THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland, Maine 04103
(207) 780-4249      usm.maine.edu/planet
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.

[image: Andromeda_Collides_Milky_Way.jpg]
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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