03 June 2012


Imagine the Milky Way galaxy ~ 200-400 billion stars ~ our sun resides in a modest position along one of its spiral arms.  Now imagine the Andromeda galaxy ~ 1000 billion stars ~ the largest galaxy of the local group.  Now imagine that the two massive galaxies are on a collision course.  Guess what ~ they are.

Last week the science journal Nature reported astronomers' announcement that the Milky Way and Andromeda will undergo a direct and complete merger ~ "Four billion years from now, the two galaxies will pass through each other and 2 billion years after that, they will fall back into a permanent embrace to form a single galaxy."

The research team reached its conclusion after eight years of intense observation using the Hubble Space Telescope, coupled with intricate computer simulation software.  "The team's simulations had to account for the Sun's motion as well as the gravity of a satellite galaxy of Andromeda called M33, which the researchers say has a 9% chance of plowing into the Milky way before Andromeda does.

"The Milky Way - Andromeda collision will almost certainly move the Sun outwards from its current position of 8,000 parsecs (26,000 light years) away from the Galactic center.  The simulation shows a 10% chance that the Solar System will be exiled to a Galactic Siberia, more than 50,000 parsecs (160,000 light years) from the center of the merged bodies."

Let's contemplate this for a moment.  When I was young (back during the Punic Wars), I recall reading that the ratio of mass to space in the universe is similar to that of placing three grains of sand inside the world's largest cathedral. If objects in space are so far apart, one might think that two galaxies might simply pass through each other unchanged.  There are three reasons why this reasoning falls flat ~ (a) a galaxy is, by definition, a concentration of mass greater than that found in the rest of space ~ (b) in the intervening years we've discovered the presence of dark matter, which is not detectable by telescopes, but which nevertheless makes up an estimated 84% of the matter in the universe (including 84% of the matter in galaxies) ~ and (c) the most important factor of all, gravitation.

Even over vast distances, a single massive object (say our Sun) exerts an enormous gravitational pull on other objects too far to be seen with the unaided eye.  The Sun holds Earth in a captive orbit 93 million miles away, and Neptune in a captive orbit 2.7 billion miles away.  The Oort Cloud?  Forget about it.  Our Milky Way galaxy is a grand, slow-motion swirl of stars held in place (and in motion) by the dynamic balance of gravitation, centripetal force, and momentum.

So imagine the titanic gravitational forces at work when the paths of two galaxies merge.  It will be like stirring melted white chocolate into melted dark chocolate.  Initially the colors remain distinct, even as they whirlpool into each other.  Stir long enough, and the colors blend into a single composite hue.  Thus with the Milky Way and Andromeda.  There may be explosive collisions of individual stars, but for the most part the component stars will act like molecules of blending chocolate ~ remaining structurally intact, but rearranged in relation to each other.

I wish we humans had life spans long enough to witness such cosmic events.  Here is a brief animated simulation of what the Milky Way - Andromeda collision may look like.

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