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Al Stahler: The Cartwheel

Al Stahler
The Cartwheel Galaxy formed as the result of a high-speed collision that occurred about 400 million years ago.
Provided by NASA, ESA, CSA, STScI

Viewing sculpture, we can almost feel how the artist’s hands pressed and shaped the work.

Car and truck bodies, too, are sculpture, sculpted in the factory by machines pressing and shaping the sheet metal.

County fair! All-Star Monster Truck races! Truck bodies now and then re-sculpted – pressed into new shapes – when two trucks meet.

Dangerous work … we’re not really hoping for a crack-up, are we?

Through a telescope, the sky is littered with sculpture … which brings up two differences between astronomers and truck race fans:

1) While pushing and pressing contribute to sculpting objects in space, the largest structures are largely shaped by pulling, with gravity. Put a finger into a jar of honey and pull it out … a tendril of honey follows our finger. Gravity pulls similar tendrils of stars, atoms and dust.

2) Astronomers unashamedly hope to see smash-ups: Whole galaxies – hundreds of billions of stars – slamming into each other at millions of miles an hour.

When metal monsters collide, the results are not pretty. When galaxies collide, the results can be beautiful, as we see in the new space telescope’s image of the Cartwheel Galaxy.

With few exceptions, everything we see in the sky at night is part of our Milky Way Galaxy. One exception, though, is the farthest thing we can see with the naked eye (come autumn): The Andromeda Galaxy. Andromeda is two-and-a-half million light years (15 quintillion … 15 million million million … 15 followed by 18 zeroes … miles) from the Milky Way … but drawing closer at a quarter-million miles an hour. As we grow closer, we fall together faster; the collision – in some billions of years – will be spectacular.

Thus, to watch galaxies collide is to watch our own future … and also our past: Our collision with Andromeda won’t be our first such encounter.

The Big Dipper is high in the sky in summer. The Dipper is a signpost to the North Star, of course, but the Dipper also guides us to other stars: The handle of the Dipper curves, to form an arc. Follow the curve of the handle’s arc, and we come to the bright star Arcturus. “Follow the arc to Arcturus.”

All the stars we see at night – all the stars in our neighborhood – move in pretty much the same direction … except for Arcturus. Our sun and its neighbors are all rolling down the same freeway, but Arcturus follows a freeway that cuts across our own.

Arcturus is an oddball for more reasons than that. Arcturus is such an oddball, it’s likely Arcturus is a visitor, from another galaxy … a galaxy that smashed into our own, millions of years ago. A couple dozen other stars move like Arcturus, all likely remnants of that galaxy that smashed into us, that still bashes through our galaxy. Arcturus will soon be gone from our neck of the woods.

Maybe half-a-billion years ago, a small galaxy punched through the center of what was probably a normal, spiral galaxy (much like our own Milky Way), transforming it into the Cartwheel.

Like our own Milky Way, the original galaxy was littered with dust (visible, if you know where to look, to the naked eye). Throughout the history of astronomy, this dust obscured the view in visible light. The James Webb Space Telescope only sees a little in the visible part of the spectrum; it largely sees infrared, which comes through the dust.

Compress dust – squeeze it – and you can make stars. Like tossing a rock into a lake, a colliding galaxy sends waves through dust … triggering the formation of baby stars. The bright outer ring of the Cartwheel is a ring of such young stars.

Imagine tossing a rock, not into a lake, but into a bathtub. When the wave reaches the walls of the tub, it reflects, heads back inward … again triggering starbirth as we (perhaps) see in the inner ring of the Cartwheel.

Invisible to our eyes, and to the space telescope, is a tendril of gas (visible only in radio waves), pulled (like honey) out of the Cartwheel. It leads to a small galaxy (not in this image), that, very possibly, flew through the larger galaxy, turning it into the Cartwheel.


The Perseid meteor shower peaks right around now – just as the moon waxes full – making this an unfortunate time for meteor-hunting. But the moon will rise later and later, one night to the next, so we should have some dark skies, and perhaps a few bright Perseids, in coming nights.


Saturday, Aug. 20, at 9 p.m. at the intersection of Highway 49 and Old Downieville Highway. Free … bring the kids … and a sweater.

Al Stahler enjoys sharing science and nature with friends and neighbors, in The Union and on KVMR-FM. He teaches classes, for both kids and grown-ups, and may be reached at a.a.stahler11@gmail.com. Get in touch, if you’d like to be on the email list for star parties.


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