Image Credit: Galaxy Zoo, ING, APOD
A couple of months ago, I was looking at the daily Astronomy Picture of the Day, when I saw the weird green thing near a normal galaxy in picture above. Even stranger was the object's name: "Hanny's Voorwerp" (which, it turns out, is Dutch for "Hanney's Object"). It was a cool picture of an object the likes of which hadn't been seen before. So, I stuck it in the "weird astronomy things" file in my brain and went on.
Now, a couple of months later, CNN.com has made Hanny's Voorwerp its lead story. It must be a slow news day, because while the discovery (and method of discovery) are good stories, I'm not sure it should rank above the news about the Olympics, or news from Iraq or Afghanistan, or the U.S. election. But it is a nice change.
So, what's the story? It starts with a project called Galaxy Zoo. Galaxy Zoo was set up to get people at home involved in a science project that requires human eyes and brain power to classify galaxies, but is too labor intensive for any astronomer to undertake. Basically, galaxies come in three basic shapes -- spiral, elliptical, and irregular. (That's an over-simplification; there are sub-classifications and many details that I won't go into, because they aren't important here). Computer algorithms for identifying galaxies do okay, but they aren't perfect, and they tend to miss interesting objects. In the Galaxy Zoo, pictures of millions of galaxies taken as part of the Sloan Digital Sky Survey have been looked at by volunteers around the world.
Hanny's Voorwerp is one of those interesting objects that I doubt a computer would have recognized, because it is unlike anything we've seen around other galaxies. It appears to be a large patch of gas near big galaxy, but this gas doesn't seem to have its own stars. It's just sitting there in space and glowing. It was discovered by a school teacher from the Netherlands, Hanny van Arkel.
So, what is Hanny's Voorwerp? We don't know! But that doesn't keep us from guessing. Initial data from the picture and from spectrographs indicate that this is really hot gas at the same distance as the galaxy in the picture above, and that the gas has probably been heated by a shock wave (like a blast wave from an explosion, or a collision with something). The best guess, according to astronomer Bill Keel's work, is that the galaxy above Hanny's Voorwerp harbors a big black hole at its center (as most galaxies do), and that it recently was "active," or swallowing dust and gas at a prodigious rate. When a black hole is gobbling material like this, the material tends to heat up, and some of it can get shot away from the black hole before it is swallowed in gigantic, high-energy jets (like these). And then, within the last 100,000 years or so (very recently by astronomical standards), the black hole ate everything within reach, and went into hibernation, turning off the visible light.
Hanny's Voorwerp was an innocent bystander, some gas that happened to either be passing by the galaxy, or perhaps falling into the galaxy, when the black hole turned on and blasted it with intense radiation and jets of material. The black hole then turned off, leaving behind a ghostly glow from the impact.
Congratulations to Hanny van Arkel on her unique and interesting find!