I’m a teacher by trade, and December can be a trying time for pedagogues. What with recalcitrant students, big stacks of papers to mark, sleepless nights — most of them cloudy and starless — and cold, sometimes snowy drives to work, December is not the most enlivening of months.
However, the occasional clear night provides a bit of long-distance inspiration.
If one comes your way, check out the Andromeda Galaxy (M31, for short), which rises high in the east by the end of evening twilight right now. From even modestly dark skies, the galaxy is visible to the unaided eye, making it the farthest object that most folks can see without optical aid.
Sadly, a whirlpool-shaped collection of 300 billion stars is reduced to an elongated fuzzy patch when it’s 2.5 million light years away. Every one of those light years is equal to about six trillion miles. To put it in scientific terms, that puts the galaxy really, really far away. And yet it is also the closest galaxy to us in the Milky Way if you don’t count a couple of puny satellite galaxies huddled close to us.
We don’t see it from the top, which means we can’t observe its round(ish), spirally splendor. The galaxy is tilted at about a 45-degree angle, making it look vaguely cigar-shaped.
Despite its mind-melting distance, the Andromeda Galaxy stretches all the way across most binocular fields. It looks big because it is big — 150,000 light years wide.
Its size and tilt create yet another mind-melting peculiarity. The front edge of the galaxy is considerably closer to us than the back end. You see the light from the back of the galaxy 100,000 or so years after you see the front end. In effect, M31 is so big that you don’t see it all at the same time.
The Andromeda Galaxy gets its spiral shape because of its spin. Stars tend to form in clumps because they are born in enormous clouds of hydrogen gas. The stars in the central portion of the galaxy travel around the galactic core quite rapidly. Stars at the periphery take hundreds of millions of years to make one revolution. The outer stars lag behind the inner ones, a process that stretches the star clumps into a spread-out pinwheel.
But that doesn’t explain how the pinwheel survives over billions of years. The same spin dynamic that creates the spiral should also destroy the spiral. As time goes by, the arms should wrap themselves into rings around the galactic center, but they do not.
How does the spiral shape survive? Increasingly, the evidence suggests that the stars are caught up in density waves. As any given clump stretches out into its spiral arm, each individual star develops its own particular velocity. In other words, a given star is moving at its own pace and direction through the spiral arm.
As the stars interact with each other gravitationally, they slow down and speed up as they reach gravitational choke points. If you saw the process speeded up, you would swear that the stars are engaged in a glorious gravitational dance as the stars merge and then separate
We all at some time or another have experienced such density waves first hand. As you drive down the freeway and approach an accident (or whatever), you are forced to decelerate and join a bunched-up section of slow-moving vehicles. After you pass the choke point, you speed up and the distance among cars stretches out again until you approach the next choke point.
Imagine the same thing happening to the stars in a spiral over billions of years. The process slows and thus prevents the spiral from wrapping itself into a ring around the galactic hub.
That the galaxies have such a revolutionary temperament should not surprise us. Our own planet revolves around the sun, and it wouldn’t be here at all if it didn’t.
The planets of our solar system move in a perfect balance between their velocity, which makes them want to fly away from the sun, and the sun’s enormous gravity, which holds them in. Stop Earth’s revolution, and our planet would simply fall into the sun.
The same is true of the Andromeda Galaxy and, come to think of it, our own Milky
Way. Stop the revolution of any galaxy, and the whole shebang would collapse by its own gravity to a very dense and compact lump. So thank goodness for the swirl, which is, in fact, one of those fundamental engines that keeps the universe in balance.
These things became very clear to me at 5 a.m. one morning as I sat far from home in front of a cup of coffee at an all-night diner after a night of observing galaxies, which were also far from home.
My 9 AM class was on my exhaustion-encrusted mind as I performed a simple experiment. You can perform it as well if you’re willing to engage in an early morning astronomical endeavor. Give your cup o’ Joe a quick, vigorous stir, and then pour in the cream.
That morning — discouraged, tired, and despondent as I poured the cream into the cup — a perfect spiral galaxy formed in the swirling black brew. The same force that stirs our morning coffee rules the universe.
And at that moment, some lines from a poem by William Blake swirled into my head:
To see a world in a grain of sand
And a heaven in a wild flower
Hold infinity in the palm of your hand
And eternity in an hour.
There was a galaxy in my coffee cup, a universe in the palm of my hand.
I was ready to teach. Astronomically speaking, inspiration sometimes comes from very far away, but sometimes we can find it in our third cup of morning coffee.
Please come to one of our Friday-night Guest Nights at Perkins Observatory!
We won’t be going them from December 22 – January 18 to spend some time with our families and do much-needed repairs at the “O.” We will resume Guest Nights on January 19. Please call (740) 363-1257 for more information or to buy tickets.
Tom Burns is director of the Perkins Observatory in Delaware.
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