Archive for the ‘Comets’ Category

Capturing a comet

Posted: January 7, 2013 in Comets, Imaging

I’ve been out of touch with astronomy for various reasons in the past few weeks.  But a brief message from a friend alerted me to a comet in the sky.  A quick check on SkyTools revealed the comet to be C/2012 K5 LINEAR – a small object passing relatively close to Earth at the moment.  Although this comet is too faint to be seen without good binoculars or a telescope I thought it would be worth trying to get some images.

The orbit of the comet around the Sun is tilted to more than 90 degrees to the plane of the Earth’s orbit.  This basically means that K5 LINEAR is moving from celestial north to south in the sky.  The following chart shows the rapid motion of the comet during the next week or so.


K5 LINEAR is tracking south through a busy region between the constellations Orion and Taurus.  I set up the 80mm Skywatcher refractor on the HEQ5 Pro mount without much hope of getting a great picture.  It was overcast and raining at one point so me and Malcolm had to throw a cover over the whole setup.  But the cloud was quick moving and eventually I got a clear spell lasting more than half an hour.


This image is a composite of 10 one-minute subexposures taken with the Nikon D80 at prime focus of the Skywatcher telescope.  The processing was carried out in DeepSky Stacker and GIMP.

The green colour of the comet is from jets spewing cyanogen (CN) and diatomic carbon (C2) into space.  Both have a distinctive green glow under the action of sunlight in space.

The sub-exposures actually show the rapid motion of the comet across the field of view.  Here is an animation showing the comet as it tracks across the sky at a rate of 10 arcseconds per minute.

K5 was about 0.3 astronomical units away (about 28 million miles) when the pictures were taken.  That rapid motion corresponds to an orbital speed of about 80,000 miles per hour!  (Since the comet is passing through the plane of our orbit, the radial component is almost zero).

Comet K5 LINEAR is already fading as it heads south and away from the Sun.  But 2013 looks like it will be a spectacular year for other comets!  In late February we’ll have a naked eye comet called PANSTARRS in our sky and towards the end of the year – a more dramatic visitor called ISON might give the moon a good run for it’s money.


Death of a giant comet

Posted: November 12, 2011 in Astrobites, Comets, Meteors

The Northern Taurid meteor shower reaches a peak on November 12th.  For many amateur astronomers the month of November is associated with the Leonid meteor shower – a much more famous shower which has occasional stormy outbursts.  But I have to say, I prefer the Taurids because they have a more interesting backstory!

About 20,000 or 30,000 years ago a huge comet – perhaps 50km in diameter or more – became embroiled in a series of close approaches to the planet Jupiter.  Nothing unusual here – Jupiter has a huge family of comets even today.

For this ancient giant comet, the increased solar heating of its large surface area would have made a spectacular sight in the night sky for our ancestors.  The new orbital period – a few years, rather than decades or centuries – subjected this giant ice-world to intense levels of activity.   The rapid pace of the comet going around the Sun also accelerated its demise and it split into fragments, each of which would have made dramatic comet.  As the centuries rolled by each fragment may have split further or simply had its volatile surface materials boiled away into space – leaving, to all intents and purposes, an object resembling an asteroid.

These days, all we have left of the original giant progenitor comet are a small, faint comet with an orbital period of 3.3 years (called Encke’s Comet), a few small asteroids and a complex series of dust streams which the Earth encounters in November each year.   The dust released by the original comet and its subsequent fragments over periods of thousands of years have gradually been spread out into a broad swathe of the inner solar system.  Actually, the Earth also encounters one of the streams during June but during the daylight hours.  It is speculated that the devastating Tunguska event of 1908 was due to a larger fragment disintegrating in the Earth’s atmosphere.  That’s another story.

On Earth, each November, we see the remains of this giant comet, streaking into our atmosphere as a shower of shooting stars and appearing to come from the constellation Taurus.  It takes the Earth weeks to cross these lanes of dust and in doing so we encounter two distinct peaks – evidence of the complex evolution of the meteor orbits.  So in November we see the Northern Taurids and the Southern Taurids.  Activity is fairly low – typically about seven or eight per hour.  Compare this to, say, the short sharp spike in activity of the Perseids in August or the Geminids in December.

Will we see another giant comet? An intriguing object called Chiron, initially thought to be an asteroid, was discovered in 1977.  It orbits the Sun from within the orbit of Saturn to just beyond Uranus.  Later observations showed it to be developing a coma as it approached its nearest point to the Sun.  Chiron seems to be a giant comet  – perhaps 180km in diameter.  Mathematical modelling showed that the orbit it follows is unstable and chaotic over long periods; in a million years it will probably have been put into a completely different orbit within the solar system or perhaps ejected completely the neighbourhood.  It’s not out of the question that Chiron will one day be a spectacular giant comet gliding regularly through the inner solar system.

For further scientific reading see here.

Along the Great Rift

Posted: October 14, 2011 in Astrobites, Comets, Deepsky

This is a great time of the year to get to know the Milky Way.  Get away from urban street lighting you’ll see this pale band of light formed from the accumulated glow of millions of distant stars.  It flows right overhead and down towards the southwest horizon and it seems to branch into two streams as it winds through the Summer Triangle.

The two streams of stars are divided by the a dark feature called the Great Rift.  A couple of weeks ago Malcolm Robinson took some pictures of the northern section of the Great Rift (commonly called the Cygnus Rift).

Actually, this is the result of stacking 16 images of the same part of the sky; the result is 4 times better than taking one picture!  The computer counted the number of stars in this picture and found more than 20 thousand.  Two stars from the Summer Triangle are visible here.  The bright star near the top-right is Vega, in Lyra and the bright one near the bottom left is Altair in Aquila.

Also visible in this picture are star clusters (tiny concentrations of stars), bright nebulae (reddish coloured patches) and the many dark dust clouds making up the Rift.

I made an annotated version of the image showing the constellations as well.  Surprisingly Malcolm also caught a comet in the picture!

Let’s take a quick tour of some of these objects.

First the Great Rift itself.  It’s made from numerous overlapping clouds of gas and dust between us and the distant haze of stars of the Milky Way.  The clouds are relatively nearby – just a few hundred light years.  Astronomers estimate that the total mass of these dark clouds is about one million times the mass of the Sun.  Because that mass is distributed over huge volumes of the space, the density of the clouds is low – you get a hundred or maybe a thousand tiny dust grains per cubic centimetre.  Sounds like a lot but these dust grains are small: think particles of smoke!

Several star clusters can be seen in Malcolm’s image.  Star clusters come in two kinds: open and globular.  It’s common to find open clusters like NGC6940 and NGC6885 along the Milky Way. They condense out of giant molecular clouds like those making up the Great Rift.  On the other hand, globular clusters, like M71, tend to be found in the halo of the galaxy – above or below the disk – and so away from the Milky Way in the sky.  Globular Clusters are more compact and broadly spherical.  They orbit centre of the galaxy and so they do occasionally pass through the plane.  M71 is perhaps a little unusual.  The stars are more loosely packed and astronomers were not sure whether this was actually a very densely populated open cluster until fairly recently.

Several types of nebula are visible.  One of the biggest planetary nebulae in the sky shows up (just: you can see it more clearly by zooming into the big version of the picture).  I’m referring to M27 – the Dumbbell Nebula.  This is an expanding shell of gas blown outwards when the central star changed from a red giant to a white dwarf.  There are pinkish regions towards the top of the picture; these are glowing clouds of partially ionized hydrogen – heated by the ultraviolet radiation from stars nearby.  Astronomers call these HII regions (spoken as “H2 regions”).  The large expanse of sky covered by the Veil Nebula is just about visible at top-right.  The Veil Nebula is the expanding wreckage of supernova which exploded between five and seven thousand years ago.  It occupies an area of sky 36 times greater than the Full Moon!

Finally, in the foreground and virtually in the back garden, a comet was captured.  It’s Comet Garradd.  Currently a binocular object and tracking westwards through Hercules, the comet will continue to get brighter over the winter and will reach peak brightness in February next year.

All this AND a celestial coathanger…!

Comet Garradd (August)

Posted: August 27, 2011 in Comets
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Now that the moon is gone from the evening sky it’s time to track down an brightening comet!

Comet 2009/P1 Garradd ideally placed for UK astronomers.  At the end of August the comet is moving westwards through the constellation Sagitta (the Arrow) and into Vulpecula (the Fox).  These constellations are are very high in the sky during late evenings at this time of the year.  Here’s a star chart show the motion of the comet over the next week or so.  Note the american style month/day for the date:

For a few days centred on September 3rd, the comet will be right next to the Coathanger cluster (Collinder 399, or Brocchi’s cluster) – that’ll make a nice picture!  Here’s a picture of the cluster taken by Malcolm Robinson:

The comet is currently around 8th magnitude, which puts it within the light grasp of good binoculars from dark sites.  Those of you with telescopes shouldn’t have too many problems seeing the comet provided you can find it.

It doesn’t matter too much if the weather doesn’t cooperate during this week.  Comet Garradd is going to be around for the rest of the year and it’ll be brightest in February 2012, at which time it could be shining at around magnitude 5 or 6 – on the fringe of naked eye visibility.