Step outside on Friday night, January 7th, between 9 and 10 o'clock, and face south. There's Orion the Hunter locked in combat with Taurus the Bull.

The star patterns are unmistakable. Just above them hovers a delicate little dipper--the Seven Sisters, a.k.a. the Pleiades. (Sky Map)

This is how the sky looks almost any evening in January. Except on January 7th there's something extra: the green cloud.

Look to to the right of the Pleiades. (If you live in the southern hemisphere, look to the left.) The tip of your pinky finger, held at arms length is about 1o wide, so 2o is two pinkies.

The cloud resembles a faint and fuzzy star, barely visible to the unaided eye, but easy to see through binoculars.

If you've followed these instructions, you've just found Comet Machholz.

The cloud is the comet's wispy atmosphere or, as an astronomer would say, its "coma." With a diameter greater than 450,000 km, the coma is at least three times wider than Jupiter. Yet the comet itself is tiny.

Comets are, basically, asteroids made of dusty dirty ice and this one is probably no more than a few kilometers wide, a miniscule nugget hidden deep inside its own atmosphere.

Astronomers have been watching Comet Machholz approach Earth since amateur comet-hunter Don Machholz discovered it in August 2004. This week it makes its closest approach to our planet: 52 million km (0.35 AU) away.

That's not very close, which is why Comet Machholz looks like a faint fuzzball and not a jaw-dropping Great Comet.

Still, it is pretty. Try looking through a small telescope. The comet not only has a beautiful green atmosphere, but also two tails.

One tail is the ion tail. It's made of electrically charged atoms and molecules (ions) blown away from the coma by the solar wind. This tail points straight away from the sun.

Gusts of solar wind can cause the ion tail to swing back and forth, to develop curlicues and temporary knots. Amateur astronomers have seen this happen in recent weeks.

The other tail is the dust tail. Comet dust is weightier than gas. It resists solar wind pressure and lingers behind the comet, tracing its orbit. Solar wind gusts have little effect on the dust tail.

Everything you see when you look at Comet Machholz--its giant coma and its long tails-- comes from the icy asteroid-sized "nugget" in the middle. Astronomers call this "the nucleus."

When sunlight hits the nucleus, fragile ices vaporize, spewing jets of the dust and gas into space. These jets feed the coma and provide raw material for the tails.

A frequently-asked question: Why do some comet atmospheres glow green?

Answer: The coma contains cyanogen (CN), a poisonous gas, and diatomic carbon (C2). Both of these substances glow green when illuminated by sunlight. This is called "resonant fluorescence."

The Pleiades, on the other hand, glow blue. Why?

The Pleiades are a clutch of baby stars 400 light-years away. They formed 100 million years ago, during the age of dinosaurs on Earth, from a collapsing cloud of interstellar gas.

The biggest and brightest Pleiades are blue-white and five times wider than the sun. Blue starlight reflecting from wisps of gas threading through the cluster give the ensemble a distinctly blue tint.

A green comet, a blue star cluster, a close encounter: don't miss it!

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