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July 2019 Sky Chart.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


In early July, we are now past Midsummer in the Northern Hemisphere and very slowly will begin to experience longer nights.  However, this change is gradual and the further north you find yourself, the longer permanent Astronomical Twilight persists.  From the 22nd of May to 19th July, there is a state of permanent Astronomical Twilight for those in Southern UK latitudes (around 51 degrees N), which means that the Sun is less than 18 degrees below the horizon all night long.  This means that the skies are never truly dark and that objects around or below the 6th magnitude are unable to be distinguished with the naked eye.  This obviously has knock-on effects for deep sky observation and astrophotography.  The further north one observes from, the longer this period of permanent Astronomical Twilight persists: in Manchester UK (latitude 53.5 degrees N) this extends from 13th May to 30th July; in Edinburgh (just shy of 56 degrees N) the period is yet longer, from May 3rd to August 8th.  Do bear in mind, if you find yourself in latitudes similar to Reykjavik, Iceland (64 degrees N) Astronomical Twilight persists from 10th April to 31st August - pretty extreme!  Naturally, those readers in the temperate Southern Hemisphere will be experiencing the exact opposite to those in the North.  Those in the equatorial regions of the planet don’t experience Astronomical Twilight extension in these extremes, as the equatorial plane of the ecliptic sets at a much more extreme angle - and during the night, there’s consequently quite a lot more of planet Earth obscuring the Sun.  In Equatorial regions, Astronomical Twilight can last as little as around 70 minutes after the Sun sets.

 

Despite the astronomical twilight conditions in the northern Hemisphere, there’s plenty of observing opportunities during July - so let’s see what’s in store for us in the skies above this coming month.

 

 

The Moon

 

The Moon begins July in Taurus as a Waning Crescent of just under 4% illumination. At barely a day before New, it’s extremely unlikely you’ll catch the very thin crescent of the Moon in the predawn sky, sitting a little to the west of the equally challenging Venus, on the morning of the 1st. The moon will make an altitude of around 7 degrees elevation (as seen from 51 degrees N) by the time the Sun rises, but will be lost in the glare of the morning.  Those in the equatorial regions of the planet might fare better in this respect, as the Moon is still 21 degrees away from the Sun at this point.

 

On the 2nd, the Moon becomes New and this coincides with a Total Solar Eclipse, which is visible from South America - the track of Totality passing over Chile and Argentina, just missing the capital of Buenos Aires. A Partial Eclipse will be visible from much of the Southern Pacific, Brazil, Uruguay, Paraguay, Bolivia and the Falkland Islands and South Atlantic bordering the southern part of Sounth America.

 

 

The Path of Totality and Partial Phase Viewing Area, Solar Eclipse July 2nd 2019.  Image Credit: NASA, Public Domain.

 

The Moon slides between Mercury and Mars in Cancer on the 3rd, though this will be during daylight hours in Europe, so unobservable from here.

 

The Moon reaches First Quarter in the 9th while in Virgo, sitting just above Virgo’s principal star Spica at sunset.

 

The Moon then gently continues its slide south through the ecliptic, until it reaches Full in Sagittarius on the 16th, which coincides with the second Eclipse of the month - this time, a Partial Lunar Eclipse.  Unlike Solar Eclipses, Lunar Eclipses tend to be visible over a huge area.  The range of visibility of some of the event stretches from South America, the Caribbean and Nova Scotia in the western extremes, to New Zealand and the Pacific Islands and Japan in the east.  From the UK and much of Scandinavia, the Eclipse full range of the event will not be visible, as the Moon will rise already just within the Umbral stage.  However, the majority of Europe will see the Moon rise in the Preumbral stage (entering the ring of shadow caused by light diffused through the Earth’s atmosphere). Much of Africa, the Middle East and the western half of the Indian subcontinent will be able to see the event in full.  

 

At the greatest extent a Umbral phase of the Eclipse, about 2/3rds of the Moon’s face will be in the deep shadow of the Earth.  This occurs at 10.32pm (BST) of the evening of the 16th.  The Umbral phase of the Eclipse will be over at just after 12 midnight on the 17th.  The last contact of the Penumbral stage of the event will be finish at 1.18am on the 17th.  This is the last Lunar Eclipse is not until until May 26th 2021, so make the most of this one, if the weather is kind.

 

The Moon Mid-Eclipse, 16th July.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 
The Moon comes to Last Quarter on the Pisces/Cetus borders on 25th July, before ending the month much as it started, on the 31st as an ultra-thin 1.6% illuminated Crescent in Gemini rising a little over a hour before the Sun rises, barely a day before New Moon on the 1st July.  

 

 

Mercury

 

After Mercury’s strong showing in June, July is a little bit of a let-down in comparison.  The Innermost Planet starts the month as a +1.2 mag, 8.5 arc second diameter, 25% illuminated target, standing just under 9 degree high in the west at sunset (from 51 degrees N).   At the beginning of the month Mercury lies 23 degrees to the east of the Sun. Mercury is drawing round the Sun towards us, after June’s greatest eastern elongation and in doing so is growing in size, but diminishing in phase.  Unlike Venus, whose apparent brilliance can stay static, as its angular size increases and phase decreases, Mercury doesn’t share the same mean albedo, size or proximity to us on Earth as Venus does.  This means that regardless of increase in angular size, as Mercury decreases in phase, it gets fainter from our perspective on Earth.

 

By mid-July, Mercury will be invisible to us on Earth as it is only at 4% phase and a metre +3.7 magnitude.  At this point, the planet will have mad it into double figures in terms of angular size, sitting at 11.6 arc seconds across.  By this point, the planet is barely 10 degrees from the Sun and this separation will only decrease until the 21st, when we experience Mercury’s Inferior Conjunction (sitting between us on Earth and the Sun).  At Inferior Conjunction, Mercury will sit 5 degrees to the south of the Sun on the Gemini/Cancer borders, after which it will gradually reappear as an evening object.

 

Mercury at Inferior Conjunction, 21st July.   Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

By July’s close, Mercury will have reached a 14 degree separation from the Sun and while it will remain largely invisible in the glare of the morning sky at +2.2 mag, will be on course for another good show in the morning sky in the first half of August.

 

 Relative positions of the Inner Planets, 21st July.   Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

 

Venus

 

Venus remains close to the Sun (12 degrees separation on the 1st) and difficult from the temperate northern hemisphere, as (despite its brilliance) it will still be very low in the sky before dawn - just over 6 degrees high at sunrise (as seen from 51 degrees N) at the beginning of the month.  The planet is headed towards Superior Conjunction - sitting on the opposite side of the Sun to us - in mid-August.  Consequently, this isn’t the best time to catch the brightest of our planetary neighbours, as Venus’ distance from the Sun will only decrease as the month progresses.

 

 

Mars

 

The Red Planet is another poor target, this time in the evening sky.  Like Venus, it is headed towards Superior Conjunction, albeit at a more sedate pace than our inner neighbour.  At +1.8 mag and only 3.6 arc seconds across, Mars is far from its best and as such, there isn’t a huge amount to recommend searching it out in the evening twilight.  As the month progresses, Mars’ angular separation from the Sun continues to decrease, which will not help any observational attempts.  By the end of the month the planet is lost in the evening glare and will be unobservable until it re-emerges on the morning side of the Sun in the latter part of the year.  It will be October 2020 before it is back at opposition and the lead up to this event will be very gradual.

 

 

Jupiter

 

The King of the Planets is just past its opposition point and is still at maximum brightness of -2.6 mag at the beginning of the month.  At over 45 arc seconds diameter it is a fine-sized target and will reward observations with just about any optical equipment.  Binoculars will show the Galilean satellites lined up around the planet and telescopes will reveal these, plus the northern and southern equatorial belts (at least, larger instruments revealing temperate belts and other cloud patterns), the Great Red Spot and satellite transits and shadow transits too.  While Jupiter is bright, it is not especially well-placed for those of us in the northern hemisphere, so tempering expectations of upper magnification limits will keep the quality of view up and may actually reveal slightly more detail as a result - sometimes less is more when it comes to planetary observations.  

 

Jupiter will stand just over 16 1/2 degrees high when it transits in the south at 11.24pm (BST) on the evening of the 1st (as seen from 51 degrees N).  Having the planet transit earlier and earlier helps meaningful observations being made at more sociable hours.  By the time the month ends, Jupiter will transit at just past 9.15pm and while it will have faded a touch to -2.4 mag and shrunk to 42.7 arc seconds diameter, will still present a magnificent sight in any telescope (seeing conditions permitting).

 

In terms of Jovian evens, there’s a nice simultaneous GRS and Europa Transit and Shadow Transit visible from Europe on the evening of the 2nd and the early morning of the 3rd; a simultaneous GRS and Ganymede Transit on the early evening of the 10th (though Jupiter will still be quite low as seen from Europe for this); another simultaneous Ganymede and GRS transit in the late evening on the 17th, a simultaneous GRS, Io and Io Shadow Transit in the early evening of the 22nd and another on the evening of the 29th.

 

Simultaneous GRS and Europa Transit and Shadow Transit, evening of the 2nd July.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

Saturn

 

Saturn comes to Opposition on the 9th July, so is at maximum brightness and angular size during the month.  Although the ring plane of the planet has now passed its point of maximum opening, the amazing sight of the rings practically wide open is there for anyone with a telescope to see. As an 18.4 arc second diameter, +0.1 mag target on the night of the 9th, Saturn is clearly brighter than any star in its neighbourhood of Sagittarius and as such, should be easy to spot in the low in the SE soon after dark.  Although as with Jupiter, Saturn is clearly still very far south in the ecliptic and thus not optimally-positioned for observers in the temperate northern hemisphere, the planet is always a rewarding target, no matter where you view it from.  However, just as we’ve recommended with observing its neighbour, Jupiter, keeping magnifications more modest will improve the quality of view.  Trying to max out the practical upper range of magnification of your telescope when observing objects lower than 30 degrees in elevation rarely pays much in the way of qualitative dividends.  Seeing as Saturn reaches maximum altitude of a little under 17 degrees high in the sky at transit point (as seen from 51 degrees N), again, less is more in regards to using higher magnifications.  High speed imaging runs and accessories such as Atmospheric Dispersion Correctors can help tighten things up considerably (the latter from both an imaging and visual perspective), but ultimately we have to deal with the vagaries of Earth’s atmosphere.  As a result of Saturn’s lower surface brightness, it is often said it is less negatively impacted by poor seeing conditions and atmospherics.  While there may be a grain of perceptual  “truth” in this, poor seeing and atmospheric extinction will affect any low-altitude target.  When you are looking at an object of Saturn or Jupiter’s altitude from around 51 degrees N, you are looking through 5 times more atmosphere than you are when looking at an object directly overhead.  Heavy red filters can really help in being a cheaper alternative to an ADC for visual use, but will require reasonable aperture to really benefit the observer.

 

Saturn and Inner Moons, Opposition Night 2019.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

By the end of the month, Saturn will transit at a little after 11.30pm (BST) and will only be fractionally smaller and fainter than it was at Opposition.  On the 31st Saturn is +0.2 magnitude, presenting a 18.3 arc second diameter disk.  

 

 

Uranus and Neptune

 

Both the outer planets are seen at their best in the morning sky.  Neptune, being a little further west in the ecliptic in Aquarius is the better positioned of the two, though always much fainter and more of a challenge to find.  Neptune reaches transit point at just before 5 am on the 15th, sitting at +7.8 mag and just 2.3 arc seconds across.  Uranus on the other hand can be found higher up in the northern sector of the ecliptic in Aries, gaining reasonable altitude for temperate northern hemisphere observers before sunrise. However, lack of true Astronomical darkness at this time of yet will make even the brighter Uranus, at +5.8 mag and 3.5 arc seconds across a tricky target to locate.

 

 Uranus and Neptune's relative positions, mid-July.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

 

Pluto

 

The former outlier of the Solar System comes to opposition on July 14th this year.  Pluto at +14.2 mag and just 0.1 of an arc second angular size is one of the more challenging objects to observe that we cover in the Sky Guide.  But technically speaking, at high magnification and given a good observing site, this diminutive world can be seen in as small a telescope as a 5 inch refractor, or 6-inch reflector.  However, many other factors come into play here: observer’s age and eyesight, altitude of target and seeing conditions.  As Pluto is currently a resident of Sagittarius, it is not the greatest target for those of us in the Northern Hemisphere and greater aperture will be needed to make a positive identification.

 

Pluto at Transit, 14th July, Opposition Night (note relative position of Saturn to the west).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

If you want a breathtaking target, Pluto definitely isn’t one - the sheer joy of identification is the ultimate reward in observing it.  Pluto is 4910 million km away from us on opposition night and while there are now stranger additional worlds which have joined it in the primordial darkness of the outer part of our planetary system, Pluto will (for those of us that grew up in the 20th Century) remain a romantic outpost.  For many years the last “planet”, Pluto is now more accurately described as one of the larger Dwarf Planets of our solar family.

 

 

Comets

 

After a little lull in cometary activity, we have the prospect of C/2017 T2 PanSTARRS to whet people’s appetite with.  This comet was discovered back in 2017, by the panSTARRS automated survey.  At time of writing it is at conjunction wit the Sun, but come the end of July will be observable in the constellation of Taurus.  Found between the horns of the Bull, not too far (around 5 degrees eastward) from Taurus’ principal star Aldebaran, Alpha Taurii, on the morning of the 31st, you will need a telescope or powerful binoculars to pick it.  Proximity to the “Winter” Milky Way through Taurus may make the comet a more difficult spot, as will its surface brightness, which will still be quite low.  However, observations made just prior to conjunction suggested the comet’s brightness was rising faster than predicted.  Brightness prediction of comets is a rather dark art and in this sky guide we are always cautious not to ramp up excitement too high.  However, the median of C/2017 T2 PanSTARRS’ predicted brightness puts it around the +2/+3 magnitudes.  The upper range of brightness prediction goes up to -5 to -6 magnitudes - brighter than Venus.  However, more sobering are the lower estimates which put it around +7 at best.  The comet is predicted to reach peak brightness in March to May of 2020, so there is still some way to go before then.  The good news for Northern Hemisphere observation is that the comet will be circumpolar for most of its peak in 2020 - though this will obviously disadvantage this in the Southern Hemisphere somewhat.

 

C/2017 T2 PanSTARRS, predawn, 31st July.   Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

Meteors

 

 August is always a bumper month for meteors, being the peak of the Perseids, but this shower does begin in later July, so it's worth keeping an eye open for some early participants in this year's shower later in the month - particularly as this year’s peak in mid-August will be affected by the Moon’s influence.  However, the peak of the Delta Aquariids on the night of the 28th/29th July is the major shower that peaks during July.

Delta Aquariid over ALMA Array - Image Credit: ESO/C. Malin, Creative Commons

 

This shower normally favours observers in the southern hemisphere somewhat, but can be seen throughout the world.  The Moon is at the end of the lunar monthly cycle and being a thin waning crescent in the morning sky, rising at a little past 2am on the night of the shower's peak, won't interfere tremendously with our potential enjoyment of it.  Best seen after midnight, the Delta Aquariids are relatively slow moving meteors, at a mean of around 41km / 25 miles per second.  This means they are not as energetic and bright as some other showers.  Despite this, the shower is pretty reliable and is actually the more southern of the two Delta Aquariid showers (the northern equivalent is much less active and peaks in mid August).  They are seeded by Comet 96/P Macholtz, a short period comet, which will next come to Perihelion in January 2023.  When observed in 2012, a couple of smaller fragments of the comet appeared to have detached themselves from the main body, which may eventually lead to an uplift in Zenithal Hourly Rates of the meteor shower, which currently stands at around 15-20 meteors per hour.  Naturally, the best way of recording the shower is by the use of multiple widefield images - a DSLR with a widefield lens, or USB imager with an "All Sky" super-widefield lens are both ideal for this.

 

 

Deep Sky Delights in the Deep South 

 

This month we visit the rich area of Sagittarius, the eastern part of Sepens (Serpens Cauda - the tail of the serpent) and the compact but notable constellation of Scutum, the Shield. 

 

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

 

We start in Sagittarius with a chain of globular clusters, which are practically as low as one can observe from UK latitudes - though the further south you find yourself, naturally the better you can see them. Northern temperate observers have to contend with atmospherics in order to make any meaningful observations of these objects - naturally, it almost goes without saying that having a clear southerly horizon is a must! These are difficult objects from the UK and Northern Europe in general, but rewarding to identify. 

 

Messiers 69, 70 and 54 are strung out in a line running between Kaus Australis , Epsilon Sagittarii and Ascella, Zeta Sagittarii, Sagittarius’ first and third brightest stars respectively - the line representing the base of Sagittarius’“Teapot” asterism M69 is the most westerly and is +7.69 mag and 1.7 arc minutes diameter.


It was discovered, along with its neighbour M70 on the same night, August 31st 1780, by Messier. This is within reach of decent binoculars, though begin compact and not especially bright will require a larger telescope to resolve individual stars. The same can be said of M70, to be found 2 1/2 degrees to the east, though it is a little fainter at +7.86 mag and somewhat more compact at 1.4 arc minutes diameter. M54 is found just over three degrees to the NW of M70 and was discovered a little earlier by Messier in 1778. It can be more easily located by working back from Ascella by 1 3/4 degrees to the west. M54 is the brightest of this globular chain at +7.59 and is 1.6 arc minutes in diameter. M54’s appearance is very smooth and uniform and it is very difficult to resolve into individual stars, even in larger instruments. M54 is a bit of a misfit in regards to its neighbours, which are both around 29,000 light years away from Earth. In comparison, M54 is around 86,000 light years away and rapidly receding from us. It was discovered in 1994 that M54 is not a member of the Milky Way’s globular family. Instead it is associated with the Sagittarius Dwarf Galaxy, a satellite attendant galaxy of the Milky Way. M54 lies right in the middle of the Sagittarius Dwarf from our perspective - though the galaxy itself is very difficult to detect. Although +4.50 mag, this galaxy is spread out over a huge area of sky - a colossal 447 x 214 arc minutes! This is the reason it remained undetected for so long. M54 is huge and extremely luminous - running a close second to Omega Centauri in terms of size and brightness. It is practically the only globular one can easily observe and say with certainty that it does not belong to our galaxy - so worthwhile looking out for. 

 

Left to Right: M69, M70, M54. Image credit: Hubble - NASA/ESA

 

9 2/3 degrees to the west of M54 lies another globular, M55. M55 is much brighter than the members of the “chain” at +6.32 mag and considerably larger at 5.7 arc minutes across. Reported by Nicholas Louis de Lacaille to Messier, after the former observed it on his 1752 trip to South Africa, Messier recovered it in 1778. At roughly 2/3rds the diameter of the Full Moon, M54 is big. Subsequently, even in binoculars, M54 appears granular and it is very easy to resolve its individual stars in a telescope. At 17-18,000 light years distance it is one of our nearest globular neighbours and a rewarding sight - if you can find it from your particular location. From 51 degrees N, this globular stands a maximum of 7 3/4 degrees high of the horizon at point of transit, so, like all the aforementioned globulars is a challenge to observe. 

 

Left to Right: M55, M75, M22. Image credit: ESO, Hubble - NASA, Public Domain.

 

11 degrees to the E of M55, lies another Messier globular, M75. Much more compact and fainter then its neighbour, M75 is +8.52 mag and 0.9 arc minutes across and is to be found around 68,000 light years away - on the opposing side of the Milky Way’s core to our part of the galaxy. Although fainter than many globulars, M75’s core is condensed and while you won’t be able to resolve individual stars in binoculars, telescopically at high power, it does reveal granulation. M75 was discovered in August 1780 by Messier’s collaborator, Pierre Mechain and confirmed by Messier a little while afterwards in the same year. Sir William Herschel found it in 1784 and was moved to describe it as a “miniature of M3 [the prominent globular in Canes Venatici] “. Although nowhere near as spectacular as the lovely M3, who are we to disagree with Herschel? 

 

Moving back westwards from M75, past Nunki, Simga Sagittarii, the second brightest star in the constellation (marking the top of the handle of the “Teapot”), we come the jewel of the Sagittarian globulars, the lovely M22. At +5.09 mag, this cluster outshines all the others in its class, bar Omega Centauri and 47 Tucanae. Lying on the plane of the Milky Way means this cluster is probably not as well-de ned and noticeable in its particular location as it would be were it in another, darker part of the sky. However, an observer can still make out M22 from a dark location with the naked eye. Through a telescope or binoculars it is stunning - an elliptical blizzard of stars, easily resolved in all types of optics, though it is true that its core is not particularly well-condensed. At 6.7 arc minutes across, M22 is larger than most globulars, including 47 Tucanae. Only the massive Omega Centauri, at 10 arc minutes across is appreciably bigger. 

 

M22 may have been recorded by Hevelius, but its discovery is normally credited to the 17th century German Astronomer Abraham Ihle, who first reported it in 1665. Halley included it as part of his 6 nebulous objects of 1715. Messier found and cataloged M22 on June 5th 1764. 

The reason for M22’s comparative brightness has nothing to do with its physical dimensions - at 97 light years diameter and 210,000 solar masses, it is quite average. M22 is so bright and large because it is close to us as globulars go - around 10,000 light years from Earth. 

 

2 1/2 degrees from M22 to the SW is the star Kaus Borealis, Lambda Sagittarii. This star marks the tip of the “Teapot’s” lid and also provides a useful star hopping point for the next globular Sagittarius has on offer - M28. This globular can be found a little under a degree to the west of Kaus Borealis. M28 is a little less bright and large than its neighbour, but is a lovely object in its own right. At +6.78 and just under 4 arc minutes diameter, M28 lurks on the very limit of human naked eye resolution. By all means attempt to find it without binoculars or telescope, but you will need a very, very dark location and good night adaption in order to make the attempt. However, in binoculars and telescopes, M28 really delivers. More compact and condensed than M22, M28 has a distinct core, surrounded by a halo of looser granular stars. Binoculars will pick up this granularity, but won’t resolve individual stars - a larger telescope (probably 8-inches +) will. 

 

M28 was discovered by messier at some point in July 1764, a month after its neighbour M22. It is now known to lie some 18,000 light years away from us and be around 60 light years in diameter. Again, like M22, M28 is a cluster well worth seeking out. 

 

M28. Image credit: Hubble - NASA/ESA, Public Domain.

 

We now take a break from the delights of globular clusters for a little while to explore one of the best parts of the sky for nebulae - the heart of the Sagittarius Milky Way. 

Moving westwards from M28, by 4 3/4 degrees, we arrive at the fabulous Lagoon Nebula, M8. At 4300 Light Years distance, the Lagoon appears as a titanic object n our skies. It is a degree and a half in length and over half a degree wide – roughly three full Moon’s width by a Moon’s width – comparable in area to the Orion Nebula M42/M43 complex, though not quite as bright. Still at +6 mag it is an easy object in large binoculars and small telescopes, though at a maximum of 14 1⁄2 degrees above the horizon at its highest for the UK, it can be a tricky object for those without a clear southern horizon. The Lagoon is so prominent, it was first cataloged by the telescopic observer Giovanni Battista Hodierna in or slightly before 1654. It was also noted by English Astronomer Royal John Flamstead around 1680 and French Astronomers de Cheseaux and Le Gentil in 1747 and 1748 respectively. Messier cataloged the Lagoon in 1764, noting both the cluster that lies within the nebula and the nebulosity. 

 

The Lagoon is home to numerous young stars and the Hourglass section of its interior is actively observed to be in the process of stellar formation. It is these stars that cause the nebula to glow its distinctive pink colour, which make the Lagoon another very attractive target for astrophotographers. 

 

The Trifid Nebula and The Lagoon Nebula. Image Credit: Ljubinko Jovanovic, Creative Commons.

 

1 1⁄2 degrees north of the Lagoon lies the magnificent Trifid Nebula, or M20. This is one of the best deep sky objects in the sky to observe and can be easily found in binoculars and telescopes. At +6.30 mag and half a degree across, the Trifid is an impressive sight. Progressively larger instruments will show the dark lanes that trisect this object and UHC/Ultrablock lters will also help isolate the lanes and enhance the brighter HII regions. It was the trisecting pattern of dark material that gave rise to the Trifid’s popular name. John Herschel was the first to describe it as such and the name stuck, though it was first discovered by the French observer Le Gentil in 1750 and later cataloged by Charles Messier, if he rediscovered it on June 5th 1764. Located around 5000 Light Years from us, the Trifid is the stellar nursery for a number of stars which also illuminate the bright blue reflection nebula to the North of the object’s edge. The beautiful range of colours in this target and the starkness of the dark lanes gives M20 an amazing three-dimensionality and makes it 

a perennial subject for astrophotography. As M20 and M8 lie so close together in the sky, they make for a fantastic pairing in wider field images. It is thought that the Trifid and the Lagoon are both constituent parts of a much larger molecular cloud (much as the separate components of the Orion Nebula are), though the Trifid lies a little further from us and is potentially somewhat younger - current estimates put it at around 300,000 years old, which would make it around 10 light years across. 

 

2/3 of a degree to the NE of the Trifid, sits the open cluster M21. At +5.90 mag and 14 arc minutes across, M21 is fairly prominent and can normally be found in the same binocular eld as its neighbour. Containing upwards of 50 stars, this cluster is thought to lie around 4000 light years away - somewhat closer than its neighbour and due to the spectral signature of its stars is assumed to be around 4-5 million years old. 

 

Just under 4 degrees to the NW of M21 sits yet another Messier object - the lovely open cluster M23. A little brighter than M21, M23 is +5.5 mag and is twice the diameter at 29 arc minutes wide and a glorious sight in telescopes and binoculars. This cluster is practically the same width in the sky as the Full Moon and its brightest members form a fan shape in its central region. M23 lies around 2000 light years from our solar system and is thought to be around 20 light years in diameter. It is a little older than its neighbour as spectral data reveals the oldest of its stars to be around 300 million years of age. 

 

Drifting eastwards, about equidistant from M23 on the other side of the +3.8 mag star Polis, Mu Sagittarii, we come to yet another of Sagittarius’ fine clusters, M25. Discovered by de Cheseaux in 1746, M25 was independently rediscovered by Messier in 1764. It is bright at +4.59 mag and an easy target for those with binoculars and small telescopes. At 29 arc minutes diameter, it is the same dimensions in the sky as M23, though a little more concentrated in brightness. There are under 40 easily observable stars in M25, though there are many more - up to 600 - in the cluster as a whole. Some of the brighter members of the cluster form a star chain that appears to be akin to the letter W on its side - or maybe more pertinently, the Sigma sign. This can be seen easily through telescopes at moderate power. As M25 contains G class giant stars, this suggests that the cluster is around the 90 million year old mark and the cluster is thought to lie similar distance from us as M23 - around 2000 light years. 

Crossing back westwards from M25, back in the direction of Polis, we come to another Messier target - M24. This object is often known as the Sagittarius Star Cloud, as it represents one of the brightest parts of the Milky Way in this any of the sky. Describing M24 as “a large nebula, containing many stars” Messier listed M24 with dimensions of 1.5 degrees across. Although a fainter cluster, NGC6603 is contained within these boundaries, it is clear from Messier’s description that this is not what he was cataloging. Easily seen in binoculars and wide eld telescopes, M24 represents the truncated end of the Sagittarius-Carina Arm of our galaxy - the arm adjacent to the Orion-Cygnus Spur which our solar system sits in. A gap in the surrounding dust clouds frame this area and this void allows M24 to appear particularly bright from our location - though this is simply a line of sight effect. Binoculars reveal a huge number of stars within this area - over 1000 visible in such a small area. Although strictly speaking not a nebula or a star cluster, M24 is a very interesting area of sky to examine and is well worth tracking down. 

 

Found 1 1/3 degrees north of the Sagittarius Star Cloud is M18 - though at +7 mag and loose conformation, it is one of the less exciting of the Messier list in this part of the sky. This open cluster contains around 30 visible members spread over a 5 arc minute eld and is thought to be around 4-5000 light years away. A comparatively young cluster at around 30 million years of age, M18 is about 17 light years in diameter. Long duration astrophotography reveals faint nebulosity surrounding this cluster - whether this is the remnants of the nebula the cluster formed from or material it is encountering in its way around the galaxy is still the matter for debate. 

Lying 1 1/4 degrees to the N of M18 is the final object of note we shall be covering in Sagittarius - and what a way to end. The Omega Nebula, otherwise known as the Swan, Lobster or Horseshoe (take your pick), or more properly, M17, is a bright nebula of +6 magnitude and a healthy 46 x 37 arc minutes in size. This object is capable of being resolved by the naked eye under ideal conditions (rarely from the UK due to atmospherics), but is easily picked up in binoculars and marvellous in telescopes of all sizes. Discovered by de Cheseaux in early 1746, Messier discovered it independently in 1764. 

 

The Omega Nebula. Image Credit: ESO 

 

While not as extensive as the Orion Nebula, M42, M17 has a brightly condensed core and as such is arguably the second most prominent emission nebula in the sky. A telescope will reveal the looped structure of the gas clouds against which are silhouetted dark clouds of material, which causes the distinctly swan like shape. The looped area of the “neck” of the Swan is what gave rise to the Omega and Horseshoe nicknames - as this section does resemble the Greek letter, or indeed the shoe of a horse. The Lobster nickname comes from the tail-like section of the nebula - the opposing end to the swan’s neck - and the red-pink colour of the nebula revealed in long duration astrophotography. The glowing gas clouds of this nebula are powered by newly-formed stars hiding in its interior. These massive stars can’t be seen optically, but studies of the nebula at other wavelengths have revealed their presence. These stars are big and extremely luminous - it is estimated they are anything up to 30 times the mass of the Sun and 6 times hotter. It is estimated there is enough material left in the Omega Nebula to form up to 800 stars the mass of the Sun - a much higher number than that the Orion Nebula is capable of producing.

M17 is thought to lie around 5-6000 light years from us. 

 

Leaving Sagittarius, we briefly cross over its northern border into the constellation of Serpens Cauda - the tail of Serpens. Just under 2 1/2 degrees to the north of M17 sits a magnificent 35 x 28 arc minute target: this object is the +6.40 mag star cluster and nebula, M16 – otherwise known as the Eagle Nebula. Made famous by the famous “Pillars of Creation” Hubble Space Telescope picture, this object is well seen in all kinds of telescope, but the larger the instrument, naturally, the more you can see of it! The star cluster formed from the surrounding nebulosity, which can be glimpsed in a sub-6-inch telescope. An instrument of the class of a 12-inch+ Dobsonian will be needed to see the “Pillars” and OIII or UHC-type filter will help considerably with this. Photographically, the Eagle Nebula is a fantastic subject. Amateur CCD images of the nebula may lack the resolution of the Hubble image, but can reveal a surprising amount of equivalent detail. 

 

The Eagle Nebula. Image Credit: ESO, Creative Commons.

 

The Eagle was discovered by de Cheseaux in 1745 or 46 - though he simply listed the star cluster as the point of focus. Messier, independently recovering it, nearly 20 years later in 1764, not only mentions the star cluster, but also the impression that the stars within it we “enmeshed in a faint glow” - a clear sign that nebulosity was evident to him in his observations. Certainly the nebulous regions of M16 start to be visible in a telescope of around 8-inches of aperture, but as previously mentioned, 12-inches of aperture will be needed to start making out structure within the nebula itself. 

 

Modern astrometry puts the Eagle at about 7000 light years from our neck of the cosmic woods - similar in distance to the aforementioned Omega Nebula. Some theorists postulate that the two objects may be linked by the same molecular cloud and form two parts of a constituent whole. Certainly, there can be little doubt that they both lie in the same part of our galaxy - the Sagittarius-Carina spiral arm, but are they more closely related? 

 

The age of the stars in the cluster seem to suggest that the M16’s stellar population itself is around 5.5 million years old. Some astronomers have pointed out that while the “Pillars of Creation” area of the Eagle Nebula is prominent from our perspective today, that stellar compression by cosmic wind and the sheer luminance of the newly formed stars has probably already eroded these completely - in 7000 years-or-so, we’ll find out if this is actually true! 

 

Moving NE of the Omega Nebula complex, we come to the diminutive constellation of Scutum, The Shield. Scutum contains two objects of note, both open star clusters, the fainter M26 and the magnificent M11, or Wild Duck Cluster. M26 is 9 degrees NE of the Omega Nebula and at +8.00 mag and 7 arc minutes in diameter is not the brightest nor largest cluster in the area. This is largely thought to be the result of interstellar matter obscuring part of the cluster - a reasonable common occurrence for objects located on or near the plane of our galaxy. If this material was not present it is likely M26 would appear much bigger and brighter than it does to us. Binoculars will pick it out and small telescopes will show its 30-or-so members well. Messier found M26 on the night of 20th June 1764 and reportedly was rather underwhelmed by its appearance - “not distinguished in a 3 1.2 foot [focal length] telescope and needed a better instrument”, he wrote in his description. 

 

M26 is thought to lie around 5000 light years away. 

 

M26’s neighbour, M11 is to be found just under 3.5 degrees to the NE. Whereas M26 is rather diminutive, the Wild Duck Cluster, as it is commonly known, is a lovely, rich object of +5.80 mag and 32 arc minutes across. The major part of the cluster takes up an area roughly a third of the diameter of the Moon, making it a prominent feature in this area of sky. M11 was discovered in 1681 by German Astronomer Gottfried Kirsch and included as an original Messier object in 1764. It was the noted observer Admiral Smyth who first suggested the “Wild Duck” moniker - describing the fan shaped structure as resembling “a flight of wild ducks”. If examined in a telescope or larger binoculars, the “V” shape of the cluster seems to point in an Easterly direction, though it is not particularly well defined. M11 is supposed to be about 250 million years old and thought to be around 6000 light years distance. Its total of stars is thought to number just shy of 3000, though only 500 of which will be visible to amateur telescopes. It is not an object that should be missed in any type of instrument. 

 

The Wild Duck Cluster. Image Credit: ESO

  

Text: Kerin Smith