Saturday, January 16, 2010

...in vacant or in pensive mood...


For those with a decent amateur telescope, spring is a good time for galaxy hunting. There is a swath of sky between the Big Dipper and Virgo that is rich in "island universes", including the magnificent M81 in Ursa Major and the Whirlpool Galaxy in Canes Venatici. M94 is in Canes Venatici too, another of Charles Messier's fuzzy spots. And that is what it looks like through an amateur telescope. It's only the bright central core of the galaxy that one sees -- a smudge on the windowpane of night.

Thursday's Astronomy Picture of the Day (APOD) is the most revealing image yet of this beautiful face-on spiral, more or less a twin of our own Milky Way Galaxy. For the first time we see the faint outer arms of the galaxy, dragging their gas and stars in a grand sweeping revolution -- perhaps turning once on its axis every few hundred million years.

What I love about this photograph is the way it reveals the three-dimensionality of space.

When we look at M94, we are looking pretty much straight up out of the flat spiral of the Milky Way; that is to say, we are looking into the deep universe with as little as possible obscuring local stuff in the way. But there happens to be three Milky Way stars in the picture, the three bright objects with diffraction spikes (caused by the telescope). These stars are less than, say, a thousand light-years away, which is about the thickness of the Milky Way Galaxy in our neighborhood of the spiral arms. These stars are much too faint to be seen with the naked eye.

M94 is 15 million light-years away. If the Milky Way Galaxy were a dinner plate (containing hundreds of billions of stars, including the Sun and the three we see here), M94 would be another dinner plate a few hundred feet away -- down the block. That makes it, by the way, a relatively near neighbor. By comparison, the closest spiral galaxy, the Andromeda Galaxy (M31), would be a dinner plate just across the room.

Except for the three local stars, the stars we see in this image of M94 are the giants of that galaxy. A mediocre star like the Sun would not show up in the picture at all.

But wait! We have another layer of universe.

Here and there among the stars of M94 you can see what look like little elongated smudges; for example, one above and one below the central nucleus. I can count a dozen or so. These are other galaxies, other M94s if you will, far, far beyond -- other dinner plates in the next town.

And so this APOD invites our imaginations to soar, up and out of our galaxy across millions of light-years of empty space to another magnificent whirl of hundreds of billions of stars, and then on -- on -- on into the depths of a universe whose vastness and hidden wonders our species has only begun to sample.