During the past week, Comet Lovejoy has been prominent in
the night sky in the Northern Hemisphere.
In my neck of the woods (latitude 54 degrees north, in western North
America) it has been high in the southern sky in the early evening, a very nice
object for binoculars or a small telescope.
In a properly dark sky it has been reported to have been observable by
the naked eye as well, but that won’t be true in a light polluted city sky.
It has been an easy object to find, as it is just a couple
of binocular fields to the right (west) of the Pleiades, the attractive star
cluster in Taurus. Just find the
Pleiades and go a couple of binocular fields to the right and sweep up and down. You should find it pretty quickly, but that won’t
be possible for much longer, as it will soon begin its long journey back to the
outer solar system.
Below is a NASA picture, though it is obviously a lot different
from what you would see with visual observing.
A long exposure photograph allows a lot more detail to come out. But for my part, there is something magical
about visual observations, knowing that the same photons that left the object
are the ones striking your eyes.
I first spotted it on Saturday night, Jan 17, 2014, through
my 7 X 50 binoculars. My initial
reaction was “there’s a globular cluster that I haven’t seen before”. I quickly realized my mistake, and that the
fuzzy, spread out source of light that I was looking at, was actually the
comet.
That was ok, but it was a far more awesome on the night of
Jan 20, 2014 when I had another look with
binoculars. The night sky was
very dark and crisp, for the city, with excellent seeing. Stars were just “popping out”. I definitely had a sense of a green colour to
the comet and a bright nucleus surrounded by a lighter aura. With just visual observations, I wasn’t able
to see a tail, but photos taken by both professionals and amateur astronomers have shown a long
wispy tail.
Interestingly, Charles Messier came up with his list of 102
Messier objects in order to discriminate comets from other extended sources of
light in the night sky (usually referred to now as deep sky objects). He was interested in finding comets (after
all, they are named after the discoverer), so he wanted to ignore any deep sky
object that didn’t move across the sky in relatively a short time, and
therefore couldn’t be a comet. Thus he
came up with his list of ignorable objects.
Later, it turned out that these other objects were actually more
important than comets. They include
everything from galaxies with billions of stars, to ancient star clusters with
millions of stars (globular clusters), to more recent star clusters with
hundreds or thousands of stars (open clusters), to the remnants of single dead
stars (“planetary” nebulae and supernova remnants).
Comet Lovejoy is thought to have a highly eccentric orbit,
that will take about 8000 years to complete.
So, it will be a long time until it comes back. That’s true of most comets – comets like
Halley’s comet, that orbit in a human life span are very rare. But the fact that comets take thousands of
years to complete an orbit is compensated for by the fact that there are millions
(maybe billions) of them in the far outer solar system, in the Oort Cloud. Every now and then one will get perturbed in
its orbit, and fall in towards the sun to visit the inner solar system. The gravitational perturbations can come from interactions
with objects outside of the solar system, like nearby stars, giant molecular
clouds or the galactic tides (large scale gravitational effects that stretch the
Oort Cloud towards toward the center of the galaxy and compress in the other
axes). Other theories that have been
proposed include an as yet unseen brown dwarf companion star or a large planet
in a distant orbit.
The green colour of the comet is due to diatomic carbon
molecules fluorescing in ultra-violet light from the sun. Often, the tail has a blue colour, due to
fluorescing carbon monoxide ions. The
tail, of course, always points away from the sun, due to its interaction with
the solar wind.
An amusing aspect of Comet Lovejoy is that a Simpson’s
episode once featured a comet hurtling towards the Earth. Springfield’s pastor is, of course, named
Reverend Lovejoy. He and his wife are
in the center-left of the picture from that episode. The Rev isn’t too optimistic about the
future.
Homer:
It's times like this I wish I were a religious man.
Reverend Lovejoy: It's all
over, people! We don't have a prayer!
The dinosaurs probably would
have said the same thing as Reverend Lovejoy, if they could have spoken, 65
million years ago, as a comet (or large asteroid) bore down on them.
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