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Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

This month ushers in Midwinter for those in the northern hemisphere - and Midsummer for those readers in the Earth's southern climes. The Sun reaches the most southerly part of the ecliptic in the sky on 21st December 2018 at 22.22 GMT.  At this point of the year for readers around the 51 degree N latitude, the Sun will be just 15 degrees above the horizon at the highest point of transit in the south.  The more northerly you are, the less the Sun's altitude at transit point will be.  For those at the Arctic Circle the Sun won't rise at all.  Those above the Arctic Circle will be already experiencing total darkness for days or even weeks surrounding the Winter Solstice. No matter where you are in the northern hemisphere, this day will be the shortest of the year and the night the longest.  This is caused by the Earth's 23.5 degree polar inclination from its orbital plane - the major cause of the seasonal nature of our planet's weather.

 

The Earth at the Winter Solstice.  Image Credit: Kerin Smith

Conversely, those in the southern hemisphere will experience the longest day of the year, the shortest night and the very height of Summer on the 21st December.  Wherever you find yourself at this time of the year, we wish all you the very best, whatever season you're experiencing.  As usual, if you turn your eyes and telescopes skyward, there's lots to see…

 

The Solar System

 

 

The Moon

 

The Moon starts December on the Leo/Virgo borders, at 36% illuminated Waning Crescent phase - rising a little after midnight GMT in the evening on the 1st.  A couple of days later, on the mornings of the 3rd and 4th, The thin Moon can be found near to the ever-impressive Venus (itself near maximum possible brightness) - the two forming a pretty pairing in the sky before dawn. 

 

The Moon, Venus and Mercury, Pre-dawn, 3rd December (from 51 degrees N).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


The Moon reaches New as it joins the Sun in Ophiuchus on the 7th, after which it becomes an evening target. Around this point in the month is the best time for deep sky observation and astrophotography, as extraneous moonlight will be at a minimum.

 

The Moon reaches First Quarter on the 15th, in Aquarius - sitting to the west of Mars by just under 8 degrees. 

 

The Moon becomes Full on the 22nd, in the northern part of the non-zodiacal constellation of Orion. Naturally, this is not the ideal time of the month for deep sky observations and imaging, without the use of narrowband filters. 

 

The Moon comes to Last Quarter phase, while on the Leo/Virgo borders on the 28th December and ends the year as a 30% illuminated Waning Crescent on the Virgo/Libra borders. 

 

 

Mercury

 

Mercury, having reached superior conjunction on November 27th, will be unobservable in the first few days of December, but will reemerge as a morning target in Libra. At the end of the first week of December, the innermost planet presents itself as a +0.3 mag, 8.2 arc second diameter target, some 32% illuminated. Standing 12 1/2 degrees high at sunrise (from latitude 51 degrees N), Mercury won't be too difficult to locate in the dawn sky, especially with the brighter Jupiter providing a useful signpost, located just over 9 degrees to the SE in the sky. 

 

Mercury grows brighter, at the same time shrinking as the month progresses. By the 15th it is -0.4 mag and 6.7 arc seconds diameter and now stands 13 degrees high at sunrise (again, from latitude 51 degrees N). The 15th also coincides with Mercury's maximum western elongation (separated from the Sun by 21 1/4 degrees - after which it begins its trek back sunward. 

 

Mercury at greatest western elongation, December 15th, Sunrise (from 51 degrees N).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


The mornings of the 21st and 22nd finds Mercury in conjunction with Jupiter for the second time this month - the two being separated by around a degree. Jupiter again providing a useful signpost to Mercury's celestial location. Mercury is a respectable -0.4 mag, presenting a 5.9 arc second, 76% illuminated disk. 

 

By the end of 2018, Mercury is still -0.4 mag, but its separation from the Sun has decreased to 16 1/4 degrees and the planet now stands just under 7 degrees high at sunrise. Looking forward to 2019, Mercury will again transit the Sun - though we'll have to wait until November 11th to observe this. 

 

 

 

Venus

 

At the beginning of December, Venus can be found in Virgo, having recently reached a maximum brightness of a dazzling -4.7 mag and is displaying a 40.5 arc second, 26% illuminated disk. It stands just under 26 degrees high in the east at sunrise (from latitude 51 degrees N), separated from the Sun by 39 3/4 degrees. If you are up early, this will be an excellent time to observe Venus, though the planet will appear extremely bright and will benefit (as it often does) the use of neutral density or coloured filtration, when viewed through a telescope. An ND filter will retain the natural colour balance of the planet, whereas coloured filters - particularly the #47 Violet filter - can help discern major cloud features in the planet's atmosphere. 

 

Venus phases and angular size through December 2018. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


Mid-December finds Venus in Libra, having decreased brightness fractionally to a still-impressive -4.6 mag. The planet is now 32.7 arc seconds across and 37% illuminated. It stands 25 1/2 degrees high at sunrise (again, from latitude 51 degrees N). 

 

By the end of the year, Venus stands just under 23 degrees high at sunrise (again, from latitude 51 degrees N). By this point in time, Venus will be -4.5 magnitude in brightness, presenting a 47% illuminated, 26.5 arc second diameter. A resident of Libra, the planet is separated from the Sun by  just under 49 degrees.  The planet will be under a week from maximum western elongation on December 31st.

 

 

 

Mars

 

Late November finds Mars transiting at a little after 6pm (GMT).  Having climbed further north in the ecliptic from where it sat at opposition, the Red Planet now stands 29 2/3 degrees high as it transits (from 51 degrees N). At this point, Mars will be +0.0 mag and displays a 9.3 arc second diameter disk, at 86% phase. 

 

Mars transiting in the early evening, 1st December (from 51 degrees N).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

Mid-December finds Mars a little fainter at +0.2 mag and 8.3 arc seconds across. It will transit in the south at just after 5.44pm (GMT) and will stand a little over 33 1/2 degrees high in altitude when doing so (from 51 degrees N). 

 

At the end of the month, Mars has faded further to +0.5 mag. The planet now displays an 7.4 arc second angular diameter and will stand just over 38 degrees high at transit (from 51 degrees N), which it will reach a little before 5.20pm (GMT). 

 

 

 

Jupiter

 

Having reached superior conjunction in late November, Jupiter is not well placed for observations during much of December. At the very end of the year, Jupiter can be found in the morning sky in Ophiuchus, standing 12 3/4 degrees high at sunrise, shining at -1.8 magnitude - the planet is flanked by Mercury, lower and to the east and Venus, higher in sky and further west.

 

 Jupiter at sunrise, 31st December 2018 (from 51 degrees N).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

Saturn

 

At the beginning of the December, Saturn is found in Sagittarius at +0.5 magnitude and is now 15.2 arc seconds diameter. As reported in last month's sky guide, the window for evening observation of Saturn is definitely coming to a close, as the planet heads towards superior conjunction on January 2nd 2019. The planet will set at just past 5.50pm on the 1st (GMT, from 51 degrees N) - and although technically visible, will be very low in the south for northern hemisphere observation, though those in the equatorial and southern reaches of the Earth will fare better.  

 

Saturn and Moons, early evening 1st December 2018. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

By the end of the month, Saturn will be a couple of days away from superior conjunction, after which it will reemerge as a morning object. 

 

 

Uranus and Neptune

 

The two outer planets are evening objects during December, though Neptune, being located further west in the ecliptic, has a shorter window for observation, setting as it does a little before 11pm (GMT) in the middle of December.  Neptune transits a touch before 5.30pm on the 15th. However, the Neptunian highlight of December occurs a week before this, when Neptune comes into very close conjunction with Mars. In the early evening of the 7th, the two very different worlds are separated by a tiny amount - by about 3 1/2 arc minutes - as the Sun sets at 0 degrees longitude.  Those in Europe and Africa are best situated to observe the conjunction at its closest. It is rare to find a reasonably bright planet so close to one of the outer lying members of the solar system, so if the weather is clear, you’re encouraged to use Mars as a waypoint for finding Neptune, using binoculars or telescopes.  Although the Red Planet is way past its best, it will certainly serve us well in this respect. Neptune will appear as a a tiny blue disk, +7.9 magnitude in brightness and 2.3 arc seconds in diameter.

 

Neptune and Mars in close conjunction, early evening 7th December 2018.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

Uranus is visible for slightly longer during the evenings as it is in a more easterly direction in the ecliptic than its neighbour.  Lurking on the Aries/Pisces borders in December (the planet actually crosses back into Pisces on the 2nd/3rd of the month), Uranus is technically of naked eye visibility from a dark observation site (if you have keen eyesight).  At +5.7 magnitude and 3.7 arc seconds diameter, it is still not a really easy target to locate, but once found, Uranus’ green-grey disk is unmistakably not stellar.  With larger instruments you may be able to pick out the two brighter Uranian satellites, Titania and Oberon - but you’ll need high magnification and a telescope of at least 8-10” of aperture to do so.  

 

Uranus and Neptune relative positions, 15th December 2018.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

Uranus transits in the south at just after 8pm (GMT) during mid-November, setting almost exactly seven hours later (from 51 degrees N).

 

 

Comets

 

This month all eyes should be on periodic comet 46/P Wirtanen as it draws very close to Earth and has the potential to be a low naked eye object during December. 

 

The comet starts the month in the southerly reaches of Cetus, the Whale - at a mere 18 million km from Earth.  On the 1st of December the comet will transit at a little before 10pm from Northern Europe - indeed, its transit time doesn’t change tremendously throughout the month, as its headed in a north-easterly direction in the sky and subsequently the comet’s Right Ascension keeps pace with the the steady shift westward of the background sky in relation to local time.  However, Wirtanen is really shifting in Declination in comparison to the background sky as it hurtles northward in the sky.  On the 1st the comet is found at -18 degrees Dec - a week later it is found at -03 degrees - a change of 15 degrees - a mean of over 2 degrees a day.  

 

By mid-month, this movement northward is even faster, as 46/P zips through Taurus.  This period will probably coincide with the comet’s peak brightness.  Wirtanen passes the Pleiades on the 16th, by around 4 degrees to the E/SE of the famous cluster, making for a terrific photo opportunity.  Unfortunately, the Gibbous-to-Full Moon will be lurking nearby in Pisces, Cetus, Aries, Taurus, Gemini and Cancer, during the first part of the second half of December, spoiling the comet’s mid-December showing somewhat.  

 

46/P Wirtenan's path during December 2018.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

On the 23rd December, the comet will pass close to the very prominent star Capella, Alpha Aurigae, seeping past by about a degree.  This will make the comet very easy to find in binoculars and small telescopes, as the two will appear in the same low power field of view.  For this in mid-northern latitudes, the comet will now be circumpolar and will not set, meaning it will be visible for the entire night - though it transits at just after 11pm.

 

Wirtanen ends 2018 in a rather bare part of the sky in Lynx, having faded somewhat from its best showing in mid-December.  

 

Remember there’s a Pro-Am Wirtanen watch going on, which all are encouraged to take part in.  Details can be found here: http://wirtanen.astro.umd.edu/46P/Meetings_telecons/DPS2016_Wirtanen_workshop.pdf

 

 

Meteors

 

December is home to the Geminid meteor shower, fed by the mysterious "rock comet" asteroid 3200 Phaethon.  The shower is expected to be visible from 6th to the 17th December this year. Reaching their peak on the 14th, when we could reasonably expect Zenithal Hourly Rates in excess of 90-100 meteors (though not all of these will be readily visible from one particular location). The Geminids radiate from an area inside the constellation of Gemini and are usually very well seen from the northern hemisphere.  2018 is a mixed bag in terms of the influence of Moonlight: the early evening of the 14th will be influenced by the Waxing Gibbous Moon at around 44% illumination.  However, the Moon will be set by a little before midnight from mid-northern latitudes, so this is the time the shower will be best seen.  

 

Naturally, the shower presents great opportunities for astrophotographic record - all you need is a solidly mounted camera, capable of timed exposures, with a reasonably wide field lens. Once set up - even in a fairly light polluted environment - you will be unlucky not to capture a couple of brighter meteors, given an hour or so’s multiple exposures.

 A Geminid meteor.  Image credit: Kerin Smith

 


Deep Sky Delights: the Winter Caldwell Objects

 

Taking a break from our usual constellation-by-contellation guide, we take a step back and look at the early evening’s most prominent constellations and some of the less well-known objects in them.  The prominent British Astronomer, Sir Patrick Moore, decided to create his own catalogue of popular “missed” objects on the Messier list.  While Messier and his colleagues were reasonably thorough in their cataloguing of objects that could be mistaken for comets, there were a number of rather surprising omissions from this list.  These are objects that are either too large and prominent to the naked eye to be mistaken for comets, or to fit into the field of view of many of the rather long focal length telescopes (with accompanying limited fields of view) that Messier and his colleague Pierre Mechain used for observation, or possibly smaller and more compact and thus overlooked.  Some of the compact omissions from the Messier list are surprising, given their locations, but we are to remember that many of the telescopes used to discover the main list members were in all likelihood considerably more difficult to use than modern instruments and no-where near their optical quality.  What experienced observers such as Messier and Mechain could have discovered with modern telescopes, we can only speculate.

 

The Winter Caldwell objects (from 51 degrees N).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

The Caldwell list fills in the gaps in the Messier list and can thus be viewed as complimentary, not so much an alternative.  The evening winter skies contain some excellent Caldwell members, many of which a well-seen in binoculars alone - though telescopes will be needed to see some of the more challenging objects.

 

Starting with the most northernly members of those depicted above, the first object of note is Caldwell 25 (NGC2419), the Intergalactic Wanderer, a rather dim globular cluster, which lies some way away from the regular haunt of the globular - the shell around the central part of the Milky Way.  The Intergalactic Wanderer was nicknamed as such, due to the fact it used to be thought as an extra-galactic cluster, wandering through space.  Observations of its motion have revealed it is not - it is indeed a satellite of our galaxy, much as the other major globulars are - in NGC2419's  case just a very outlying one.  At 270,000-300,000 light years from us, it is almost twice as distant as the Large Magellanic Cloud, but intrinsically very luminous.  It almost matches the King of Globular clusters, Omega Centauri, for true brightness, but appears as a rather feeble +10 mag object at just 1.8 arc minutes across, simply because it is so far away.  Larger telescopes will be needed to see much of it, though it is possible to pick out in smaller instruments from dark locations.  Lying in the rather barren constellation of Lynx makes this among the more challenging of the targets to find this month.

 

Caldwell 25, The Intergalactic Wanderer.  Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona, Creative Commons


Moving into neighbouring Auriga, the Charioteer, we come to Caldwell 31, the Flaming Star Nebula, IC405.  This object is a partial emission, partial reflection nebula, meaning that one  part of its structure glows under excitement from radiation, whereas the other part merely reflects light from the stars imbedded in the object.  Measuring around 30 x 19 arc minutes, IC405 is centred around the star AE Aurigae, a star which was ejected from the nearby Orion Nebula under 3 million years ago.  At +10 mag, it is not an intrinsically bright object, but condensed enough to be seen in small telescopes from a decent location.  It is unsure if any of the material that makes up the Flaming Star Nebula was once a part of the Orion Molecular Cloud - it is more likely that it is material that the star is merely passing through.  As previously mentioned, this is an area rife with gas and other star forming material.  IC405 lies some 1500 light years from Earth.

 

Caldwell 31, The Flaming Star Nebula. Image Credit: Mark Blundell

 

Moving into neighbouring Gemini, 2 1/3 degrees East of the star Wasat (Delta Geminorum) is the fabulous Caldwell 39, otherwise known as the Eskimo Nebula, (NGC2392).  This Planetary Nebula supposedly resembles an Eskimo's head, surrounded by the fur of an Arctic Parka hood.  A reasonably compact 0.8 arc minute across (about 2/3rds the size of the Ring Nebula, M57), the Eskimo is only +9.19 mag, though its compact size makes its surface brightness quite high and it takes magnification well.   Discovered by William Herschel in 1787, it is perhaps surprising that it wasn't noticed by earlier observers - though this is most likely down to its small size.  OIII filters reveal more of the two stages of the object: it's tenuous outer shell and the gleaming, brighter interior.  Larger instruments reveal more of the complex structure of the internal part of the Eskimo -  its radial double shell of expanding gasses and fine filaments blown by cosmic winds from its central star.  This central star shines at +10.5 mag and is relatively easy to spot in most instruments.   The nebula is thought to lie at 2800-3000 light years distance.

 

Caldwell 39, the Eskimo Nebula. HST Image - NASA/ESA. Public Domain.

 

The next Caldwell object, in neighbouring Taurus, needs very little introduction: the Hyades star cluster, or Caldwell 41.  So prominent to the naked eye - and so clearly not of cometary origin - its understandable why this outstanding “V-shaped” cluster of stars was left off the Messier list.  The Hyades are in all probability the second-closest star cluster to our Solar System (after the Ursa Major group, containing the Plough, or Big Dipper) and as such have presented astronomers with a great opportunity to study.  The group is so close to us that parallax measurement of distance of individual members of the cluster from one side of Earth’s orbit to the other is possible.  Both HST and Hipparcos measurements have put the cluster at a mean distance of around 153 light years from us and has also confirmed a common motion with the relatively nearby M44, or Praesepe (The Beehive), in Cancer - which suggests a single birthplace for both clusters.

 

Caldwell 41, the Hyades (lower centre), plus the neighbouring Pleiades and Geminid Meteor. Image credit: Kerin Smith

 

The Hyades is estimated to be around 625 million years old, which means it should have dispersed by now.  However, being a resident of an outlying arm of our galaxy, clusters such as the Hyades can avoid being broken up by the massive tidal forces of galactic rotation for longer.  The Hyades’ mass is estimated to be around 400 times that of the Sun, as we observe it at present, though it may have been considerably larger in its earlier history.

 

The Hyades distinctive “V-shape” forms the head of the Bull which Taurus is meant to represent, which can be seen with the naked eye, even from fairly light polluted environments.  Aldebaran, Alpha Tauri, sits towards the apex of the more southerly tip of the ‘V”, it’s distinct red colouring is often seen as Taurus’ gleaming eye.  However, the Red Giant K5 class Aldebaran is not a member of the Hyades and simply sits about half the distance of the Hyades, in a line of sight to the cluster when viewed from our galactic locale. Binoculars will reveal many more members of the Hyades than are visible to the naked eye - quite a few of which are G-type stars like our Sun.  However, the predominant populations are A and K types and quite a few red dwarfs.

 

To the east and slightly to the south of Taurus, through the top of Orion, we come to Monoceros, the Unicorn.  This is the location of Caldwell 46, otherwise known as Hubble's Variable Nebula, or NGC2261.  This nebulosity was first discovered by Sir William Herschel in 1783 - its fan-shaped appearance making him think it to be a comet.  Further observations showed it to be of a fixed position - this was no comet.

 

Caldwell 46, Hubble's Variable Nebula.  Image based on HST data (NASA/ESA, Public Domain), additional processing Judy Schmidt, Creative Commons.

 

The nebula's surface area is reasonably bright, as it is a compact 4x2 arc minute in size and surrounds the star R Monocerotis, which is thought to be a T Tauri variable - unusual "Lithium burning" primitive stars, which have not yet started Hydrogen nuclear fusion in their cores - though the star itself is rarely directly observable.  Although listed as +9.19 mag brightness officially, NGC2261 varies in brightness along with R Monocerotis, sometimes appearing two magnitudes difference over a matter of months.  R Monoceros may be surrounded by a toridal disk of dark material which frequently obscures the star's light.  It is also likely that the star is accompanied by a smaller companion star, which my in turn be influencing the disk's orbit.  The internal structure of NGC2261 has been noted to be changing rapidly, though this is best observed via the medium of astrophotography.  The object holds the distinction of being the first object imaged by the groundbreaking 200-inch Hale Reflector, in 1949, by Edwin Hubble, who had been fascinated by this object throughout his astronomical career, previously studying it at Yerkes Observatory and through the 100-inch reflector at Mount Wilson.

 

Hubble's Variable can be seen easily as a comet-like fan-shaped light patch in an 8-inch telescope.  It is not impossible to observe the nebula with a smaller instrument when using a gentle LPR filter to boost contrast with background sky brightness, though it is suggested an observer keeps their magnification reasonably high to further aid this.  Larger amateur instruments will be needed to much of the internal structure.  NGC2261 is thought to be around 2500 light years distance and occupies about 4-5 cubic light years

 

Four degrees south of Caldwell 46 is the spectacular Rosette Nebula and Cluster system - Caldwells 48 and 50 respectively. The Rosette's central cluster C50 is easily seen in binoculars and small telescopes and is no challenge to larger instruments whatsoever - this was discovered by the first Astronomer Royal, John Flamsteed in the early 1690s.  The nebulosity surrounding the cluster is slightly trickier and while it can be seen with larger binoculars from a very dark site, requires a larger instrument of the 8-inch + class to resolve well.  Variations and the darker lanes in the nebulosity are best seen with more substantial telescopes, using filtration - UHC, OIII and H Beta Filters all help isolate differing areas of the Rosette. This nebulosity will also require a low power widefield eyepiece as the extent of the Rosette Nebula is huge: 80 x 60 arc minutes, over 5 times the area of the Full Moon.  The nebulosity was first noted by 19th century astronomers John Herschel, Albert Marth and Lewis Swift.  The Rosette's 30 light year wide central hole has been created by the solar wind from the stars of the Caldwell 50 cluster, this wind has created compression fronts in the outer nebula, leading to the Rosette's radial petal-like appearance.  Astrophotography will reveal the whole of the Rosette's structure and its deep pink and red colouration.

 

Caldwells 48 and 50, the Rosette Nebula and star cluster.  Image credit: Mark Blundell.

 

Another Caldwell object of note in Monoceros, is Caldwell 54, or NGC2506 - a very reasonable triangle-shaped collection of around 150 observable stars.  At +7.59 mag and being around the 12 arc minutes size, it is a pleasant-enough object to pick out in larger binoculars or a reasonable aperture of telescope.  As a cluster, NGC2506 is very old, being about 1.1 billion years of age, but hasn't as yet dispersed - being. Like the aforementioned Hyades an outlier in our galaxy.

 

Across the border into neighbouring Canis Major, lies yet another Caldwell open cluster, Caldwell 58.  Though not quite as bright or as large as some of its more illustrious and well-known neighbours, the is cluster is an attractive +7.19 mag, 13 arc minute diameter object.  It was discovered by Caroline Herschel, sister of William, who was a very skilled and methodical observer and astronomer in her own right - and a great organiser and cataloguer of her brother's work.  This is significantly thought to be her first independent discovery, though included on her own Deep Sky list as number 2.  There are more than 100 stars of observable magnitude packed into this compact areas, of which the Western section is more populous than the Eastern half.  There are many fine chains and voids within this cluster and it can easily be found in binoculars and smaller telescopes.  It is probable that this cluster would be more conspicuous were it not appearing to merge with the Milky Way to its Southern reaches.  The rich star clouds of our background galaxy appear to somewhat swamp C58 in this area.  This cluster is thought to be around 6100-6200 light years away.

 

Caldwell 58. Image credit: Sloan Digital Sky Survey, Creative Commons.

Not far from C58 is another another excellent cluster, Caldwell 64, otherwise known as NGC2362, or the Tau Canis Majoris Cluster.  This cluster was discovered in the 1650s by the early telescopic astronomer and forerunner of Messier, Giovanni Battista Hodierna, who first catalogued it in 1654, in his book “De systemate orbis cometici, deque admirandis coeli characteribus” (Of the systematics of the world of comets, and on the admirable objects of the sky).  This work covered much the same topic as Messier’s later catalogue and was, as such, principally interested in comets and the objects that could be mistaken for them.  At +4.1 mag Caldwell 64 is a bright object so it is uncertain how the likes of Messier and his collaborators managed to miss it.  Sir William Herschel discovered it independently in 1783.

 

Caldwell 64, the Tau Canis Majoris Cluster - Spitzer Space Telescope Image. Image Credit: NASA/JPL, Public Domain.

 

Caldwell 64 is a compact cluster - barely 5 arc minutes across, though it is reasonably numerous in population.  We can see many of its 60 stars in amateur telescopes, though the most prominent of these by far is the star Tau Canis Majoris, which often gives this cluster it unofficial name.  Tau CM is a very unusual star - a spectroscopic binary with obsoletely enormous components of spectral type O8.  This system is thought to be amongst some of the largest and most luminous Supergiant stars known, with an absolute magnitude of -7.  Lying some 5000 light years away, Caldwell 64 is very luminous as a cluster and is surprisingly bright.   It is thought to be a mere 5 million years of age, so its component stars are very young and vigorous - indeed, Caldwell 64 is amongst the youngest of all star clusters to be observed.

 

The next of the Caldwell objects in this part of the sky can only be observed from the southern parts of Europe and North America and Asia, though observers in the southern hemisphere and northern Africa and the middle East should  be able to observe it reasonably.  This is the rather lovely globular cluster Caldwell 73, or NGC1851, located in the constellation of Columba, the Dove.  At mag 7.3, it isn’t as bright as some of the more famous globulars in the sky, but it is not difficult to find in small telescopes, if it as a target can get enough elevation from the horizon from your particular location.  

 

Caldwell 73, GALEX image.  Image credit, NASA, Public Domain.

At 12 arc minutes across, Caldwell 73 is a shade smaller than M92 in Hercules and while a little fainter, shouldn’t be a difficult target in larger binoculars, given a dark location and enough separation from the horizon.  If you’re based in Northern Europe though, don’t bother looking for it, as it never rises from this part of the world.

 

The last of the Winter Caldwell Catalogue is arguably the most attractive, though sadly one of the hardest to find and observe, due to its southerly location and relatively lowly brightness.  Caldwell 67, otherwise known as NCG1097, is a beautiful barred spiral galaxy, of mag 9.7 brightness.  The galaxy sits in the constellation of Fornax, the Furnace, which is a faint little constellation, tucked under the rather expansive Eridanus, The River, which sits adjacently to the west of Orion.  You can find Caldwell 67 in a smaller telescope, as its core has a reasonable surface brightness, though much larger instruments will be needed to see more of the galaxy’s fantastic structure.

Caldwell 67, NCG1097 by the VLT.  Image Credit, ESO, Creative Commons.

 

As with any target sitting this low in the sky for northern temperate observers, atmospheric extinction will lead to an apparent drop in perceived brightness, so don’t be disappointed if you don’t see a huge amount of this target.

 

Text: Kerin Smith