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Comets

 

Comet 17P / Holmes,   October - December 2007

 

Comet 17P/Holmes is an extremely faint periodic comet that returns every 6.88 years without anyone taking much notice. Its arrival last year gained it world-wide attention, for it exploded on 24 October 2007. A vast sphere of dust and debris was ejected in an ever-growing cloud. Though the comet’s head is only some tens of kilometres across, the cloud rapidly reached the size of Jupiter by November 9 grew larger than the Sun. It has continued to enlarge until it exceeded two million kilometres in diameter.

Before the eruption, the comet could only be seen through large telescopes, but the explosion caused it to brighten a millionfold within 36 hours, making it an obvious naked-eye object. It is possible that there could be a second explosion, as occurred in 1892 and led to its discovery by Edwin Holmes.

Since the explosion was first detected, the comet expanded dramatically, to become the largest object in the solar system. It reached a size in the night sky a little larger than the diameter of the Moon.  How a small comet could produce such an enormous cloud has not yet been explained.

In mid-November 2007, Comet Holmes experienced a ‘disconnection event’ - its faint, beautiful blue ion tail became detached from its head. Comet tails can be disconnected by gusts of solar wind which trigger magnetic storms around the comet similar to the geomagnetic storms which cause aurorae on Earth. Such a storm and disconnection was observed earlier this year in the tail of Comet Encke.

Was it really the largest object in the Solar System? In diameter, yes, but of course the Sun is the most massive object by several orders of magnitude. Some comets produce tails many millions of kilometres long, so they would be longer, but not 'bigger'. Photographs taken from Starfield Observatory, Nambour appear below.

 

This image and those following are all taken with the same equipment and have the same plate scale, except when indicated otherwise. They therefore show how the ejecta cloud surrounding the nucleus has expanded from night to night. The sphere of ejecta surrounding the comet's nucleus is most clearly defined in the direction of the Sun, In the picture above this direction is towards the lower right. The magnitude 11.4 star GSC 3321:602 can be seen shining through the cloud at upper left. The diameter of the expanding cloud had reached 15 arcminutes and was still growing.

In the remaining images, the direction of the Sun is to the right. This image was taken ten nights later, on 13 November. The cloud of dust surrounding the nucleus is much larger - in fact the cloud itself was larger than the Sun and appeared in the sky about the same size as the Full Moon.

 

 

This image was taken three nights later, just after midnight on 17 November. The cloud of dust surrounding the nucleus continues to grow, and the comet is now the largest object in the Solar System.  It appeared to the unaided eye like a faint ghost of the Full Moon. The bright star at lower left is Mirfak, a yellow-white F5 star of magnitude 1.79.

 

 

This image was taken two nights later, on November 19. The coma of Comet Holmes appears to swallow the much more distant star Mirfak. At this stage the comet is fading, and becoming swamped by moonlight from the waxing gibbous Moon.

 

 

This image was taken ten nights later, on November 29. The cloud is still expanding, and has reached a diameter of 46 arcminutes (cf approximately 30 arcminutes for the Full Moon. A newly developing tail can be seen extending from the spherical cloud to the left-hand margin.

 

 

This image was taken at the prime focus of the RCOS reflector, and has a much larger plate scale than the other images above. It shows the interior of the ejecta cloud, which fills the frame and has now become the comet's coma. The nucleus or head is just right of centre, and the beginnings of the tail stream off to the left. Image acquired on December 3.

 

 

 

Comet 2006 P1 (McNaught),   January - February 2007

 

Comet McNaught provided a magnificent display in early 2007, the best since the 1910 apparition of Halley's Comet. At its brightest, the head outshone nearby Venus, and the tail developed great fan-shaped streamers of dust and gas that spread over a large span of the night sky. The following images were taken on January 20 and 21, when there was a break in persistent overcast weather.  Thanks to Nambour Plaza's Digital Dog for careful processing.

 

Comet McNaught faintly appears out of the twilight shortly after sunset. Photographed from the Maleny-Conondale Road on January 20.

 

 

As twilight fades, Comet McNaught becomes easily seen.

 

 

The comet becomes clearly visible as darkness falls.

 

 

A cloud obscures part of the comet's tail.

 

 

The great tail does not become visible until twilight fades. Unfortunately this happens after the comet's head has passed below the horizon. Photographed from Starfield Observatory in Nambour on January 21. The house lights in the foreground are at Image Flat. The short curved lines in the sky are star trails caused by the Earth's rotation.

 

The full extent of the tail is revealed after darkness falls. A faint line of dots crossing the frame is the trail left by the strobe lights of the local rescue helicopter on its flight path to the Nambour Hospital.

 

There are over a dozen synchronic bands or streamers visible in the comet's tail in this photograph. The lights on the skyline are private homes built on Kureelpa Falls Road, on the edge of the Highworth Range escarpment. The Dulong Lookout is at the left margin. The brightest star trail at upper right was made by the first magnitude star Fomalhaut.  The bright star behind the comet's tail (above left centre of photograph) is the star Al Nair in the constellation Grus. Taken from Starfield Observatory with a standard lens which has a field width of 43 degrees.

The glowing tail remains visible hours after the comet's head has set, like a dim glow in the south-western sky. Taken from Starfield Observatory with a wide-angle lens which has a field width of 63 degrees.

 

By February 6 the synchronic bands have merged into a wide, triangular fan tail covering an angle of about 55 degrees. The star just below the comet's coma (the glowing gas and dust surrounding the nucleus) is SAO 247006, magnitude 7.47. The faintest stars on this image are of magnitude 13. None of these stars is visible to the unaided eye. Taken from Starfield Observatory - the field width is 4.5 degrees.

 

Comet 2007 N3 (Lulin),   February - March  2009

 

Comet Lulin was discovered in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers. It moved rapidly from Scorpius to Gemini, the head reaching a magnitude of 5. It had bright ion and dust tails, and an anti-tail. 

 

Comet Lulin at 11:30 pm on February 23, 2009, in Leo.

 

 

Comet Lulin at 11:25 pm on February 28, in Leo. The brightest star is Nu Leonis, magnitude 5.26.

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Transit of Mercury, 9 November 2006

 

Sunrise on November 9 occurred at 4.54 am. Soon after, starting at 5.19 am, Mercury was visible through the telescope as a black dot moving across the Sun’s disc. This transit of Mercury’ lasted until 10.12 am. These transits are quite rare, and the next one will occur on 9 May, 2016. Unfortunately, the next five transits of Mercury will not be visible from Australia, and we will have to wait for 46 years to see the next one from Nambour. The November 9 event occurred on a cloudy morning with some rain, but despite the weather, the following images were captured at Starfield Observatory:

 

  

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The Moon                                        to:  The Stars

 

Mare Imbrium, with the large walled plain Plato (centre left, 100 km diameter) and the 130 km
long Alpine Valley (centre right).


 


Mare Imbrium, bounded by the Apennine Range to the south-east.


 

 

Mare Imbrium, with the large walled plain Archimedes (80 km across) and the two craters
Aristillus (56 km) and Autolycus (38 km) to its north-east.
 


 

The south-western end of the Apennines, with the large impact crater Eratosthenes (61 km across).
Note above Eratosthenes, a crater that has been almost totally overwhelmed by a flood of lava.
Named Wallace, it is a good example of a 'ghost crater'. To its left are solidified ripples, over 100
km long, in the lava flow that makes Mare Imbrium. Such ripples are known as 'wrinkle ridges'.

 

The craters Eratosthenes (top) and 90 km wide Copernicus (left). Between the two are numerous
craterlets gouged out by debris thrown out from the impact when Copernicus was formed.
 


 

At centre is the 6 km wide crater Hyginus. Its northern wall is deformed by a 2 km wide smaller
crater. Passing through Hyginus is a notable valley or rille, rather disjointed in outline, appearing
in parts to be made up of chains of craterlets. To its right is the long Ariadæus Rille. This area of
the Moon shows numerous clefts as well.

 

Above centre is the large walled plain, Ptolemæus, which has a diameter of 145 km.  Its flat floor
is marked by some smaller craterlets. South of Ptolemæus is another crater plain, Alphonsus,
which has a central peak. Four dark markings on the outside edges of the crater's floor are
deposits of dark grey ash from volcanic vents. To the right of these two large craters is a third,
Albategnius. This area is damaged by material blasted across the surface by a cataclysmic
explosion called the Imbrium Event. The damage appears as grooves crossing the image
from north-north-west to south-south-east.

This image adjoins the one above.  Thebit is the deep crater just left of centre, 48 km in diameter.
Above it is the crater Arzachel and below it Purbach and Regiomontanus.  To the west (left) of
Thebit is a remarkable formation called the Straight Wall. This is a cliff or fault 250 metres high
that runs across a lava plain called Mare Nubium (Sea of Clouds) for 100 km.

 

This image adjoins the one above.  At centre is a very large ancient walled plain called Hörbiger.
Over 160 km across, its walled are largely ruined and its floor is dotted with craters, craterlets,
crater chains and hills.


 

At bottom right is the magnificent crater Clavius, 233 km in diameter.  The walls rise in places over
3.6 km above the floor. The slopes at lower right exhibit massive land slips. A remarkable series of
five craters begins on the southern wall (top) and trends in ever-decreasing size towards the north,
 then west  (right).

 

This image adjoins the one above. Just below centre is the famous crater Tycho, 90 km in diameter
with a cluster of peaks at its centre.  At Full Moon, this is the most prominent crater, as it is surrounded
by white rays of ejecta blasted out from the impact centre.  As it is quite crisp in outline and overlaps
other craters, it is obviously one of the most recently-formed lunar features.  The rim and floor are
covered in blocks of rock. South is at the top, west to the right.

 

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The Stars

 

Achernar is the ninth brightest star in the night sky. Its visual magnitude ( mv ) is  0.45, and it is a hot blue-white star of B3 spectral type. The width of the field is 24 arcminutes and the faintest stars are mv 15.

 

Arcturus, an orange K2 giant star, magnitude -0.05.

 

 

Gamma Crucis is a red star at the top of the Southern Cross. It shines at magnitude 1.59, and is a giant star of type M4. The white star seen just to lower right of the main star is a sixth magnitude companion, in orbit around Gamma Crucis.

 

 A typical nebula, where hydrogen gas is condensing into stars. The Great Nebula in Orion, M42, with its smaller companion at left, M43.

 

 

The central section of the Great Nebula in Orion. At the brightest spot is a famous multiple star system, the Trapezium.

 

 

The brightest spot in the centre of the nebula is illuminated by a famous multiple star system, the Trapezium.

 

 

The Trapezium.

 

 

The brightest star in Orion is Rigel, a large white supergiant. It is 500 times brighter than its faint companion Rigel B, which is itself composed of an extremely close pair of Sun-type stars.

 

 

The Trifid Nebula, M20, is a combination of blue reflection nebula, red emission nebula, and dark molecular clouds.

 

 

Sirius (Alpha Canis Majoris) is the brightest star in the night sky. This image was taken in heatwave conditions on January 31, 2017. The image below is at much higher magnification than the image above, and is a shorter exposure to reduce the glare of the primary star.


 



Sirius is a binary star. Sirius A is a white main sequence star, larger and brighter than our Sun. It has a faint white dwarf companion, Sirius B, which is 63000 times fainter than Sirius A. Sirius B is seen at position angle 62º from Sirius A (roughly east-north-east, north is at the top), in the photograph above, as a faint dot. As Sirius A is also known as the Dog Star (as it is the brightest star in the constellation of Canis Major - the Great Dog), Sirius B has been called 'The Pup'.
 

 

The optical double star Mu Scorpii, halfway along the body of Scorpius, is a useful test of keen eyesight, being only 6 arcminutes apart. Though the two components look similar, it is only a chance alignment, not a true binary system. The stars are not related in any way. The upper  star of the two is an eclipsing binary 822 light years distant, while the lower star, a blue-white subgiant, is only 517 light years away.

 

A planetary nebula, the 'Ghost of Jupiter', NGC 3242, formed when the central star exploded.

 

 

A galactic cluster in Scorpius, M7, also known as Ptolemy's Cluster.

 

 

The Ring Nebula, M57, a planetary nebula.

 

 

The star Gamma Crucis, at the top of the Southern Cross.

 

 

 The 'Wishing Well Cluster', NGC 3532.

 

 

The Eagle Nebula, M16, a star-forming area.

 

 

 This nebula has almost entirely contracted to form a cluster of new, hot, blue stars. Small amounts of wispy nebulosity remain around the brighter stars. This is the Pleiades star cluster, M45. Such clusters are called 'open clusters' or 'galactic clusters'.


 

The red supergiant star Betelgeuse (Alpha Orionis)

 

 

 Star clouds in Sagittarius, with the dark Snake Nebula obscuring the stars behind. A satellite trail crosses the image.

 

 

The cluster IC 2602, known as the 'Southern Pleiades'.

 

 

The star Regor.

 

 

The globular cluster Omega Centauri

 

 

The central core of Omega Centauri

 

 

There are over 120 globular clusters like this one, on the outer fringes of our galaxy. They contain hundreds of thousands of old stars. This example is named NGC104, but is popularly known as 47 Tucanae.
 

 

 The centre of our galaxy, the Milky Way, in the constellation Sagittarius.


 


 Proxima Centauri (circled) is an 11th magnitude red dwarf star 4.2 light years away. It is the
nearest star to the Sun. It orbits the bright binary star Alpha Centauri (at left).


 

Close-up of the star field around Proxima Centauri

 

 

The binary star at the centre of the Alpha Centauri triple system. Both stars are solar types.



 

The binary star Albireo (Beta Cygni), which is well known for its colour contrast.



 

The central part of the Eta Carinae nebula, showing dark lanes, molecular clouds, and glowing clouds of fluorescing hydrogen

 

 

The Keyhole, a dark cloud obscuring part of the Eta Carinae Nebula

 

 

The Homunculus, a tiny planetary nebula ejected by the eruptive variable star, Eta Carinae

 


 

 The Cat's Paw Nebula



 

 The star Zeta Scorpii and the open cluster Caldwell 76.



 

The Dumbbell Nebula, M27.

 

 

The Lagoon Nebula, M8, in Sagittarius, adjacent to Scorpius

 

The eastern half of the Lagoon Nebula, M8, showing dark Bok globules where protostars are forming

 

 

The centre of the Lagoon Nebula

 

 

Nebulosity in Scorpius.

 

 

The two bright stars at centre form the sting of the Scorpion's tail. Their names are Shaula and Lesath.
 


Shaula and Lesath are both hot, blue B type stars.

 

This cluster of new, hot B type stars surrounds the star Theta Carinae, and is sometimes called the
 'Southern Pleiades'.


 

 Halley's Comet, photographed as it passed in front of the stars of Scorpius in April, 1986.

 

 

The Great Spiral M33 in Triangulum.

 

 

The Great Galaxy in Andromeda, M31, photographed at Starfield Observatory with an off-the-shelf digital camera on 16 November 2007.

 

NGC 4945, an edge-on spiral galaxy in Centaurus.

 

 

 

The Quasi-Stellar Object 3C-273 is extremely remote.  It lies at a distance of 2440 million light years, over one sixth of the way to the edge of the universe. It is 1000 times further away than the Andromeda Galaxy shown above. 

 

 

NGC 5128 was once believed to be a dusty spiral galaxy in collision with an elliptical galaxy.


 

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