galaxy, the Milky Way, are traveling through space towards each other at a speed of 216,000 miles per hour. The coming collision will be the biggest event in our region of space in eons.
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With a visual magnitude of 3.4 Andromeda has always been bright enough in the night sky for humans to observe it with the naked eye. In the year 964 the Persian astronomer al-Sufi described it as a "nebulous smear."
Long thought to be just another nebula in our Milky Way (although the biggest) it was Immanuel Kant in 1755, who first conjectured that it was an "island universe" like our own galaxy. That was right on the money, but it took astronomers 170 years to confirm it. That happened in 1925 when Edwin Hubble used the Hooker Telescope (largest in the world then) to show it was indeed another galaxy.
So, let’s examine the big picture some.
Our own galaxy lives in a neighborhood called the "Local Group" of galaxies. The Group is isolated from other galaxy groups scattered here and there in deep space. Our neighborhood contains over 54 galaxies. Most of them, discovered in recent years, are dwarf galaxies. The two biggest by far are the Milky Way and the Great Andromeda Galaxy.
The geographic center of the neighborhood is about half way between our two giant galaxies. Space/time is already warped in that region as the two galaxies have "gravitationally detected" each other. Now they are rushing at each other down the warp in space/time like two stags in a celestial field.
We lie about 2.5 million light years from each other right now.
Both galaxies contain massive black holes in their cores, and both are barred spirals. The two are about the same size in total mass based on a study in 2018. Andromeda appears to be more spread out with a diameter of 220,000 light years, compared to our 100,000.
The make-up of stars is somewhat different between the two. Our galaxy generally has a younger population than Andromeda's. Accordingly, the rate of star formation is larger in the Milky Way at 3-5 new ones per year vs only about one per year in Andromeda.
Despite the speed of approach, the distance between the two galaxies makes the collision date about four billion years from now. So, you can sleep tonight. Our sun can too, because by then it will be well into its own death cycle, and the earth long since bleached and dried up. This is a good place to discuss the influence of vast distances in space. We can use a spectrograph to measure the blue shift of Andromeda (meaning its coming at us, and not away from us) to predict when the collision will occur. However, we still cannot detect its advance towards us optically, or even measure the increase in its brightness as it approaches. To see a change optically you would examine two photographs taken say 10 years ago, and now. You would be looking for a slight increase in separation in the stars in Andromeda from each other – meaning it’s getting closer.
At the current speed and distance, the separation would only be about 30 micro seconds in a hundred years! We don't have instruments yet that can do that.
Similarly, with brightness. To detect a slight change, we would need an instrument so sensitive that it would detect the drop-in brightness of a distant sun-like star being eclipsed by an earth sized planet. The planned LUVOIR type telescope could do it, but it’s not scheduled for deployment for a couple of decades.
Yet we are left though with the awesome vision of the two giant galaxies hurtling toward each other in the silence of space as we sip our morning coffee.
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Ted Wolfe is a member of the Everglades Astronomical Society. Organized in 1981 it serves the Naples community, providing information in all aspects of amateur astronomy. Its goals include educating the public, school children and other groups to the wonders of the universe. The society meets at 7 p.m. every second Tuesday of the month at the Norris Center (public invited). Regular viewing visits to a special, dark sky site in the Everglades are held each month, allowing the public to observe the night sky through telescopes, under pristine conditions. For more information, visit the website at http://naples.net/clubs/eas. A Blu-ray disc for viewing on TV is now available which features 70 of Ted's deep space images with original background music. For more information, go to www.naples.net/clubs/eas/sales.html.