THE SOUTHWORTH PLANETARIUM

70 Falmouth Street Portland, Maine 04103

43.6667° N 70.2667° W Altitude: 10 feet below sea level Founded January 1970

2021-2022: LXXII

"We are all in the gutter, but some of us are looking at the stars." -Oscar Wilde

THE DAILY ASTRONOMER

Wednesday, January 26, 2022

Approaching Andromeda

We devote today's article to a subscriber question pertaining to the Andromeda Galaxy, the nearest large spiral galaxy to the Milky Way.

"I heard that the Andromeda Galaxy and the Milky are going to crash into each other. How do astronomers know that? What will happen to the stars within both galaxies?" - W.H.

First, some back story.

The Milky Way and Andromeda Galaxies, currently 2.2 million light years apart, are moving toward each other at the speed of 300,000 miles per hour. Within 4 -6 billion years, the two galaxies will begin the first in a series of "collisions" that will eventually cause them to merge into one super galaxy containing approximately one trillion stars. Because the Andromeda and Milky Way Galaxies are relatively close together, their mutual gravitational attraction overwhelms the Universal expansion.

Astronomers know that the Andromeda Galaxy is approaching because its light exhibits a "blue shift." When a wave-emitting body moves, the wave frequencies change. If the body moves away from an observer, the waves are elongated and the frequency decreases. When light waves experience elongation, they are said to be red-shifted because red light has a lower frequency than blue light. If the body moves toward an observer, the waves are compressed and the frequency increases. When light waves experience compression, they are said to be blue-shifted.

By measuring the amount by which Andromeda's emitted light shifts, astronomers can determine its approach speed. Or, more correctly, the speed at which the two galaxies are approaching each other.

The Andromeda and Milky Way Galaxies are due to collide in about four to six billion years! They reduce their separation distance by 300,000 miles every hour. At this rate, they'll be one light year closer together in about 2,100 years.

As for the stars within both galaxies: millions of them will likely be propelled into the gulfs of intergalactic space. In fact, astronomers believe that billions of stars are careening through the vast space outside of galaxies largely as a result of galactic collisions and mergers. Others will remain within the immensely large galaxy that the merger will create. Moreover, the compression of gasses caused by these collisions will likely create millions of more stars!

Tomorrow....twilight.

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