Astro Bob: Tau Herculid meteors show up right on time
No one was sure what to expect. While we didn't see a "meteor storm," observers with dark skies got a nice show.
DULUTH — I had to get the heck out of town to see it but was glad I did. Clouds packed the sky from home around 10 p.m. but a check on the latest satellite photos showed a band of clear skies to the southwest. I got in the car and found gold one hour and 55 miles (90 km) later just outside the town of Barnum, Minnesota.
Locating a suitable observing site out of the blue can be fraught. But when my headlights revealed a muddy pullout without a yard light in sight I knew I was home. Nearby, an oversize rut, filled with water from recent storms, hosted a very vocal colony of frogs. Passion takes many forms. While I flopped out on a lawn chair and faced the sky, they focused on arguably more essential earthly business.
Not 5 minutes out of the car I spotted my first Tau Herculid, sliding down the western sky. Then another. And it was still just 11:15 p.m., 45 minutes before the predicted peak. All told during my 2-plus-hour stay, I counted 23 shower members and three sporadic or random meteors. Their brightness ranged from magnitude 1 (about as bright as Deneb in the Northern Cross or Spica in Virgo) to about magnitude 3, one level fainter than the Big Dipper stars.
Besides a common radiant point near Arcturus, what set them apart was their speed. As predicted, the dusty bits shot across the sky fairly slowly, leaving short streaks ending in small, moderately-bright, yellowish "heads." They weren't like the zippy August Perseids — more like tears that roll down the cheek and fall to the ground. Other observers who've posted here reported similar experiences.
The shower was most active between 11:15 p.m. (May 30) and 12:30 a.m. (May 31), then it dribbled away to nothing. But there were caveats. Waves of clouds occasionally passed by preventing full views of the sky for short periods of time. One of those waves featured a spectacular lighting display from a line of thunderheads several miles east of my location. When your eyes are used to the dark and suddenly lightning flashes — whew! It might as well be nuclear bombs detonating.
I stayed horizontal and watched the sky rapidly flicker like an old Bela Lugosi movie. One moment it was black and overflowing with stars. Then lightning would flash, and wipe away all but the brightest. Moments later, they'd rematerialize. Absolutely mesmerizing. I've said it before: if you're looking for adventure you'll find it in this hobby. Sometimes the best part has nothing to do with why you chose to loose sleep in the first place.
As if meteors, lightning and frogmania weren't enough, the aurora borealis did a little dance along the northern horizon, and parallel plumes of green airglow streaked much of the sky. I like to call airglow nature's light pollution. While the aurora concentrates in ovals centered on Earth's geomagnetic poles, airglow enshrouds the entire planet. Few are acquainted with it however, because dark skies are required to see it.
During the day, ultraviolet (UV) light from the sun breaks apart oxygen and nitrogen molecules in the upper atmosphere about 60 miles (100 km) high. At night, they recombine (or combine with each other) and emit a faint light as they return to their original, "unexcited" state. Oxygen creates the lovely green tendrils in the photos.
Ordinarily, airglow demands the darkest conditions, but last night's version was not only relatively bright (I thought it was clouds at first) but also covered more than half the sky in great green waves. While the structures were faint they were still obvious with the dark-adapted eye. But in the camera — oh gosh! Just look at the color and majesty of it.
The pattern was likely induced by the thunderstorms. Powerful storms can create gravity waves — undulations of air that propagate all the way up into the upper atmosphere. Like ripples spreading in a pool, they shape the airglow and sometimes even the aurora, which lies at a similar altitude, into dunes, waves, plumes or even noodles like those in a classic lasagna.
On the way home at 2 a.m. I had the freeway to myself. Now and again, I'd peer up through the windshield to watch the progress of the shower, still hoping to catch a stray fireball. I hope you were able to spend some time with the shower and see a few. While a storm never occurred (at least not visually), the fact that it happened was incredible enough.
Let us know what you saw (or didn't see) by adding a comment on my Facebook page . Thanks!