AOST2 summarises this nature of this object perfectly; "This unique object also has a unusually rich emission spectrum and a very high expansion velocity, characteristics similar to the Bug Nebula, NGC 6302."

The Mystery of a Strange "Star" Seen in 185AD.

In the year 185 AD, the Chinese recorded a strange star, that appeared very close to the southern horizon. Some debate has occurred regarding what is the true candidate. Most seems to centre on the interpretation of a 52-word paragraph in the astronomical book known as the "Houhanshu" (The History of the Later Han Dynasty). The original written text was compiled by the dynastical scribe Sima Biao in 300 AD, and first translated for western eyes by Fan Yeh in 1971. For centuries, this "new" star 200-day reign remained as a "celestial oddity", until they realised that the object may correspond to several possible known objects - including He2-111.

Connection to He2-111.

Reports and observations in the historical record come from a provincial city known as Lo Yang. (Latitude +34.7^ON) Invisible from this location today, precession places this "new star" at declination -51^O36' S (185AD) whose southerly culmination is some 3.6^O above the horizon. (NOTE: The 2000.0 declination is -60^O 50'S; while He2-111 position today is 14h 38' 18" -60^O 49' 44", and in 185 AD. 12h 34'25" -51^O 38' 46") Today, the culmination date of the "new star" is about; 3pm on 15th Feb; Midnight on 2nd April and 17th May at 9pm.

Over the eighteen centuries that have passed since the object appeared, and accounting for some 24^O of precession (some 6.8% of the entire ecliptic circle), suggests that culmination of He2-111 occurs some c.24.5 days earlier than the dates given above. If this position does match, then the culmination dates would be; 3pm on 22nd January; Midnight on 8th March and 9pm. on 23rd April.

If they saw a supernova on the 7th December 185, then it would have appeared in the morning sky, close to the beginning of twilight, with a 7.45am being the daylight culmination. Within three weeks the SNe would have culminated during darkness around 5.30am in the morning. The disappearance of the guest star was around the 5th July 186, corresponding to a helical rising at 7.40pm - just on the end of twilight and before setting. Visibility therefore correlates well with the alleged position of this object. It is tempting to think that both these positions and times are not stated just by pure chance.

As mention earlier, the giant halo shell has a very high velocity that is akin to what we see in the outer envelope of the Crab Nebula in Taurus. However, the gaseous outflow is not behaving like a SNR. Using a graphical representation, first given by Sabbadin and D'Odorico (A&A, 103, 119 (1976)) of the [H/NII] ratio versus the [H/SII] ratio, clearly shows that He2-111 is clearly beyond the range of all SNR and the "standard" planetaries. (Note: The reason that these ratios are important is that each class of nebulae has quite differing elemental abundances compared with their hydrogen contents - a clue to their origins. These two elements seem good indicators to differentiate one gaseous object from another.) This lead Gutiérrez-Moreno, Moreno and Cortéz (1994) to conclude;

"He2-111 is far from the supernovae remnants region...with He2-111 clearly showing a bipolar structure...but agree[ing] well with that of other type I PNe."

Some debate still exists about the supernova's location. In recent decades, we have identified a number as possible candidates that can be found in a few significant papers.

Webster discovered the largest, brightest and best candidate RCW 86/ MSH 14-63/ G 315.4-2.3 (14429-6230) in 1977. (Galactic Position L=315.4; b=-2.3) [Record #155] Studies by Clark and Stephenson "The Historical Supernovae" Pergamon Press (1977), also state considered that the age, distance and position matched that of SN 185AD. According to "A Catalogue of Galactic Supernovae" (Green 1998) published in Cambridge, several candidates of SuperNovae Remnant (SNR's) exist. Although this SNR is completely invisible in all amateur telescopes, the location is 40' due east of the double star R243. (Discussed in NSP XVII) Here they give the area as 42'arc.min² (0.9x??), as a class "S" SNR - a remnant showing a shell(ed) radio structure. RCW 86 has a flux of 49Jy. @ 1 GHz.

RCW 86 is associated with the pulsar PSR 1449-64 (14535-6413), but does not match very well the central place of the SNR. PSR 1449-64 has a period of 0.179848389392 seconds, which has a currently slow spin down rate of 6x10^-17 sec.sec^-1 - about one second in two billion years! Unlike PSR 1509-58 mentioned below, it is moderately bright in the radio (400 MHz.) The discrepancy of 20'min.arc. in both positions against the proper motions, which they have not measured, leading some researchers to claim that they are two separated objects. Secondly, the mean expansion velocity of the SNR would have to be > 13 000km.sec.^-1 - greater than any known SN II event! (Schaefer, B.E., PASP, 105, 1238-1239; November (1993))

According to Schaefer (1993), the maximum brightness was -11. Yet because of its southerly location, based on the extinction of light close to the horizon at Lo Yang, the visual brightness could not have been brighter than -3. Schaefer is claiming an extraordinary seven magnitudes of extinction - which Schaefer at least claims is a slight overestimate! Thorsett, S.E. (Nature; 356, 660 (1992)) states a much lower value for the -1.8 extinguished magnitude; giving a more plausible magnitude of >-5.2. Estimates of the expansion rate by radio observations, find a distance somewhere between 0.55kpc. (Thorsett (1992)) or 0.95±0.4kpc. (Kaastra et.al.; A&A, 264, 654 (1992))

Schaefer (1993) states that;

"The counter of SN 185 cannot be conclusively determined on the basis of visibility or other grounds. [In] my judgement...leads to a preference for the [RCW 86] candidate."

In my own "gut" opinion, my money on He2-111!

Schaefer, B.E.; Astron.J.,110,4, 1793-1795 (October 1995) speculates that the object seen by the Chinese could have been either an unknown comet, or periodic Comet Swift-Tuttle. This is based on the astrological interpretation of the word "Houhanshu" in the middle of the Chinese asterism known as Nanmen, inferring an observed diameter of c.5^O. If it were a comet, and it was visible for 200-odd days, this places severe restrictions on this hypothesis. The experts seem to feel that this theory has many problems.

[Incidentally, Comet Swift-Tuttle is the source of the Perseids meteor shower, which is due to make a very close approach to the Earth in August 2126.]

Connection to a Nova.

It appears that the translated word "twinkling" in the original text infers that the source was stellar. If we eliminate all possibilities, then a nova is only left, which conveniently does not leave a lasting remnant. If we apply the same restrictions as we did with our supernova candidates, then the magnitude would have to reach a maximum magnitude of -3. A nova has a range during an outburst of between ten and fifteen magnitudes, so the progenitor would have to be magnitude seven to twelve. A cataclysmic variable star this bright would be easily visible. In a survey made by Downes and Shara (1993) found no variables that match these criteria. However, this does not mean that a candidate may not exist.

Further Reading;

1. Schaefer, B.; S&T, 70, 261 (1985)

Postscript: An Odd Occurrence.

Against this is that the He2-111's position is also some 9.4' in declination more northerly than its true placement. Secondly, no other deep-sky object can match this given description. Open clusters are the only objects that pepper the surrounding fields - and nothing else.

Innes reputation as a visual observer is the finest - so it is unlikely this unknown object is a mere telescopic aberration. His accuracy, for example, with observations of southern double stars is an exemplary testament to his meticulous approach. We now reflect this with the "unknown" classification of the object in the IC catalogue rather than just being "non-existent".

NGC 5617 has quite an interesting history. Dunlop discovered the cluster from his backyard in Hunter Street, Parramatta, which he describes, after observing the cluster twice, as; "A small minute group of small stars of mixt magnitudes, considerably congregated towards the centre, 4' or 5' diameter."

His description is a little odd, because the distribution of stars a 20cm. using a 167X (12.5mm. Ortho) shows a "boat-shaped" collection of stars to the south of the field, followed by two wider curved arcs at the top of the field. Nor do I see any central condensation. I thought the distribution made it appear roughly pentagonal in shape. In 1834 John Herschel, who observed this cluster twice, more accurately described this cluster as; "...very rich, irregularly round; pretty much condensed in the middle, but scattered at borders; 15'; there are three stars of 10th mag, 5 or 6 of 11th; the rest below 11th."

Other references state;

RNGC (1973) lists the cluster as;
"Large, Pretty Rich, Pretty Compressed in middle, star 8th..."

AOST1 & 2 states;

"A fine open cluster about 15'min.arc. across with some central condensation, in a rich and beautiful region; it is well shown with a 10.5cm and D is about 1000 pc."

Bailey was the first to measure this cluster in 1908, and noted a diameter of 15', containing 40 star between magnitude 10-12. Later in 1922, Raab found that the diameter of the outliers, giving a distance of 625pc., and counting 122 stars with spectra between B5 and M0. Trumpler's survey in 1930 found a distance of 1.0kpc., which is the value often quoted in references today. In the next year Collinder classified it as Cr 282.

Little observation data was made of NGC 5617 until the new surveys made from Stromlo Observatory during 1950's and 60's. Here the investigation into the component stars began. Wallenquiest identified 288 stars between magnitude 10.7 and 14.7, and using Stock's observations of several late giant stars, investigated the density distribution of the stars. A second investigation began in 1964 by Lohmann, then Hogg in 1965. Hogg's observations eventually proved the most important of the data thenceforth, by combined the density studies and velocities to decide the dynamics. He also discovered some 194 stars in the cluster, of which, 153 were determined as true members.

Next was the southern sky survey by Ruprecht, who classified the cluster as "I 3 m" - "Detached with strong central condensation, highly luminous compared to the surrounding field, medium rich in stars."

The first variable discovered as a possible member of the cluster NGC 5617 was the "Lb:" late spectral class - slow irregular variable V798 Centauri (14347-6058), that Carevsky (1966) had suspected from his investigating of both the variable's and cluster's proper motion. Variations of the deep red, M6III spectral class star, varies between magnitudes 8.7 and 9.3 in an estimated period of 180 days. (See Description and He-111's chart of TX Centauri below to find this star.)

Another survey by Lindoff (1968) produced a colour-magnitude diagram that determined the age as 45 million years, which he later revised in the same year to 46 million years. Using photometry of the 180 odd stars, which he thought were associated members, the total magnitude of the cluster became 9.66. Reevaluating this data, and yet another investigation, especially investigating the evolution of the red stars in this cluster, Barbara (1969) found the age to be closer to 60±10 million years. The "standard" colour-magnitude diagram was finally produced by Hagen in 1970, using the photometric data of Sarov produced earlier in the same year. The data being limited to the inner portions of the cluster extending to c.4'min.arc. Stellar magnitudes in this region were estimated using HD126593 in 1968 (Arkiv.f.Astr.,4,587.) Other studies of the 1970's provided information on the metal abundances and absolute magnitudes. (The Total M_v=1.53 (Sol=4.5)) The most interesting investigation was by Dzigvašvilli who looked at the relative orbits of the stars within NGC 5617.

Last of any significance to the cluster's history was the reported discovery of four "ultra-hot" Wolf-Rayet variables near the central parts. Lynga (1987 and 1991) thought that these are true Wolf-Rayet's- and this is not reported anywhere else in the literature. (I searched for evidence of these stars, but without success!) Brightest of these is the 8.81 magnitude star HD126593 (also a comparison star), while the total visual magnitude of the open cluster is 6.3. The measured O8 spectra reveals a hot star.

A.H. Hogg in "Catalogue of Open Star Clusters South of -45^O Declination." Mem.Mt.Stromlo.Obs. (1965) says that this cluster is "quite elongated", along PA 25^O -205^O

Latest information says the cluster contains about eighty stars, while the present age by Lindoff remains at 46 million years. The proper motion of Alpha Centauri is aimed 3' north of the heart of this cluster, which will be hidden by its brightness in the year 3085AD. Only a dozen or so of brightest stars in the cluster will be visible in a 30cm. - though it will certainly make a neat little multiple!

Pismis 19 (Pi 19)/ VHA 160/ OCL#921 OSC

This cluster was discovered by Pishmish in 1959, and appears in Uranometria 2000.0. Containing 26 stars, at or below 12th magnitude, this compact cluster subtends a mere 3'min.arc. across. It requires at least a 30cm. to see the small luminous haze against the background field, and it reminded me very much of a faint extragalactic globular star cluster. Pismis 19 really requires a dark-site to see clearly, and even using averted vision made the cluster a bit more prominent. With care, I suspect a 25cm could see this open cluster, though AOST2 says;

"Some 12' south-east is the very faint cluster Pismis 19 which 30cm shows as a faint patch of haze about 2' across."

Ruprecht classifies it as "II 2 p". The last professional paper on this cluster was by van der Burgh and Hagen; "Uniform Survey of Clusters in the Southern Milky Way."; (Astron., J.,80,1. (1975)), and this reference incorrectly gives the position some 6' north of its true location.

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I have added some fainter clusters in the same field as the PN He2-111, but observers should note that these are uninteresting yet perversely rather alluring objects. These make good contrast with the more obvious nearby clusters, NGC 5617 and Pismis 19. In a 20cm., the stars in each of the cluster(s)were hard to identify without a suitable finder chart. This entire field lays some 30' SSW of the planetary He2-111, and all appear on the maps of Uranometria 2000.0.

The history of these clusters is an odd story of identification, as initially both were co-discovered as the object "CR283." Earlier references call this a single cluster, however, the references, like the Lynga, G. "Catalogue of Open Cluster Data, 5th Edition." (1987/1991) show them as separate clusters, while Megastar 4.0 has the clusters as two separate objects. In reality they are the same cluster - but on differing edges

Trumpler 22 (Tr22) / CR283 Ocl#920 & CR283/ Hogg 17 Ocl#920

Trumpler discovered CR283 as a possible cluster in 1930, and estimated the cluster's general appearance and total magnitude to be c.12th. Distance is given as 2.2kpc, but the next year (1931), Collinder estimated the distance to be 2.5kpc. Furthermore, he identified the brightest star as Vmag=10, which was later measured photographically as 9.8. By 1957, Stock stated that he doubted this being a true cluster. In 1966 Ruprecht classified Trumpler22/ CR283/ Hogg17 using the Trumpler System as 'II 3 p -' - Detached with little central condensation, moderate luminosity and mediumly rich. Observations by A.R.Hogg looked closely at the proper motions of Trumpler 22 - finding 53 surrounding stars, with 38 to have similar proper motions. Here he found a significant split in the stars, and deemed that they were two dynamically different clusters, but were associated. Ie. A Double Cluster. For his own "discovery" of Hogg 17, he identified 46 stars with 31 having similar common proper motions. Both these results were published after this, within Hogg's "Catalogue of Open Star Clusters South of -45^O Declination." Mem.Mt. Stromlo.Obs. (1965) Observationally, this reference states "Not an obvious group, but confirmed by count. 'Edge Effect.'"

Lindoff's studies in 1968 suggest some 80 stars are associated, while the range of spectral classes vary between B3 and K6. Lindoff also determined the Mag_ph=11.68 [E(B-V)=0.53] and estimated the age as 100 million years. He estimated the distance to be 1.7kpc. Investigation by Lohman observed the nature and structure of the cluster, finding 83 attached stars.

Hagen (1970) in a new study produced a colour-magnitude diagram of this cluster. Visual magnitudes vary between 10 and about 13.7. Colours are bluish to orange-yellow. [(B-V)_O=-0.7 to 1.2, with a spectral type averaging B4. The absolute magnitude of the brighter stars is between -3.0 and 1.0, though the entire cluster averages about 1.65.]

The cluster has also been suggested to be associated, as it has with NGC 5617, with the deep red irregular variable V798 Centauri. Presently the entire cluster(s?) is estimated to be about 25' across (0.4^O)

Wray 16-155/ PK 314-0.1 (14333-6055) is a planetary nebula some 5.6' south of He2-111 along PA 195^O. Since discovery, little information is known about this object. Although being seen by the unaided eye in all telescopes is unlikely, I have included it because of its proximity to He2-111. The PN's position is shown in the STScI image in Figure 1.

TX Centauri (14352-6059) is a Cepheid variable that lies 16.3' ESE of He2-111. I mention it only because it features in the same field as the planetary, and it appears in the small insert for Proxima on Map 25 of Sky Atlas 2000.0. It can be quickly identified by drawing a line between a 7.5 magnitude star (The brightest star in the field (SAO252809) and the PNe. This Cepheid being a mere 3' along this line. Although the period is moderately long 17.0936 days, the brightness fluctuations are small, varying between 9.85-11.14 (JDE 2434964) The rise between minimum and maximum takes six and a half days to rise a mere 0.29 magnitudes in c.6.5 days. Amateurs may like to observe TX Cen a week apart to see if they can see any differences in brightness.

Along the same line mentioned above, another 3' closer to the planetary is an obvious c.8.5 magnitude deep red star. This is V798 Centauri, mentioned earlier in the text with NGC5617.

Some problem exists on the discovery date by H.C.Russell. This pair could have been seen by Russell as early as the 11th June 1871, as it is close to the 11th magnitude pairs; R245 (2"sec.arc.) and R246 (15"sec.arc.) As no evidence is written on this earlier observation, then the stated discovery date must remain ten years later on the 9th August 1881.

R 249 is an orange pair (KO III) with magnitudes of 8.2 and 9.8, who Russell positions at (1428-6203 (1880)) with a Separation 2.77"sec.arc. at PA 33.633^O (Mags. 8 and 10)

Other than Russell's positions, little has changed in the 3.0"sec.arc. separation along PA 24^O, which was last measured in 1960. I have used this double in the past for finding 11.3 magnitude Proxima Centauri/ V645 Cen (14302-6242), which is 49' WSW (@PA 260^O) from R249. (Details on Proxima appear in NSP XVII.) Listed as HIP 71475/ PPM360885/ SAO252821, the combined magnitude is measured at 7.5±0.4, but was unresolvable using the Hipparchos's instruments. The parallax is measured as 4.77±1.58m.sec.arc., giving a distance of 209pc.± 69pc. (680±225lty.) Using this distance to estimate the separation, the two stars are some 94 billion kilometres (630AU) apart. If this is a true binary, then period is around 15 000 years. Little wonder we have seen little motion over in the last 120-odd years!