Wednesday, September 26, 2012

Naked eye

Can you see a galaxy with the naked-eye?

Well, of course, you can see our own galaxy, the Milky Way. We are inside it, and see it as a band of milky light sweeping across the sky in a grand 360, breathtaking on a clear, dark summer night. Still, I used to be surprised when teaching a general studies astronomy course how few students had any memory of seeing the Milky Way -- an indication, I suppose, of just how dramatically we have polluted the night sky with artificial light.

Then there are the Milky Way's two satellite galaxies, The Large and Small Magellanic Clouds, prominent in dark skies of the southern hemisphere. I've seen them but once, under the best possible conditions, in the unpolluted skies of central Australia. To the naked eye they look like stationary clouds of hazy light.

The closest spiral galaxy to our own is the Andromeda Galaxy, 2.6 million light-years away, which takes its name from its constellation. It fills a part of the sky that you could cover with a finger held at arm's length (six times wider than the full Moon), but with the naked eye you can only see the bright core of the galaxy as a fuzzy blur on a dark summer night under clear skies. It's an easy object if you know where to look.

Not much further away (2.9 million light-years) is the Triangulum Galaxy, reportedly easy to see under ideal conditions, although I have never been successful. A few other nearby galaxies have reportedly been observed with unaided vision, but not by me. I've seen the object Omega Centauri often enough, but always considered it a globular cluster, and part of our own Milky Way, but I believe a few years ago it was reclassifed as a dwarf galaxy.

So that's it. Of the billions of galaxies visible to telescopes, only Andromeda gives us a naked-eye glimpse of the greater universe away from our own galaxy and its immediate neighborhood.

The Andromeda Galaxy and the Milky Way are destined to merge.

We are separated by more than 2 million light-years, but gravity is pulling us together. The most recent work suggests that the collision will not be head-on. In about 4 billion years, the two galaxies will sweep past one another at 600 kilometers per second, caught in a gravitational pas de deux, like the two galaxies in the Hubble photograph above, NGS2207 and IC 2163 (click to enlarge). They will twirl around, ripping their neat spiral arms asunder, and gradually merge into a giant elliptical galaxy.

Which is about at the same time the Sun will have used up it nuclear fuel and expired.

It will be a stunning sight, that approaching spiral filling the night sky -- if there is anyone around to watch. Unlikely to be our descendents, but surely among the hundreds of billions of stars among those two fine galaxies, some sort of sentient life will have appeared on the scene to witness what must surely be one of the grandest naked-eye spectaculars in the universe.