to: 2006 transit of Mercury to: The Moon to: The Stars
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. I
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 eight nights later, on November 29. The cloud is still expanding, and has reached a diameter of 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.
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:
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.
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.
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.
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 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
Proxima Centauri and surrounding stars.
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 binary star Rigel (Beta Orionis) is a large white supergiant. It is 500 times brighter than its companion Rigel B, which itself composed of an extremely close pair of Sun-type stars.
The emission nebula
surrounding the eruptive variable star Eta Carinae is one of the brightest
of
its type and is visible to the naked eye.
Here are two nebulae
contracting to form star clusters. Hydrogen gas surrounding the newly-
formed
stars is caused to fluoresce by their radiation and glows red. It is called an
emission nebula.
Dust surrounding new stars does not glow, but is simply lit up
by the light of the hot, new stars.
It is called a reflection nebula, and
appears blue.
The galactic cluster known
as M 7 (No. 7 on Charles Messier's list) has formed from contraction of
a
molecular cloud of gas and dust.
The Lagoon Nebula (M8) in Sagittarius, is a place where new stars are being formed out of a nebula. The area contains many dark Bok globules and protostars.
The centre of the Lagoon Nebula.
The Trifid Nebula, M20. Red or pinkish areas are fluorescing hydrogen, while bluish areas are gas that is being illuminated by nearby stars.
The Cat's Paw Nebula in Scorpius.
The Eagle Nebula, M16, is a nursery for star formation. The dark molecular clouds are known as 'elephant trunks'. Those at the centre are also known as the 'Pillars of Creation'.
The 'Saucepan' lies in the
centre of the constellation of Orion. The three bright stars forming the
side of
the Saucepan are properly called 'Orion's Belt' (right of picture). The naked
eye will show
a fuzzy star in the handle of the Saucepan. This fuzzy spot is
actually the brightest part of a vast,
glowing cloud of hydrogen, known as the
Great Nebula in Orion (M 42). Binoculars will reveal its
true nature, as in the
photograph above.
The red supergiant Antares (Alpha Scorpii) is at upper left, with the globular cluster M4 below it.
A close-up of Antares
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.
The young, bright, open cluster Caldwell C76 also known as NGC 6231 is at centre, and the stars Zeta 1 and 2 Scorpii are at right. Zeta 1 is a hot, blue-white B1 star, while Zeta 2 is a cooler orange K4 star.
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.
The Snake Nebula is a dark molecular cloud in front of the star clouds of Sagittarius. A satellite trail crosses the field of view at right.
The planet Jupiter and its family of satellites passes before the background stars of Sagittarius.
This cluster of new, hot B type
stars surrounds the star Theta Carinae, and is sometimes called the
At the
centre of the Dumbbell Nebula (M27) is the explosive star that ejected it.
The Ring
Nebula in Lyra (M57) is a planetary nebula ejected by the unstable star at its
centre.
This example is named NGC104,
but is popularly known as 47 Tucanae.
'Southern
Pleiades'.
The Great Galaxy in Andromeda, M31, photographed at Starfield Observatory with an off-the-shelf digital camera on 16 November 2007.
The Great Spiral M33 in Triangulum.