"One Ring to rule them all." Join me as we explore a unique class of galaxies forged in the chaos of collision.
As Orion tilts to the west, the approach of spring offers a wonderful diversity of galaxies for telescopic viewing. Spirals. Ellipticals. Irregulars. Here and there among them are representatives of a rare breed — the ring galaxies. Shaped like onion rings, their formation is rooted in catastrophe.
Ring galaxies form when a small neighbor galaxy plunges through the disk of a larger spiral galaxy. The collision rarely results in any stars smacking into one another, but the gravitational shock acts like a tsunami that alters the orbits of stars and gas clouds, pushing them outward in excess of 150,000 mph (241,000 km/h). When the gas clouds collide, they compress and transform into a expanding ring of new star formation — think of an enormous wave crashing ashore in a frothy chaos of white foam.
Simulation of the collision between an elliptical and spiral galaxy to form the recently discovered "Auriga's Wheel" ring galaxy.
A typical ring galaxy still hangs on to its dense nucleus, but the once-majestic spiral arms have been recast as a striking stellar halo sparkling with young, massive blue stars. In Hubble Space Telescope images, ring galaxies appear temptingly bright. Unfortunately, most are faint and rather small, but not so much so that we can't track down at least a few with telescopes with 10-inches (25 cm) of aperture or more.
We'll check out three of them, Arp 143, Arp 141, and NGC 2793, all well-placed for viewing on March evenings. It's no surprise that two of them made it into astronomer Halton Arp's fascinating Atlas of Peculiar Galaxies, which includes numerous interacting pairs.
Our first stop will be Arp 143 in Lynx located high in the southern sky above Castor and Pollux at nightfall. Known also as NGC 2445 and NGC 2444, each is listed at magnitude 14, but they're clearly brighter than this. I would estimate 13. Through my 15-inch (37-cm) Obsession scope, both are immediately visible at low power as two separate objects.
The intruder, NGC 2444, is compact with a bright nucleus; its ring galaxy neighbor is one of the oddest galaxies I've ever set eyes upon. The nucleus appears is offset well to the north end of galactic center. Upping the magnification to 257x I could discern four glowing knots — HII star-formation regions — within the galaxy's amorphous, hazy outline. The knot south of the nucleus was easy enough, but the other three required averted vision and concentration to tease out.
At first glance the disturbed galaxy's clumpy appearance resembles a raisin pudding far more than a wheel. Where's the ring, you ask? Give it some time, say astronomers who've studied it closely. What we have here is a nascent ring on the cusp of a massive wave of star formation. We're witnessing the chaos of collision by neighboring NGC 2444, the distorted shape of which tells us it did not escape unscathed from its dashing pass.
Once you've cut your teeth on Arp 143, you're ready for UGC 3730, or Arp 141, a 13th-magnitude ring galaxy in Camelopardalis located about 10° northwest of Ursa Major's bright M81-M82 galaxies. Arp 141's core is a strange beast with an obscured nucleus and tortured ring. Like Arp 141, it's faintly but immediately apparent as a fuzzy patch about 2.5′ long and extended north-south at 64x. Closer inspection at 257x shows the elliptical member as a "fuzzy star".
With averted vision I could clearly see the fainter ring galaxy and a bright patch (HII region?) where they touch. The ring shape itself was visible only fleetingly. I bet an 18-incher would reveal it.
Our last stop is NGC 2793, conveniently located less than 1° west of 3rd-magnitude Alpha Lyncis. This little button of a galaxy measures just 1′ across and glows at a modest magnitude 13. As with the others, it's easy to spot but takes concentration to ferret out the details that reveal its nature. Many ring galaxies aren't that difficult to find, but seeing the contrast between the ring and the rest of the galaxy can be challenging.
With high magnification I can see a brighter, non-stellar nucleus off to the galaxy's east side; honing in by alternating averted with direct vision, I glimpsed several tiny "sparkles" or knots of what are doubtless new star clusters within its southeastern border. Overall, the galaxy's eastern third appeared brighter and denser than the hollowed-out portion seen in the photo above.
This trio of rings is seasonal and samples only a small number of an already small class of galaxies. I encourage you to explore them as well as others. A simple click will download a .pdf with photos of 40 others in this illustrated ring galaxy catalog, enough to guarantee many nights of delicious exploration. You may also find this Color Atlas of Ring Galaxies helpful.
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