Nebula and Cluster of the Month Archive 2023
In this series of articles we draw your attention to Nebulae, Clusters and other Galactic objects that are particularly worthly of an observer's time.
Messier 67 in Cancer
February 2023 - Nebula and Cluster of the Month
It has to be admitted that February is a very poor month for nebulae and clusters. The Milky Way has passed its winter zenith, and the faint galaxy fields that precede the richer hunting grounds of Leo dominate the midnight sky in the middle of the month.
Culminating this month is the ancient constellation of Cancer, the Crab. From a naked-eye point of view, it’s a fairly unremarkable area of sky. Dark skies show a wonky ‘K’ shape made of fourth-magnitude stars. A dark sky will also show one of the few deep-sky objects known to the ancients, the open cluster we now call M44.
Charles Messier catalogued another open cluster in this constellation, number 67 in his list of nebulae and star clusters published in the Almanac Connaissance des Temps (‘Knowledge of the Time’), first including M67 in the list of 68 objects published in 1783.
Messier was not the first to see M67, nor the first to publish its presence. That honour goes to Johann Gottfried Koehler (1745-1801), a contemporary of Messier who discovered 20 objects, including M59, M60 and M 67. Although it isn’t clear exactly when he first discovered the object, it must have been between 1772 and 1779. He published his discoveries in the Astronomisches Jahrbuch in 1780. His description reads
A fairly discernible nebula of oblong shape near alpha Cancri.It would seem that Koehler’s telescope wasn’t up to resolving the cluster into stars.
Messier observed the cluster on 6th April 1780, writing
A cluster of small stars with nebulosity below the southern claw of the Crab.Messier’s telescope, then, was slightly (but only slightly) superior to Koehler’s.
William Herschel observed it in 1783. He wrote
A very beautiful and pretty much compressed cluster of stars, easily to be seen by any good telescope and in which I have observed above 200 stars at once in the field of view of my great telescope with a power of 157.
Curiously, in a later note, penned in 1809, he wrote
A cluster of very small stars. There seems to be a faint milky nebulosity among them.
M67 lies at a distance of about (measurements vary) 800 – 900pc (say 2,700 ly). It is well-populated with stars. Again, estimates vary, but Archinal & Hynes give a very precise 324. It covers a diameter of about 25’ on the sky, though the ‘obvious’ members cover only about 12’. The Trumpler classification is given usually as II2r or II3r (detached from the background, little central condensation, moderate (or wide) range in the brightness of the individual stars, rich). Older catalogues occasionally class its richness as medium, instead of rich. These can be safely ignored. The late Kenneth Glyn Jones (a former president of the Webb Society) in Messier’s Nebulae and Star Clusters (1968) writes
M. 67 is known to contain 500 stars between mag. 10 and mag. 16 and a very large number of stars which are fainter still.
M67 lies well above the plane of the galaxy, which has preserved the cluster during a lifetime that would see clusters that live within the disruption of the galactic plane long evaporated. It is not the oldest open cluster known, but it is one of them. It’s the closest old cluster to us and as such has been extensively studied. Many of the stars within it are solar mass stars about the same age as the Sun, making it an ideal laboratory for the study of Sun-like stars. Over 100 stars of this type have been discovered in M67. About 15% of those stars are relatively quiescent, showing far less magnetic activity than our Sun, whilst 30% show activity significantly greater than that of the Sun at maximum.
A survey of 20 solar-like stars within the cluster determined the spin rate of each to be approximately 26 days, very comparable with the Sun’s 25.4 days.
Three exoplanets have been found in M67, around different stars, one of which is of solar type. All these planets are hot Jupiters.
Visually, M67 is a very pleasing object. It is easy to find, being just 1.5° due west of α Cancri. With a total integrated magnitude of 6.9, it stands out well in binoculars. In a small telescope the cluster is compressed and spangly, with many stars of around 10th magnitude. Larger apertures will reveal a wealth of fainter stars, down to around 15th magnitude. The overall shape, I find, is reminiscent of a goblet or a fruit bowl. The main part of the bowl is represented by a reverse ‘C’ shape which opens up to the west and in a small telescope appears almost devoid of stars, with the exception of a delicate little triple star. In larger apertures this shape is rather obscured by a number of fainter stars. Chains and curves of stars abound here, making M67 a very satisfying cluster to observe.
Object RA Dec Type Magnitude M 67 08h 51’ 19” +11° 49’ 17” Open cluster 6.9
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Cluster of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.
NGC 2245 and Herschel 1 in Monoceros
January 2023 - Nebula and Cluster of the Month
I would like to start by wishing you a very happy New Year. 2022 was pretty rubbish, and while it might seem optimistic to hope that 2023 will be any better, this is at least the time of year for optimism.
The sky in January continues to be dominated by the bright Winter constellations. Culminating at midnight throughout most of the month, the dim constellation of Monoceros, the Unicorn, will be our focus for this month. If you’re not sure exactly where Monoceros is, it’s to the left of Orion, roughly the area between Sirius and Procyon.
Monoceros, although not visually stunning, is packed with interesting Milky Way objects. We’re going to be looking at two of the smaller objects this month, although technically, our second object is marginally over the border in Canis Minor.
Firstly, a reflection nebula, NGC 2245, situated in the northwest corner of the constellation, close to the borders with Orion to the west and Gemini to the north, 9½° ENE of Betelgeuse. It lies in a field wreathed in faint nebulosity punctuated with small bright sections like itself. It was discovered by William Herschel on 16 January 1784. He described it as
Pretty bright. Much like a star with an electrical brush.He placed it in his catalogue of planetary nebulae as number 3.
Although it must be made absolutely clear that Herschel was the first person to use the term ‘planetary nebula’, and so would be entitled to use the term to describe whatever he wanted, he ascribed no physical properties to it, beyond
stars with burs, with milky chevelure, with short rays, remarkable shapes, etc.It was simply a useful tag for certain objects that looked similar to each other. This idea of a ‘brushed star’ is typical.
The term ‘planetary nebula’ is now defined much more scientifically. Of the 79 objects in Herschel’s class IV – planetary nebulae, only 20 fall within the modern definition.
NGC 2245 is in actuality a reflection nebula. It is small, 5’x4’. At its heart lies a 10.4(var) magnitude star. The star is variable and bears the designation V699 Mon. It is an Orion-type variable of spectral type A or B and varies between magnitudes 10.3 and 10.8.
The region more-or-less defined by the three bright nebulae NGC 2245, NGC 2247 and IC 446 contains the Monoceros R1 association, a region of young stellar objects (YSOs). Supersonic outflows from YSOs collide with the interstellar material and produce shocked excitation zones known as Herbig-Haro (HH) objects. Mon R1 contains at least 30 YSOs, and many HH objects, 20 of which were discovered by a survey conducted with the 1m Schmidt telescope at Byurakan Observatory in 2020.1
I observed NGC 2245 with my trusty 12” (300mm) reflector in December 2015. My observation is reproduced below.
The description reads ‘A small, bright nebula around a tenth-magnitude star, stretching away from it like a short cometary tail.’ In fact, NGC 2245 is a representative of a type of reflection nebula called ‘cometary nebulae’ because of their resemblance to comets.
Our next object is a little-known open cluster bearing the unusual designation of Herschel 1. I first came across this object when scanning the Catalogue of Optically Visible Open Clusters. OVOC 578 is listed with the alternative designation of Herschel 1, but no other.
Initially, I thought that this object must have been discovered by an astronomer (maybe an amateur) whose name just happened to be Herschel. On digging a little deeper, I found that the discoverer was in fact John Herschel (the famous one). He discovered it in 1827 but for some reason, it didn’t make it into his own catalogues. He first alluded to its existence in a very brief mention in a paper published in 1871. Such scant fanfare meant that the object was subsequently never picked up for inclusion in the NGC.
In Archinal & Hynes, it is listed as ADS 6866 cl, which simply means a cluster associated with double star ADS 6866 (also known as Σ1141). This double star (actually a triple) was discovered by Herschel at the same time as he first saw the cluster. Receiving the same neglect as the cluster, it was rediscovered and subsequently catalogued by Struve, hence his credit as its discoverer.
The upshot of all this is that here is a little open cluster that should have been in the NGC but very few people have ever heard of.
I have looked at the proper motions available to me in this area. At least seven stars across an area of about 13’ square have very similar motions, a strong indicator of physical association. At the centre of this is a small group of five brighter stars, four of which (including the bright double star) share a common proper motion. There is a high likelihood, therefore, that this is a real cluster.
Intrigued by this object, I observed it in January 2017 with my 12” reflector. Although I made no drawing, I did make a written observation of it. I found a tight little group of nine or ten stars dominated by a fairly bright double star, which I later found to be Σ1141.
Whilst very few people have heard of this little open cluster, even fewer have actually seen it. Go on, join them!
Object RA Dec Type Magnitude NGC 2245 06h 32’ 41” +10° 09’ 24” Reflection nebula - Herschel 1 07h 47’ 02” +00° 01’ 00” Open cluster Br * = 8.4
- New Herbig-Haro objects and outflows in the Mon R1 association, T. A. Movsessian, T Yu Magakian, S. N. Dodonov. Mon. Not. R. Astron. Soc. 500 (2), 2440-2450 (2021)