Andromeda Galaxy is big, really big


By Tom Burns - Stargazing



In January, I will begin what will probably be my last semester of teaching at Ohio Wesleyan. One of my classes will be the First-Year Writing Seminar, which we used to call Freshman Composition in the ancient, unenlightened past.

I’m a teacher by trade, and the “spring” semester, as we euphemistically call it, can be a trying time for pedagogues. What with recalcitrant students, tall stacks of papers to mark, sleepless nights ­— many of them cloudy and starless — and the frigid, snowy drives to work, the “spring” semester can be a trying time of year.

Most teachers of the course feel a strong sense of responsibility teaching the class, and I am no exception. We must prepare students for the rigors of college writing and guide them into writing habits that we fervently hope will stick with them for the rest of their lives.

I ask myself the same question today that I asked when I taught my first “Freshman Comp” class 48 years ago. How do I teach college-level writing in the space of a 15-week semester?

To that, I add a more pressing question. How do I help my students to overcome their post-pandemic lethargy?

Luckily for me, the occasional clear night provides a bit of long-distance inspiration.

If one comes your way, check out the Andromeda Galaxy (“M31” in Charles Messier’s influential catalog), which rises high in the east by the end of evening twilight. The galaxy is visible to the unaided eye from even modestly dark skies. As a result, M31 is the farthest object most folks can see without optical aid.

Sadly, the whirlpool-shaped collection of 300 billion stars is reduced to an elongated fuzzy patch because it’s 2.5 million light-years away. Every one of those light-years is equal to about six trillion miles.

Today, that distance seems to be set in stone, but historically, it was not. For centuries, most astronomers assumed that objects like M31 were well inside the Milky Way. Some astronomers thought those “spiral nebulae,” as galaxies like M31 were called, were swirling gas clouds trying to collapse into individual stars.

During the early 20th century, astronomers began building telescopes large enough to resolve a few of M31’s stars. Some of those stars, called Cepheid variables, pulsated in their brightness over a predictable and measurable period.

Groundbreaking astronomer Henrietta Levitt discovered that astronomers could use the length of a single pulsation to tell how bright the stars actually were, i.e., how much energy they produced. Astronomers call that quality the star’s “absolute brightness.”

By comparing a star’s apparent brightness with its absolute brightness, astronomers like Edwin Hubble concluded that M31 must be so far away that it must be outside our Milky Way. Given its apparent size in the sky, it must be a galaxy like the Milky Way.

Over the decades of the last century, estimates of M31’s distance steadily increased from Hubble’s original estimate of 900,000 light-years to the current 2.537 million.

To put it in scientific terms, M31 is 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 the Milky Way.

We don’t see M31 from the top, so 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 has survived over billions of years. The same spin dynamic that creates the spiral should also destroy the spiral. As time passes, the arms should wrap themselves into rings around the galactic center, but they do not.

How does the spiral shape survive? The evidence suggests that the stars are caught up in density waves. As any given clump stretches out into its spiral arm, each star develops its own velocity. In other words, a given star moves at its own pace and direction through the spiral arm.

As the stars interact 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 stars approach each other and then separate.

At some time or another, we all have experienced such density waves firsthand. As you drive down the freeway and approach an accident, you must decelerate and join a bunched-up section of slow-moving vehicles.

After you pass the choke point, you speed up, and the distances from vehicle to vehicle stretch 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. Earth revolves around the sun in a stable orbit. Our planet wouldn’t exist 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 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.

Thank goodness for the swirl, which is, in fact, one of those fundamental engines that keep the universe in balance.

That perfect balance of gravity and velocity is a kind of miracle that replicated itself at least six trillion times in a universe of at least six trillion galaxies.

However, a problem arises. Try as they might, astronomers cannot find enough matter in a given galaxy to generate the proper amount of gravity.

Starting in the 1960s, groundbreaking astronomer Vera Rubin began a careful study of galactic rotation and the amount of matter detected in galaxies.

Her conclusion was startling. A galaxy must contain five to ten times more material than astronomers can find.

If the detectable matter is any indication, the galaxies should never have formed in the first place or should have torn themselves apart after they formed.

Thus was formed the notion of dark matter, whatever it may be. The issue is still unresolved. Many such questions beg for answers, despite the exponential growth of knowledge about the universe — or perhaps because of it.

Those matters became apparent to me at 5 AM 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 a.m. First-Year Writing Seminar was on my exhaustion-encrusted mind as I performed a simple experiment. You can also do it 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 you can find it in your second cup of morning coffee.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.