Monday, June 23, 2008


brome grass drew our attention recently to this APOD (Astronomy Picture of the Day) of the galaxy NGC 5907 in the constellation Draco (click to enlarge). We see the galaxy edge-on. It is a spiral galaxy, much like the Milky Way, containing surely more than 100 billion stars, as well as dark banks of dust and gas. What the new photograph shows are ghostly bands of light looping around the galaxy. These are presumably the light of stars shed by smaller galaxies that were gravitationally captured, torn apart, and assimilated by the more massive spiral. Galaxies grow by attracting less massive neighbors into dancing orbits, then gobbling them up.

Dancing galaxies shedding stars like fairy dust!

NGC 5907 is 40 million light-years away. The stars you see in the photograph as discrete points of light are in the foreground; they are part of our own Milky Way Galaxy. We are inside, looking out at the universe of galaxies beyond. Some of the blurrier "stars" are perhaps galaxies more distant than NGC 5907. This is certainly true for the oval blur in the upper right-hand corner of the photograph, which could be as far as a billion light years away.

The region of the sky shown in the photo could be covered by a pinhead held at arm's length.

Stand outside on a dark night and hold a pinhead at arm's length and imagine how many pinheads it would take to cover Earth's entire sky, of which you can see at any one time somewhat less than half. How many photographs such as this one showing wonders galore! Once, at the Siding Spring Observatory in Australia, I had the opportunity to look at one of the hundreds of glass photographic plates that were part of a survey of the southern sky. The emulsion-covered glass was thin enough that it could bend to conform to the focal plane of the telescope. It was about the size of a newspaper page. From a few feet away it appeared to be covered with dust. But bending low, with a magnifier to my eye, the "dust" became stars and galaxies. Hundreds and hundreds of galaxies, island universes, scattered across the glass. I had to catch my breath. I felt like I was falling, falling, falling into a universe so sublime that no adequate words could describe it.

Intellectually, the Copernican Revolution is over. We know we are not the center of the universe. Psychologically, the Copernican Revolution has just begun. We have yet to fully grasp what it means to live in a universe where galaxies shed stars in their dancing tracks like fairy dust.