Nebula and Cluster of the Month Archive 2024

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 5 in Serpens Caput

  May 2024 - Nebula and Cluster of the Month

  May is another very poor month for nebulae and clusters. Galaxies still reign supreme in the target lists of northern hemisphere deep-sky observers.

  This month we’re going to have a quick look at one of the few objects mentioned in these articles not discovered by William Herschel. This object was discovered in 1702 by Gottfried Kirch (1639—1710). After a varied and not always successful series of career choices, Kirch became director of the Berlin Observatory. His other discoveries include Messier 11 (an open cluster in Scutum) and the Mira-type variable star, χ Cygni. He has a crater named after him on the Moon and asteroid 6841 Gottfriedkirch is named in his honour.

  Charles Messier observed this globular cluster, which was to be catalogued by him as Messier 5, on 23 May 1764 and described it as A fine nebula which I am sure contains no star. Round; seen well in a good sky in a telescope of 1-foot (focal length). Superior telescopes in later years corrected Messier’s impression that the object contained no stars. In May 1791, William Herschel inspected it with his 48” reflector. Although very famous and for many years the largest telescope in the World, the 48” was never very satisfactory. It was very cumbersome to use and the mirror (Herschel made two, only one of which was usable), was less than optimal. Nevertheless, Herschel described seeing about 200 stars involved and stated that ...the middle was so compressed that it is impossible to distinguish its components.

  Our own Reverend T W Webb described it as A beautiful assemblage of minute stars, 11—15 mag. Greatly compressed in the centre.

  

  When it was included in the 1888 New General Catalogue, it was given the designation NGC 5904.

  M5 lies in the constellation of Serpens Caput, in an area devoid of bright naked-eye stars. It is about 7¾° south-west of 2.6 magnitude α Serpentis, and even less usefully, 29° south-east of Arcturus (α Boötis).

  M5 has an apparent diameter of about 20’ (well over half a full moon) and shines at a very respectable magnitude 5.7, making it technically a naked-eye object under good skies. At declination 2°N, however, it never gets more than 37° above the horizon in mid-Britain. It is, however, an easy binocular object.

  Over 100 variable stars have been identified within M5, 97 of which are the familiar ‘cluster variables’, RR Lyrae-type variables. RR Lyr stars are similar to Cepheid variables and also act as ‘standard candles’ for distance measurement. The distance derived from these standard candles is 7,500 parsecs or 24,500 light-years.

  Studies of the stellar population of M5 have provided some unexpected results. It is often assumed that all the stars in a globular cluster are of the same age; that is, they all formed at the same time. It’s not an unreasonable assumption, given the relative lack of potentially star-forming gas in globular clusters. Studies showed that in fact, M5 hosted not one, but two populations of stars. A more recent study¹ has revealed that there are actually three populations. The majority of stars, spread evenly through the cluster, form the first population. The second and third populations are concentrated close to the centre of the cluster and display higher-than-normal abundances of nitrogen.

  How did these later populations of stars form? It seems that they may be formed from the gaseous wreckage left by the supernova explosions of the most massive members of the first generation of stars. The stars in M5 are literally recycling themselves.

  M5 is a glorious sight in a modest amateur telescope. One feature that stands out is that it is noticeably elliptical – an unusual feature in globular clusters. Stars are scattered across the field like salt on a black cloth. The brightest stars in the cluster are around 12th magnitude. As I have noticed in other bright globular clusters, the individual stars at the centre are lost against the brilliant glow.

  If you tire of little fuzzy galaxies this month, give your eyes a treat and seek out this staggeringly beautiful object.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  M 5	15h 18m 34s	+02° 04’ 58”	Globular cluster	5.7

  References:

  1. Formation of Multiple Populations of M5 (NGC 5904), Jae-Woo Lee 2021 ApJL 918 L24.

  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.

• Melotte in Coma Berenices

  April 2024 - Nebula and Cluster of the Month

  This month, we look at the question ‘what does an open cluster look like close-up?’ We have a small collection of nearby star clusters to choose from. The closest is the Ursa Major Moving Group. Arguably no longer a true open cluster, it appears that this stream of stars, all moving in the same direction and at the same speed, were at least once part of an open cluster. At the centre of the group, what would originally have been the core of the cluster, are five of the seven stars of the Plough. The stars at the ends of the asterism, α UMa (Dubhe) and η UMa (Alkaid or Benetnash) are not members, and can be measured as moving in entirely different directions.

  In total, there are 35—40 naked-eye stars (depending on your sky conditions and visual acuity) which are members of the stream, the brightest of which is β Aur at magnitude 1.9. In total, nearly 60 stars are known to be part of the group. The brightest star in the sky, Sirius, was once thought to be a member, but this has now been disproved. The sun lies at the edge of the moving stream but is not itself a member, being many times too old. The stream stretches from Cepheus to Triangulum Australe, so cannot be said to have a great visual impact.

  The closest still-intact open cluster is the Hyades, in Taurus. Lying around 150 light-years from Earth, the Hyades contains about 380 stars, the brightest of which is magnitude 4.0. It covers 5.5° of sky, 11 times the diameter of the full moon. It’s a familiar sight, representing the face of the bull. The first magnitude star α Tau (Aldebaran) is not a member, lying as it does, less than halfway between us and the Hyades.

  Perhaps a better example of what an open cluster looks like close-up is the next closest, Melotte 111. This open cluster lies 100 light-years further away from us than the Hyades, at 260 light-years distant. However, even at this remove, it spans a greater amount of sky, being 7.5° across, giving it a true diameter of close to 35 light-years.

  

  Melotte 111 is of course the asterism that makes up the most prominent part of the constellation of Coma Berenices, and is sometimes referred to as the ‘Coma Star Cluster’. In today’s constellation, it represents the hair of the Greek-Egyptian Queen Berenike II Euergetis, who reigned in her own capacity as queen of Egypt from 258 to 246 BC and as co-ruler with her husband, Ptolemy III Euergetes from 246 to 222 BC. The legend attached to the constellation relates how after the safe return of Ptolemy III from battle, Berenike made an offering of her famed hair (which was amber in colour, apparently) on the altar in the temple of Aphrodite at Cape Zephyrion. This offering subsequently disappeared, most likely stolen. In order to assuage the fury of the king and queen, the court astrologer, a man by the name of Conon convinced the royal couple that the hair had been accepted by the gods and placed in the heavens.

  Before the hair incident (and occasionally for some time afterwards) the open cluster represented the tuft of hair at the end of Leo’s tail or a sheaf of wheat held in the left hand of Virgo. The name ‘Coma Berenices’ was fixed by Tycho Brahe in his catalogue of 1602, and has been in use ever since.

  Current star counts give Melotte 111 some 273 member stars. The brightest, designated 12, 13, 16, 17, 21 and 22 Comae are all visible to the naked eye under good conditions, with the brightest, 12 Com, shining at magnitude 4.8. A curious feature of this open cluster is that it seems to contain no faint stars. There is a surprising cutoff at about magnitude 10.5 (for comparison the Sun at this distance would shine at magnitude 9.2), leading to the conclusion that, most unusually, it contains no red dwarf stars. There are also no giant stars, though the H-R diagram shows some stars just beginning their journey to the giant stage.

  The age of the Coma cluster is calculated at 450 million years, barely a tenth of the age of the Solar System.

  The best option, then, if you want to know what an open cluster looks like close-up, is to step out on an April evening and look up towards Coma Berenices. There, lying midway between α CVn (Cor Caroli) and β Leo (Denebola) lies a twinkling, misty patch. The darker your skies, the better you will see it, of course. See how many individual stars you can discern. If you can see four or five, you’re doing well.

  Then look through a pair of binoculars. It’s a wonderful sight, a northward-pointing arrowhead fills the field of view of standard 10x50s. There it is: a lovely open cluster that is, at least cosmically speaking, right on our doorstep.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  Melotte 111	12h 25m 21s	+25° 59’ 28”	Open cluster	1.8

• NGC 4147 in Coma Berenices

  March 2024 - Nebula and Cluster of the Month

  March is the month when the nighttime side of Earth begins to face away from the Milky Way. Ahead of us are the Spring months, stuffed with distant galaxies. This is good, of course. We all like a nice galaxy.

  It’s not so great for those of us who have to present a cluster or nebula of the month. They are vanishingly rare in the northern hemisphere at this time of year. The southern hemisphere is far better served. Due south of Virgo, and permanently below our southern horizon, lie the glorious Milky Way fields of Crux Australis and Centaurus.

  Here in the north, March offers but four objects relevant to this column, and one of those is a mere asterism (NGC 3231 in Ursa Major).

  Our object for this March is a globular cluster in the constellation of Coma Berenices - more famous for its galaxies. It is somewhat isolated from its fellow globular clusters, being about 70,000 light-years from the galactic centre. The object is NGC 4147 (also catalogued as NGC 4153). It was discovered by William Herschel on the night of 15th February 1784 (and then again, accidentally, on the night of 14th March of that year, accounting for the double entry in the NGC). He thought it to be bright, placing it in his first class (bright objects) as 11H.I and as 19H.I. He anticipated the descriptions of many observers to come by describing it as Very bright, pretty large, gradually brighter towards the middle.

  A century later, the NGC description had barely changed: Globular, very bright, pretty large, round, gradually brighter towards the middle, easily resolvable. From a visual perspective, that about says it all.

  

  A recent study¹ by Sneh Lata of Arybhatta Research Institute of Observational Science in India et al identified 42 variable stars in the cluster, 28 of which were new discoveries. Most were RR Lyrae stars (pulsating variables of spectral type A and F). RR Lyr stars are often called ‘cluster variables’ and are used as standard candles, like delta-Cepheid stars. They are common in globular clusters. The study enabled the team to reassess the distance of the cluster from Earth. Previously believed to be 61,000 light-years, this figure has now been reduced to 56,000 light-years.

  The location and relative remoteness of NGC 4147 suggests that it was once a member of the Sagittarius Dwarf Spheroidal galaxy, which was once a satellite galaxy of the Milky Way before being drawn into our Galaxy and tidally disrupted. Most of its former mass now lies as an elongated stream in Sagittarius. The Sagittarius Dwarf Spheroidal galaxy was believed until quite recently to have contained at least nine globular clusters, the largest and brightest of which is Messier 54. M54 may, indeed, be the actual core of the galaxy. Analysis of Gaia data has, however, revealed at least twenty more.

  For those who like contrasts, NGC 4147 is well placed, not quite equidistantly, between two galaxies, NGCs 4064 (1.4° to the west) and 4293 (2.6° to the east). Whilst these three objects are of comparable magnitude, they are not equally easy to see. I briefly quote here from my own observations:

  NGC 4064 (barred spiral galaxy, mag 10.7) ‘Pretty faint, though... visible to direct vision at x83.’

  NGC 4147 (Class VI globular cluster, mag 10.4) ‘Very bright... brighter middle.’

  NGC 4293 (barred spiral galaxy, mag 10.3) ‘Only very dimly visible to direct vision.’

  This is largely due to a quantity known as ‘surface brightness’. Surface brightness is measured in units of magnitude per square minute or magnitude per square second. I find the former far more useful, though unfortunately, magnitude per square second is widely quoted. Magnitude per square second tends to be down in the 20s, whereas using square minutes seems far more meaningful. For example, the surface brightness of NGC 4064 is 13.3, and of NGC 4293 is 13.2.

  I have noted before that the most important measurement of how visible a globular cluster is, is its concentration class. The classes run from I to XII, in decreasing order of concentration (note that this is incorrectly reversed in the Field Guide to Uranometria).

  NGC 4147 is a class VI globular cluster, placing it right in the middle of the concentration classes. It has a small angular diameter (just about 4’). Taken together, these suggest that NGC 4147 should be fairly easy to see, and so it proves.

  

  My observation of this object was made on a particularly cold night in March 2015 (the object glass of my finder scope was iced over) from my home location in the happy days before the advent of LED street lighting. I recorded a NELM of 5.8 that night. I can barely get to 3.0 these days.

  I followed the path trodden by many previous observers, describing the globular cluster as Pretty bright but not particularly large. Brighter middle. Not resolved but spangly. No resolution even at x375.

  The brightest star in the cluster is of magnitude 14.5, so resolution was a little beyond my equipment and conditions. Can you do better?

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 4147	12h 10m 06s	+18° 32’ 32”	Globular cluster	10.4

  References:

  1. Photometric observations detect 28 new variable stars in NGC 4147 (2019, May 29) retrieved 26 February 2024 from https://phys.org/news/2019-05-photometric-variable-stars-ngc.html

• Messier 44 in Cancer

  February 2024 - Nebula and Cluster of the Month

  February is a month particularly devoid of clusters and nebulae in the northern hemisphere. Apart from the two Messier-listed clusters, there is virtually nothing to be seen. Those of us with access to the southern skies have a much better deal of it this month. If you are in the UK but have exceptionally good skies and a clear southern horizon, you may just be able to glimpse the lovely NGC 2627 in Pyxis, though at a declination of -30°, this will be available to very few in the UK.

  We UK-bound observers therefore have a choice of two open clusters to observe this month. I covered M67 last year, so this year we turn to the second and final object, the wonderful M44, called since ancient times ‘Praesepe’, a strange-sounding Latin word that means ‘manger’ or ‘cot’. The two stars that nestle alongside the cluster, one to the north and one to the south, γ and δ Cancri, are called Asellus Borealis and Asellus Australis, respectively. The word ‘Asellus’ is Latin for donkey or ass. Apparently, these two stars and the cluster (which remember can only have looked nebulous to pre-telescopic eyes) represent the two donkeys and the manger from where they came, on which Dionysus and Silenus (both presumably terribly drunk at the time) rode into battle against the Titans in Greek legend. The braying of the terrified donkeys so scared the Titans that they scattered and were easily defeated by the gods (at least according to Eratosthenes). Make of that what you will.

  

  The object is listed by Hipparchus in his star catalogue of 130BC as a ‘cloudy star’. Other prominent pre-telescopic observers who catalogued it were the Greek Ptolemy, the Persian Ulugh Beg, the Danish Tycho Brahe and the Polish Johannes Hevelius.

  Galileo turned his little telescope to it in 1609. He was the first to see its true nature as a cluster of stars, writing ‘The nebula called Praesepe contains not one star only but a mass of more than 40 small stars. We have noted 36 beside the Aselli.’ This demonstrates that the small telescope constructed by Galileo could reach down at least to magnitude 9.5.

  Charles Messier published his first catalogue in 1771. It contained 45 objects, the final two of which seem to have been added (some may say cynically) to make up the numbers to a multiple of five. Messiers 44 and 45 had both been known since antiquity and there really was no need to include two such obvious objects that no comet-hunter worth their salt would ever confuse for a comet.

  On to the cluster itself, then. M44 is nestled between and slightly to the west of γ and δ Cancri, magnitudes 4.7 and 3.9, the aforementioned ‘Aselli’. These two stars in turn lie at the centre of the wonky-K shape of Cancer. Even if it wasn’t so bright, M44 would still be easy to find.

  M44 shines at magnitude 3.1, making it a naked-eye object from all but the worst light-polluted sites. Diameter estimates vary from 70’ to 95’, so this object will only fit in the lowest-power fields. Rich-field telescopes show it best, but it looks fabulous in binoculars as well, especially if they are mounted.

  The number of member stars varies considerably from source to source. Uranometria states a mere 50, Archinal & Hynes stretch to 161, whereas Wikipedia plumps for a far more impressive 1,000.

  The Trumpler classification is II3m (detached with little central condensation, wide magnitude range of member stars, moderate richness (50 – 100 stars)).

  The brightest star in the cluster is magnitude 6.3.

  The distance has recently been refined by the Hipparcos satellite to 182 parsecs, or about 590 light-years, making this one of the closest open clusters to us. Interestingly, another nearby open cluster, the Hyades in Taurus, shares the same proper motion and age profile as M44, though the Hyades might be slightly older (625My as opposed to 600My for M44). This suggests to some that the two clusters have a common origin.

  Visually, M44 is one of the more impressive open clusters, although as mentioned, it does need a rich-field or very low power, as it’s so big.

  The first impression I get is always that the centre of the cluster is defined by a keystone-shaped asterism, very much like a smaller version of the Hercules keystone. In fact, the whole cluster looks a little like a smaller version of Hercules. The brightest stars, of sixth and seventh magnitude, are brilliant, but as the Trumpler classification suggests, there are stars of all brightnesses. The brightest stars are of spectral type A, F and G, giving the cluster a little colour. Doubles and multiples abound, making this object a delight to spend a little time studying. The higher power eschewed for the overall view can now be employed to investigate the many doubles and multiples.

  Richard Hinkley Allen in ‘Star Names and Their Meanings’ reports that the Chinese name for M44 translates as ‘The Exhalation of Piled-Up Corpses’. Which is nice.

  M44 sits in front of a rich stream of galaxies, though the brightest of these, NGC 2624, is just magnitude 14.9. All the others are below 15th magnitude. If you have good skies and a large telescope, there is much here to investigate, though of course, that’s beyond the scope of this article.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  M 44 (NGC 2632)	08h 40m 22s	+19° 40’ 17”	Open cluster	3.1

• NGC 2281 in Auriga

  January 2024 - Nebula and Cluster of the Month

  Let me start by wishing you all a very happy new year. Let’s hope for dark, clear skies and a complete ban on unnecessary outside lighting. You may say I’m a dreamer, but I’m not the only one.

  As the new year opens, we’re going to look at a modest but pleasing little open cluster in Auriga. It culminates at midnight on the 5th January, when it is at an altitude of 77° from mid-Britain.

  Like so many of the objects featured in this column, it was discovered by William Herschel. Herschel discovered over 2,500 deep-sky objects, and naturally, these tend to be the brightest and most easily distinguishable. Hence so many of the objects we look at are his discoveries.

  He first saw this cluster on 4th March 1788. At that time, Herschel described it as A cluster of coarsely scattered, pretty large stars. Pretty rich. He placed it in his class VIII, ‘Coarsely scattered clusters of stars’ as no. 71.

  

  A study¹ was made of the cluster during 2020, during which the distance from Earth was calculated as 500—560 parsecs (1630—1830 light-years) and its age as 630 million years.

  The study also identified an eclipsing binary star, GSC 2945-01857. This (generally) 12.3 magnitude star lies 17’ from the centre of the cluster, and the authors of the paper admit that the probability of this star actually being a member of the cluster is only 18%, statistically speaking.

  NGC 2281 lies in a patch of sky rather barren of bright stars. It forms an isosceles triangle with and to the east of β and θ Aurigae, magnitudes 1.9 and 2.6 respectively, and sits amid the scattering of stars that all bear the designation ψ Aurigae. Specifically, the cluster is just about quarter of the way between ψ7 and ψ3. These two stars are magnitudes 5.0 and 5.3.

  Most authorities agree that the combined magnitude of the cluster is 5.4, and generally concur that the Trumpler classification should be I3m (well detached, range of bright and faint stars, moderately rich (50—100 stars), though The Night Sky Observer’s Guide assigns it a ‘poor’ richness rating (fewer than 50 stars).

  Uranometria and The Night Sky Observer’s Guide both assign the cluster 30 member stars, of which the brightest is magnitude 7.3, whereas Star Clusters by Archinal and Hynes give it 119 members, the brightest of which is given a magnitude of 8.0.

  Estimates of its angular size on the sky also vary, with Uranometria and The Night Sky Observer’s Guide giving 14’ whilst Archinal and Hynes nearly double that at 25’.

  It’s always worth checking the proper motions of stars in and around open clusters. This can give a much clearer insight into how large the cluster is and, indeed, whether it is a true cluster or simply a coincidental line-of-sight arrangement.

  The proper motions around NGC 2281 (below) show clearly that this is a true cluster of gravitationally-bound stars and that the bounds of the cluster lie well outside that marked on the chart.

  

  Visually, NGC 2281 is bright and despite the lack of bright signpost stars, is simple enough to find. At magnitude 5.4, it should be easily visible in a finder scope.

  Moderate telescopes (8”—16” aperture) will reveal two to three dozen stars, the brightest four clumping at the centre.

  I made a written observation of NGC 2281 in November 2013 with my 12” (300mm) Newtonian. The description in my records reads A pretty little cluster maybe 15’ across. There seem to be two dozen members or so. It looks like a cavorting fish with the snout pointing north and the thrashing tail pointing east. The brightest stars (9m) form a rhomboid at the junction of the body and the tail. The stars of the body are about mag. 10.

  Although I didn’t make a drawing of the cluster at the time, it has remained quite vivid in my memory because of this fish-like appearance.

  Have a look. See if you can see the fish.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 2281	06h 48m 20s	+41° 04’ 48”	Open cluster	5.4

  References:

  1. Terrell D, Gross J, Cooney WR Jr. Analysis of the Open Cluster NGC 2281. Galaxies. 2021; 9(1):7. https://doi.org/10.3390/galaxies9010007