Stargazing: Do the stars really move?


By Tom Burns - Stargazing



Here’s a deceptively simple question we recently got from a fifth grader at one of our daytime fieldtrips at Perkins Observatory: Do the stars move?

Not too long ago, asking such questions got you burned at the stake. These days, I can give the answer in the newspaper.

Yes, they move, but not in the way you might think.

If you go outside tonight or in the next few weeks at around 8 p.m., you can’t miss Sirius flickering low in the south-southeast. It is, after all, the brightest star in the nighttime sky and anchors one of the sky’s most distinctive constellations, Canis Major, the Greater Dog.

Like most bright stars, Sirius twinkles. You will see it flash, change colors, and even seem to jump to a slightly different place.

Twinkling is caused not by the motion of the star, but by the blanket of air through which we must view it. The currents in our great sea of air lift the light and move it to slightly new locations. The star is not moving. The medium through which we view it is moving.

Now go take a nap and observe Sirius again at midnight, but don’t look in the same spot you saw it before. It has moved to the south (to the right) and is now higher in the sky.

But it hasn’t moved at all. We have moved. We see the universe from the surface of a spinning top. Earth has turned a bit on its axis, and you are thus looking at the stars from a slightly different vantage.

Now take a really long nap — until, say, March. If you look for Sirius at 8 p.m. again, you will see it low in the south-southwest. Over the intervening days, Sirius has moved in an arc from southeast to higher in the south to low again in the southwest.

We are moving in yet another way. The Earth spins like a top, but the top is perched in the seat of a Ferris wheel at our planet moves around the sun.

Astronomers have been able to account for these motions for centuries and can detect with great accuracy where the stars are in the universe. They should be able to tell whether the stars really move.

In the 16th century, Giordano Bruno speculated that the stars move on their own through space, and he became a human shish kebob as a result.

Bruno’s hypothesis was finally verified a century later by Edmund Halley of Halley’s Comet fame. In 1718, he compared the current position of Sirius with positions that had been recorded by the Greek astronomer Hipparchus centuries before. Sirius had indeed moved with respect to the position of nearby stars.

In general, the stars of the Milky Way, both the sun and nearby Sirius included, are rotating around the hub of a flattened disk of 300 or so billion stars we call the Milky Way galaxy. The Milky Way’s spin has slowly stretched out its outer stars into spiral arms as the stars on the outside of the disk lag behind the inner hub. A single revolution out in the Milky Way’s periphery, where both Sirius and the sun are located, takes about 100 million years.

However, stars like our sun are not fixed in their position in the spiral arms of the Milky Way. Stars have their own distinct motions, which are affected by their gravitational interaction with other stars.

Eons ago, the sun was not in its current arm, and eons from now it will wander to other parts of our galaxy.

So, my dear fifth grader, the stars do indeed move. If humans survive as a species, many long years after you are gone from the planet, children like you will look up at the stars and see totally different constellations as the stars shift around in the galaxy and the sun and Earth travel to distant realms of our galaxy.

We live in an age where change happens quickly and sometimes unexpectedly. Humans seem to fear death and change most of all.

Life is indeed unpredictable, but it has always been so. There was a time when he fixed position of the stars in familiar constellations provided a bit of comfort when change could be far more devastating than it is today.

The fixed positions of the stars were looked upon as a sign of God’s perfection. Anyone suggesting otherwise committed an unforgivable affront to God.

Times have changed and with them our understanding. We have come to realize that we are caught up in a series of swirls within swirls within swirls. Our Earth spins once a day, and it moves once a year around the sun. Our sun swirls around the center of our galaxy, and the myriad galaxies are flying apart from each other at ungodly speeds.

Some very religious people reject such notions because such seeming chaos seems foreign to their notion of God’s perfection. But others, myself included, see it as a sign of God’s power and intelligence — if indeed that God exists at all.

Yes, the stars move, and we move with them. Change is not the bane of humanity. It is the ever-present constant that rules the universe and our much smaller lives.

Instead of looking for changelessness, we must embrace change and understand the laws that make change more predictable. Science is not just for nerds without better things to do. It speaks to our deepest fears and our deepest hopes. It helps us understand and harness the changes that rule our lives.

Yes, the stars move. If we want to continue to live in this universe, we have to learn to go along for the ride with eyes wide open and with joy.

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By Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.