The European arm of the Hubble Space Telescope coordinating facility has just released a new image of the weird Comet 17/P Holmes, taken on November 4, almost two weeks after the comet suddenly expelled a huge quantity of dust (as usual, click the image to embiggen):
The color image is a deep ground-based observation, and the grayscale one is from Hubble (note the scale bar; Hubble could see details as small as 54 km across). Some things are obvious immediately from the picture. There is a lot more dust along the horizontal axis then the vertical; that's why it looks like a funny vertical bow-tie. There is about twice as much dust horizontally (east/west) than vertically (north/south). That means the dust was expelled in a preferred direction, and not isotropically, that is, in all directions like an expanding spherical shell.
Even nearly two weeks after the outburst, the nucleus was still enshrouded in dust. A series of images taken over the course of seven days shows the central region of the comet dimming as the dust cloud expanded and cleared out:
Remember, all the action is coming from the nucleus of the comet, a chunk of ice and rock only a few kilometers across. If comets stayed that way, all tucked into a giant boulder, we'd never see them. But they heat up when they approach the Sun, and the ice sublimates (turns into a gas), expanding outward, carrying dust with it. This reflects sunlight, brightening the comet. In the case of Holmes, it was farther out from the Sun than Mars is when it suddenly brightened by a factor of a million. Some event on or under the crust of the comet nucleus caused a catastrophic release of material. The expanding cloud of debris is now bigger than the Sun itself (about 1.5 million km across).
Two factors compete to account for the brightness of the cloud. Initially, when the cloud is dense, it gets brighter as it gets bigger because it can reflect more sunlight. But eventually the cloud starts to thin out and becomes less efficient at reflection. It then gets fainter. For a while the comet cloud from Holmes got brighter, but has since been fading. I haven't had a chance to see it for a week or so (weather, travel, and such) but I'm hoping to get a chance again soon. A few days after the outburst the cloud got so big it was obviously non-starlike to the naked eye, which was astonishing.
We understand a lot about the basics of comet behavior (despite some kooky claims), but each comet is an individual, and can be difficult to predict. Holmes has shown us precisely why we need to keep observing each comet as they present themselves to us. Not only can we learn more about these objects and satisfy our scientific hunger, but they also appeal to our sense of grace and beauty.