A cloud of gas bigger than the planet Jupiter, glowing alien green, is
about to sweep past the Seven Sisters in Taurus. Got binoculars? You can
watch it happen.

Step outside on Friday night, Jan. 7^th^, 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.

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

Look 3deg to the right of the Pleiades. (The tip of your pinky finger,
held at arms length is about one degree wide, so three degrees is three
pinkies.) The cloud resembles a faint and fuzzy star, barely visible to
the unaided eye, but easy to see through binoculars.

If you 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. 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 the gassy coma.

Astronomers have been watching Comet Machholz approach Earth since
amateur comet-hunter Don Machholz discovered it in Sept. 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,
thin and filamentary, 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 simply 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 comet atmospheres glow green?

Answer: The coma contains cyanogen (CN), a poisonous gas, and diatomic
carbon (C2). Both of these substances, in gaseous form, 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!