One of my fondest memories of South Africa (apart from being shot at by police) is the bird of paradise. Not a bird: the birds of this name live in a different continent and a very different habitat. In South Africa, birds of paradise are plants with banana-like leaves growing a meter or more tall. Their official name is Strelitzia reginae. The plant is statuesque, but it comes to life in winter or early spring when flowering. The flowers are flamboyant and spectacular: huge and combining bright orange with purple. Although not hardy in the UK, it is popular (and expensive) nonetheless. But the garden centres promise too much: Strelitzia rarely flowers in the UK. Paradise needs sunlight and whilst this is plentiful in South Africa, it is not quite as frequent or powerful in the UK. Manchester is certainly not the ideal place to get a Strelitizia to flower. But we were lucky: after almost a decade, our specimen flowered for the first time last year and it put in a repeat performance this year. We are wondering whether to put it on ebay as a miracle Strelitzia, being Manchester-proof. But we would not be surprised if it will never flower again. This is a plant to keep in the forlorn hope that one day it will suddenly go from statuesque to spectacular.
(You can also buy Strelitzia nicolai which can be a bit cheaper. Be aware that (1) the flowers aren’t as colourful; (2) it doesn’t flower until it reaches full size, after about a decade; (3) that full size is the height of a tree, which is not ideal when kept as a house plant; (4) if you live in a frost-free climate it can be grown outdoors but keep it far away from your house as the roots can be destructive; (5) buy Strelitzia reginae instead.)
My daytime occupation is astronomy. (You may see a slight contradiction in terms here.) There are types of events in astronomy that are in some ways comparable to this bird of paradise: they are like a star or asteroid suddenly flowering. One such event is a comet. Most of the time it is an icy body in the frozen outback of the solar system. But every now and then one approaches the Sun, and like the plant it becomes spectacular. A tail forms which can be as long as the distance from Earth to Mars. If the comet gets close to Earth, the tail can stretch across our sky. Hale Bopp, the comet of 1997, and Halley in 1987 may be familiar names. As comets go they were rather disappointing. Hyakutake, the Great Comet of 1996, was much more impressive: it came very close to Earth (ok, 15 million kilometers) with a bright tail more than 50 degrees long. I was lucky enough to see it from both hemispheres. By far the best recent one was Comet McNaught, the Great Comet of 2007, especially as seen from the southern hemisphere. One hundred times as bright as Hyakutake, I saw people stopping their car on the motorway (traffic rules being a bit more flexible in South Africa) to gaze at the multi-tailed spectacle.
There was another comet three years ago, and in fact another one will be faintly visible in the next few weeks. But those are no competition for the Great Comets. Those are the spectacle of a life time, as beautiful, as rare and as unpredictable as a bird of paradise flowering in Manchester. One day we will have another Great Comet. We live in hope.
Comets are not the only astronomical spectacle. Supernovae are, as the name suggests, also superb. A supernova happens when a star is utterly destroyed in a nuclear explosion. There are two different types, which with the nominative creativity commonly seen in astronomy are called type I and type II. Both types end the lives of their star in a thermonuclear catastrophe. The debris of the explosion leaves an expanding cloud of gas which remains visible for thousands of years. This is the type of event that astronomers live for but which few of us will ever witness. It is rarer than a Great Comet, and even rarer than that flowering bird of paradise in Manchester. Bright supernovae are gold dust, approaching the rarity of a VEI-7 eruption. Centuries may pass before one suddenly and entirely unexpectedly brightens our skies.
When they do happen, they get deserved attention. Suddenly a star brightens and brightens. It may become so bright as to be visible in the day time, reconciling my daytime job and occupation. Since the year 1000, there have been six or seven bright supernovae in our skies, in 1006, 1054, 1181, 1572, 1604, possibly 1670, and 1987. The last one was faint and was in a satellite galaxy to ours (the Large Magellanic Cloud) rather than in our own Milky Way, but it is included here because it was visible with the naked eye. It was spotted near-simultaneously by people in three different continents and even made the front page of Newsweek. The one in 1670 (called CK Vul) was originally classified as another type of object but is now thought to have been a supernova.
Supernovae of history
Which are those bright supernovae? Almost all predate the invention of the telescope. They were found by people looking up at the sky and noticing something new. In some countries, this would have been the job of a court astronomer. (The UK still retains the title of Astronomer Royal.) Elsewhere it was unorganized and left to chance, a bit like an NHS appointment in the UK. A star bright enough to rival Venus (ten times brighter than any other celestial object other than the sun or the moon) would draw attention anywhere. One competing only with the brighter stars could easily be missed, or reported only by word of mouth. The advantage of court astronomers was that the court’s chroniclers would make note of any discovery, in documents more likely to have survived.
In May of 1006, such a bright star appeared in the constellation of Lupus. It became the most dazzling astronomical event in written history. The star became more than ten times brighter than Venus and was easily visible in the day time. It was recorded worldwide, possibly even in the ancient rock art of Arizona’s White Tanks regional park. The star remained visible for two years.
1054 was the year of the Crab, or at least the Crab nebula. Again this supernova was exceptionally bright, nearly as bright as Venus. It remained visible (though fading) for more than a year. It appeared in July in the constellation of Taurus and was recorded in Asia and possibly America, but not, strangely, in Europe.
In August 1181, a new star appeared in Cassiopeia. It wasn’t quite as bright as the previous ones – probably comparable to Saturn. It remained visible for 6 months but was recorded only in China and Japan.
Almost 400 years later, in November 1572, Tycho Brahe saw a supernova in the same constellation of Cassiopeia. It became as bright as Venus and remained visible for 18 months. It is now known as Tycho’s supernova.
Johannes Kepler, Brahe’s protege, found a supernova in October 1604 in the constellation of Ophiuchus. It was visible for one year. It was almost as bright as Brahe’s supernova. Brahe and Kepler are still the only people to have supernovae named after them.
In the spring of 1670 Johannes Hevelius found a new star. It was fainter than the previous ones, and not as bright as the brightest stars. The new star remained visible (intermittently) for two years. The remnant of this new star was finally found in the 1980s. It was long thought to have been a nova (a much more common but far fainter type of explosion) but recently it was found to have been a supernova, albeit a faint one.
The case of the missing remnant
Supernovae leave a lot of debris. Much of the material that made up the star is blown into space and it takes a long time before they disperse. Astronomy can be like archaeology: sifting through the debris to find out what happened.
There are many nebulosities (clouds) known on the sky which are identified as supernova remnants. They can be especially bright in radio waves, detected with radio telescopes. Many of these remnants are old and large, and come from supernovae that exploded more than 10,000 years ago.
All the bright supernovae known since 1000 should have clear remnants. In fact almost all do. The most famous is the Crab nebula, which is the remnant of the 1054 supernova. We also have one young remnant without a supernova: it is called Cas A and probably dates from the 16th or 17th century but the supernova was not seen, for unknown reason.
The supernova of 1181 is the outlier. It was identified with a bright radio source called 3C 58 (number 58 in the third Cambridge catalogue). However this identification became shaky, as this particular remnant appears to be some 5,000 years old. That age is problematic as the remnant should not be older than the explosion which caused it! All the other post-1000 supernovae have undisputed counterparts – this one doesn’t.
But the case of the missing remnant is now solved. We found it.
The discovery began in 2013 in Los Angeles. The amateur astronomer Dana Patchick (in his daytime job working at the California Veterans Home) was looking through archives of astronomical images, and came across a round nebula which was not in any catalogues. He is quite a prolific discoverer: this was his 30th find. As is common in astronomy, the catalogue he produced was named after him, and the object became known as Pa 30. The nebula was small, round and seen only at infrared wavelengths. Nothing much was visible in the optical.
Patchick passed it on to the experts and went on with his work. It turned out, this nebula was anything but normal. The expert was Quentin Parker, and he began his own work and turned telescopes on to this target. There was a faint star in the centre. It turned out to be extraordinary. The star was extremely hot (240 thousand Kelvin, according to the most recent research) and showed no evidence for either hydrogen or helium. The two elements together account for 99% of the mass of stars. They can’t just go missing!
Vasilii Gvaramadze found the object independently and first published it. He argued that the star came from a collision of two stars. Lidia Oskinova subsequently identified the nebula as a supernova remnant. In the mean time, our own team led by Quentin Parker had managed to measure the age of the nebula by determining how fast it was expanding: 990+-250 years.
It was time to put everything together. The lack of hydrogen and helium could only be explained by nuclear fusion. The nebula must have been ejected in a high-energy event. We found that the gas was fleeing the star at over 1000 km/s! That takes quite an explosion. The combination suggested a thermonuclear explosion: a supernova. A type-I supernova, to be precise.
So I began to wonder whether this explosion might have been seen. European astronomical records are very patchy for the middle ages. There however records from the Far East: China, Korea and Japan. Korean records are also intermittent, with some large gaps, but the other two are quite complete. Events in the sky were thought to relate to events down here, at least where emperors were involved. Astronomers had an important part to play in the court proceedings, keeping an eye on the sky and observing (and predicting) solar and lunar eclipses. Detailed records were kept (albeit not with very accurate descriptions).
During the lock-downs I found myself with a spare day or two and went busy with the old Chinese catalogues (translated, of course). The goal was to find recorded events in the correct part of the sky.
The Chinese records list the events, the durations and the approximate location on the sky. They are classified into three types: ‘guest stars’, ‘bushy stars’ and ‘broomed stars’. The last two types are mostly comets: a ‘broomed star’ is a comet with a tail, and a ‘bushy star’ is a blob but has no long tail. Guest stars are stars that appear and disappear, and unlike comets do not show a tail or move on the sky. So I was looking for guest stars in the approximate direction of Pa 30. ‘Approximate’ because the records are not particularly good at describing position. They normally give an indication where on the sky it was or on which constellation but not in great detail. They may even just say ‘in the north’ (which can mean circumpolar), but they can also state which other stars it was near to.
I found two possible identifications in the Chinese records. One was in the year 722 and one in the year 1181. The 722 event seemed to be a normal nova and was seen for only a few days. The one in 1181 was different. It had remained visible exceptionally long (compared to other guest stars), at 185 days. And of all the events I had looked at, it was the only one not classified as a nova or comet. It had been a supernova.
It was one I remembered. Years earlier I had looked up publications on this supernova. There was a remnant in the literature but it did not convince me. It seemed to me that the characteristics of the remnant, 3C58, did not fit well with the event. So when this one jumped out from the catalogues, it seemed plausible that there was another remnant. That was true for no other supernova in the list. I had a prime suspect.
We published our result in 2021 (Andreas Ritter et al, https://iopscience.iop.org/article/10.3847/2041-8213/ac2253/pdf). There has been a lively discussion since, but the result that we found the remnant of the most recent unidentified galactic supernova has so far survived. Let’s see.
The Chinese constellations are very different from the western ones. Our familiar constellations started about 4500 years ago, probably in the region of Iraq. We know the date fairly well because the location of constellations on the sky change over time. Stars in our constellations are those that were visible from Iraq at that time. The constellation of Taurus (the bull) was the start of the zodiac: at that time it was the constellation that heralded spring. The Chinese constellations are not as well described, and often used fainter stars than we use and separate smaller parts of the sky. The term ‘asterism’ is used for a grouping of stars that is not one of our constellations.
The chart shows the relevant part of the sky. The size of the dots indicates the brightness of each star. Green indicates our constellations: Cassiopeia and Cepheus. Red indicates the Chinese asterisms: Hugai, Chuanshe, Kotao and Wangliang. There are stories behind them.
These particular asterisms are made up largely by quite faint stars. They describe the area of the imperial palace. The palace itself is represented by Ziwei, an asterism just to the top-right of the area shown. Huagai is the canopy of the emperor. (It may seem to be upside-down, but the palace Ziwei is to the upper right from there, so shielded by the canopy). Wangliang represents the charioteer of the emperor. Chuanshe is the guest house of the palace. There is a historical connection here: just before the new star appeared, an ambassador from the rival northern Chinese Jin empire had arrived and was staying in the guest house. Finally, Kotao (‘mountain road’) connects a number of brighter stars. It is unusual in that this asterism crosses another one, Chuanshe.
The exact shape of these asterisms is not known and may have varied over time! We don’t have drawings from those times: the ones we have were made much later. Huagai has another brightish star just above the left end of the canopy: if this was part of the original canopy it would be better aligned with the palace. Kotoa is quite certain because it has brighter stars. Wangliang is also quite clear. But Chuanshe connects very faint stars (barely visible to the naked eye): we know there were nine stars but we don’t know which nine. We know that Chuanshe was between Wangliang and Huagai, but there are several possibilities. The chart shown here depicts two options.
There are 6 separate documents with useful information regarding the location of the guest star, 4 Chinese and 2 Japanese. The longest one is the Wenxian Tongkao, the diaries from the southern Song dynasty. Two of these state that the star appeared in the lunar lodge of Kui. This is not shown on the chart: it covers Huagai, most of Chuanshe and the area below it. One document states it was ‘beside Ziwei’ and one puts it vaguely ‘at the north near Wangliang’. The most specific locations state that it was ‘guarding’ or ‘invading’ Chuanshe. And one document even states that it was guarding the ‘fifth star of Chuanshe’ – sadly we do not know which star this was!
The important statement is that of ‘guarding’ or ‘invading’. This was used in the sense of ‘adjacent’ or ‘very close’. It implies that the guest star was not in the constellation, but very close to it, just like a guard or invader stands just outside.
The combination of the statements implies that the guest star had appeared in between Huagai and Chuanshe, probably closer to the latter.
The previous candidate 3C58 is indeed there, at the left edge of the possible range. Pa 30 is also in the range but at the right edge.
So which one is the correct location? Either fits the descriptions. 3C58 though is in a special location, at the point where Chuanshe and Kotao cross. It is also close to a brighter star. None of the documents mention this. The ‘fifth star’ could point at 3C58, as it is right next to a star that could be the fifth one. But depending on which stars to count, there are ten (!) other possibilities, two of which would agree with Pa 30. The reference to Ziwei favours a location to the right of Huagai. On balance, Pa 30 has a higher likelihood of being the counterpart.
The chart shows the combination of all constraints from those records. The blue region is the area that agrees with all descriptions. Pa 30 is in this area whilst 3C58 is not.
Celestial events were thought to be associated with the divine (at least considered divine) leaders of the empire. That is not unique to the Eastern world. It was prevalent in the near east, probably from the inception of our constellations 4500 years ago. ‘Wise men’ (i.e. astronomers) from the east even brought it into the bible, a book which was otherwise mostly robust against linking celestial events to our lives. If you call yourself the Sun King, a sunspot can be a sign of a disaster. So obviously, a new star on the sky next top the symbol of the imperial palace would have been a concern, especially when an important ambassador from the rival northern empire was just visiting. Whose side was the new star on? Was there a reason it was said to be ‘guarding’ or ‘invading’ the guest house where the ambassador was staying?
The Japanese were equally concerned. The diary of Fukiwara Kanezane, of the imperial court, reads ‘a guest star has been present in the inner sky, a sign of abnormality indicating that at any time we can expect control of the administration to be lost’. This new star was serious business.
The one thing we don’t know well is how bright this star was. It was bright enough that it was noticed easily, and as it remained visible for 185 days, it must have been fairly bright to begin with as these events fade over time. It was also bright enough to be seen as an omen. On the other hand, it was noticed by court astronomers but did not create a great stir elsewhere. That suggests something say as bright as Vega: not quite the brightest star in the sky, but not too far off. That would be quite faint for a supernova in the Milky Way. Even at its distance of 8000-9000 lightyear, it should have been 10 times brighter or more.
There is one comment in a Japanese document that compares it to Saturn: ‘At the hsui hour [7-9pm] a guest star was seen in the north-east. It was like Saturn and its colour was bluish-red and it had rays’. We don’t know what this mean: Saturn was nowhere near the supernova at the time and is not ‘blue-red’. It is often taken as indicating that the new star was as bright as Saturn. But this description was written a century later. It may have confused things: the entry is clearly about the 1181 star, but the description about the colour and rays and the specific time sound more like aurora.
The case that we have found the remnant of the supernova of 1181 seems convincing – at least to me. It fits the location and age of the remnant well, and the lack of hydrogen and helium is strongly suggestive of a thermonuclear explosion – a supernova. But why was it rather faint?
And why is there still a star? Supernovae are destructive events which the star does not survive. You may get a neutron star, a black hole, or there may be nothing left at all. Pa 30 still has a star at the centre. How is that possible?
The most likely explanation is the one by Gvaramadze: this was not a normal explosion, but was caused when two dead stars (white dwarfs) collided or merged. Many stars exist in multiple systems – in fact our sun is a bit uncommon in being alone. Once a star has finished its life, other than by supernova, it leaves a white dwarf behind: very compact, and consisting mainly of carbon and oxygen. A double system can leave a double white dwarf, often orbiting very close to each other. Over time the two stars spiral in, and eventually some may merge.
Such a merger would lead to a partial supernova: there is a thermonuclear explosion, but it does not blow up the entire system. In fact, models suggest this leads to a faint supernova. About 10% of the supernovae in the Universe may be of this type.
We had never seen one in our Milky Way. But it fits well with the supernova of 1181, with its brightness, its remnant nebulae and with the surviving, very strange central star. And with 10% of the supernovae being like this, it is not unreasonable that there would be one hiding among the historical supernovae.
The nebula itself is very faint in visible light. However, there are ways to bring it out better. Robert Fesen managed to obtain a deep image through a narrow-band filter that isolates light from sulfur. Supernova remnants can be brighter in such a filter. The result was spectacular. It was just published (Fresen et al. 2023, https://arxiv.org/pdf/2301.04809.pdf)
The new image shows a very large number of stripes. These are more normally associated with planetary nebulae and have not before seen in supernova remnants. The stripes are material that is either swept back by the strong wind from the star, or are shadow regions behind a dense globule where they are shielded from the hot radiation from the star. Either case requires that there are dense globules in the remnant, which can not be seen directly.
In fact, every single stripe looks a bit like a comet! Of course they aren’t (they are 10,000 times larger, for one). But what a way to get the two most special astronomical events in one.
This has been an adventure. It started with two Great Comets and one great flower and ended with a Great Explosion, or if not the explosion itself then at least the debris of one that happened more than 800 years ago and which caused great concern at the time.
All that is still needed is a major (but safe) volcanic eruption. Or was that perhaps Hunga Tonga?
Albert, January 2023