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The Hoods of Hale Bopp

Week of March 31, 1997
picture of comet Hale-Bopp
Clicking on the image sends you to a larger version at Terry Platt's comet page.

By now, pretty much everybody has heard about the comet Hale-Bopp or seen it for themselves. The pictures shown on the news and in newspapers, though, are usually taken without magnification (that is, with cameras mounted on tripods) or they are taken through a telescope, but overexposed to show the fainter tail.

So when I first saw Hale-Bopp through my own telescope, I was stunned. What I saw was (and is!) very much like the photograph seen here. The nucleus, or central region of the comet, is surrounded by several arcs of glowing material, and attached to the nucleus itself is a bright "comma" shaped object (these features can be seen in the image presented here; click on it to see it with more detail). What is all this?

A comet is basically a giant chuck of rock and ice, and by ice I mean frozen water and various gasses. As the comet approaches the Sun, that ice melts and even vaporizes. It boils off the comet head and the moving comet leaves it behind. This is basically what we mean by the comet's tail. But in this comet things are a bit different. For one, the comet head is rotating rapidly, completing one rotation every 11 hours or so. Plus, there must be a large pocket of gas just under the surface of the comet. When that gas heats up it erupts out of the head like a geyser. The gas heats up whenever it is facing the Sun, and cools when it faces away (just like day and night on the Earth). So as the comet rotates, local sunrise turns that jet on, and local sunset turns it off.

Since the head is spinning, that gas shoots off in an apparent spiral from the comet. Imagine taking a garden hose, turning it on and then spinning around. Someone looking at you would see the water apparently forming a spiral around you. Actually, each droplet of water is shooting straight out from you, but as you rotate the stream appears to form a spiral. That is why the erupting gas forms a "comma" shape near the nucleus.

But what about those arcs? Remember, the jet is only "on" when it faces the Sun. So instead of forming a complete spiral, the jet is only on long enough to form those arcs. As the Sun sets on the jet, it shuts down, and then turns back on 5.5 hours later, forming another arc! Note also that near the nucleus, the gas and dust is thick enough to actually cast a shadow on material as well, adding to the unusual effect. Finally, the tail is streaming off the comet head, but appears to be off to one side since the comet's motion around the Sun makes it leave behind that material.

All in all, a very striking effect. If you get a chance to see this comet through a telescope, take it! This is a relatively rare feature in comets and you may never get another chance to see it!

My thanks to Terry Platt for kindly sending me that picture.

Wanna know more about Hale-Bopp? As always, you can simply do a web search on keywords. "Hale Bopp" will list hundreds of sites! Many of these are going to be swamped (which is why I won't list individual sites here) so be patient, or try a different one if you can't connect. If you want to know more about what comets are or what they are made of, always always always try Bill Arnett's The Nine Planets.

©2008 Phil Plait. All Rights Reserved.

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