All About Space

WHAT LIT UP THE SKY IN 1054?

The Crab Nebula was formed by a supernova, and astronomer­s now think they know which type caused it

- Reported by David Crookes

In 1054 CE, Chinese astronomer­s looked towards the sky and discovered a star which had suddenly appeared in a position 6,500 light years away where no star had previously been observed. They watched as it shone six times more brightly than Venus in the constellat­ion of Taurus for 23 days from 4 July, and their observatio­ns were backed by astronomer­s in Japan and the Arab world.

Reported as a ‘guest star’ by the chief of the astronomic­al bureau at K’ai-feng, it remained visible until 17 April 1056. It would then be another 872 years before Edwin Hubble became the first to associate the observatio­n with another discovery – that of the Crab Nebula, which was spotted in 1731 by English astronomer John Bevis.

Hubble suggested that what the Chinese astronomer­s had seen was a supernova explosion that had caused stellar remains to be scattered across an area of space some six light years wide. It was a controvers­ial theory, primarily because the study of supernovae was still in its infancy. But in 1939, American observatio­nal astronomer Nicholas Mayall demonstrat­ed beyond doubt that the two were linked.

Having establishe­d that the ‘guest star’ of 1054 was a supernova – a term first used in the 1930s by Walter Baade and Fritz Zwicky at Mount Wilson Observator­y – astronomer­s had many questions. Was the date of the explosion correct, for instance, and were reports of the celestial fireworks consistent. In those cases, possibly yes and not entirely are found to be the general answers.

“If you watch the remnant expand today and work out when all that material was back together, the answer comes out to be 1054,” affirms Andy Howell, a staff scientist at Las Cumbres Observator­y. “The position is also roughly consistent with the reports from Chinese astronomer­s, although they said it was ‘to the southeast of Tianguan [Taurus], perhaps several inches away’ when it’s actually to the northwest.” The biggest question, however, was whether

SN 1054, as it became known, was a Type Ia or Type II supernova. And this would be a good time to recap the difference between the two.

Type Ia is also known as a thermonucl­ear supernova, and it occurs when a star of up to eight solar masses in a binary star system dies and becomes a dense white dwarf. The dwarf’s strong gravity begins to siphon material from its companion star, and when its mass gets close to the Chandrasek­har limit, it begins to shrink under the weight and becomes unstable. A star made of carbon and oxygen undergoes a runaway fusion, exploding it.

Type II supernovae, on the other hand, are caused by the death of a massive star – greater than ten solar masses – as it runs out of fuel and collapses in on itself, a reason why they’re also called iron core-collapse supernovae. This demise releases large quantities of energy and leaves behind either a dense neutron star or a black hole. So which type was SN 1054? The event fits within the realm of a Type II because the Crab Nebula has a pulsar – a spinning neutron star – at its centre.

Yet the ejecta in the remnant is moving slowly, indicating low kinetic energy.

“It didn’t produce very much radioactiv­e 56Ni [nickel], and the chemical signature of the ejecta is strange,” says Howell. “There’s also about five solar masses of material in the ejecta because some of it went into the making of the neutron star and some was lost earlier in the star’s life.” This actually points to a different type of supernova – one proposed in 1980 by Japanese physicist Ken’ichi Nomoto of the University of Tokyo. The problem? Nobody has ever discovered another supernova that could fit Nomoto’s theory in a convincing or conclusive way… at least until now.

Howell is the leader of the Global Supernova Project, a three-year program to obtain light curves and spectra for 500 supernovae. He’s one of many scientists contributi­ng to a study led by Daichi Hiramatsu, also of Las Cumbres Observator­y. They have discovered the very first evidence that points to the existence of an electron-capture supernova, the type that Nomoto has advocated for 40 years.

According to Nomoto, electron-capture supernovae sit in between Type Ia and Type II, with the progenitor star being about eight to ten times the mass of the Sun. The star is not light enough to prevent its core from collapsing, and it’s not heavy enough to go the way of a Type II. The star also experience­s excessive mass loss before the supernova explosion, resulting in the circumstel­lar material with an odd compositio­n. The supernova explosion produces little radioactiv­ity and low kinetic energy. The core needs to contain neutronric­h elements, too, which means it has more stable nickel compared to radioactiv­e nickel.

“I think that Nomoto and his team came up with the theory independen­t of SN 1054, but realised soon after that it might explain the Crab Nebula,” Howell says. “He wrote a Nature paper about it in 1982 that was a big deal, and he’s been talking about it ever since, at almost every supernova conference.”

Howell says many others have since added to the discussion, “especially Nathan Smith, who had a great paper in 2013 adding more evidence that SN 1054 could be an electron-capture supernova”. But while the Crab Nebula fits many expectatio­ns of electron-capture supernovae, some natural questions were going unanswered. “Why was it brighter than expected?” Howell quizzes. And crucially: “Why haven’t we seen a supernova

in real life with the properties of an electronca­pture supernova?”

Back in 2018 those questions could finally be answered, since in March of that year Japanese amateur astronomer Koichi Itagaki discovered a new supernova roughly 31 million light years away in the galaxy NGC 2146. It caught the attention of researcher­s because it was nearby and bright, and they were able to use their global network of telescopes, as part of the Global Supernova Project, along with other facilities such as NASA’s Swift satellite, the Hubble Space Telescope and the Keck Observator­y to examine it further.

“Daichi, a graduate student working with me, noticed that this supernova – which came to be called SN 2018zd – had some unusual properties that we’d never seen in a supernova before, like really strange colours just after the explosion,” says Howell. “After a few months, the supernova’s brightness dropped like a rock – the largest drop we’d ever seen. It indicated there wasn’t much radioactiv­e material produced in the supernova.” As with every nearby supernova, the team checked to see if the Hubble Space Telescope had observed its galaxy before. “We were hoping it might have a prior image of the star before it exploded,” Howell explains. “We found a few images, but the star was only visible in one of them, in the infrared. If the star would have been a red supergiant – the progenitor­s of normal corecollap­se supernovae – we would have seen it in more filters.

“However, the detection was consistent with the properties seen in another star in the Milky Way suspected of being a super-asymptotic giant branch star, which are thought to be the progenitor­s of electron-capture supernovae.”

Indeed, it soon became apparent that SN 2018zd was the only supernova ever detected to have all six of the characteri­stics said to form an electron-capture supernova. Just as importantl­y, the astronomer­s were also able to use the study of SN 2018zd to answer some of the questions they had about SN 1054.

Up until that point, they were only part certain that an electron-capture supernovae had resulted in the Crab Nebula. “It has the right chemical compositio­n of the ejecta, it has low kinetic energy and it produces neutron-rich elements, but we don’t know anything about the progenitor since it exploded and was only around before telescopes were invented,” Howell explains.

What astronomer­s had done in their studies of SN 1054, however, was reconstruc­t a rough light curve based on brightness comparison­s to other

stars and the Moon and the observatio­n that the ‘guest star’ had been visible during the day for 23 days, finding it had an apparent magnitude of -6.0. This light curve had been suggested to be too bright for an electron-capture supernova. “But if we infer that there was a lot of circumstel­lar material cast off by the progenitor, just as there was in SN 2018zd, the interactio­n of the supernova ejecta and that material may have brightened the light curve.” It would seem that there really are electronca­pture supernovae, but what significan­ce does this have?

“Before that we knew the distributi­on of pulsars was bimodal – some with short spin periods and some with longer spin periods,” says Howell.

“It was also speculated that maybe the short

“NOW WE HAVE GOOD EVIDENCE FOR ELECTRON-CAPTURE SUPERNOVAE, WE CAN REVISIT THOSE QUESTIONS”

ANDY HOWELL

spin-period systems came from electron-capture supernovae. There’s a similar bimodality in the neutron star sizes: there’s a peak in the neutron star mass distributi­on at 1.3 solar masses, and another at 1.5 solar masses.

“Some have speculated that the low-mass neutron stars come from electron-capture supernovae. Now that we have good evidence for electron-capture supernovae, we can revisit those questions. And since we also have direct evidence that supernovae produce more neutron-rich material, it tells us more about how these elements are produced in nature.”

Studies of SN 2018zd are set to continue. For starters, the astronomer­s want to be sure the distance to the host galaxy is correct. “It’s rather uncertain,” says Howell. Fresh observatio­ns of the galaxy will be undertaken using Hubble to nail down the distance to confirm how bright SN 2018zd is. “We tried to do most of the analysis in a distance-independen­t way because of this,” Howell explains.

“We also need to find more electron-capture supernovae,” he continues. “They look enough like normal supernovae that we might have been confused before. There was also speculatio­n that a small class of supernovae called SNe IIn-P might be electron-capture supernovae, and that might be correct.

“Now that we know what they look like, we can reanalyse old data, and knowing the fraction of supernovae that they make up will help us narrow down the progenitor mass range. The observatio­ns may lead to new theoretica­l studies as well, now the theorists know better which aspects of their prediction­s are realised in nature.” In that sense, the findings around SN 2018zd really are proving to be a ‘eureka’ moment.

After all, the discovery and subsequent study of SN 2018zd is helping astronomer­s find out more about the Crab Nebula, something we’re still a way off fully understand­ing, and it’s also helping to shed fresh light on what happened nearly 1,000 years ago. Howell says SN 2018zd is letting us discover more about how some neutron stars are made and how extreme stars live and die, too. “It is very exciting indeed,” he concludes.

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 ??  ?? Below: A composite image of the Crab Nebula – a remnant of the supernova SN 1054
Right: Andy Howell is a staff scientist at Las Cumbres Observator­y and leader of the Global Supernova Project
Below: A composite image of the Crab Nebula – a remnant of the supernova SN 1054 Right: Andy Howell is a staff scientist at Las Cumbres Observator­y and leader of the Global Supernova Project
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 ??  ?? Left: A star would need to be at least eight times heavier than our Sun to create this special type of supernova explosion
Left: A star would need to be at least eight times heavier than our Sun to create this special type of supernova explosion
 ??  ?? Above: This Hubble image captures the beating heart of the Crab Nebula – a rapidly spinning pulsar
Above: This Hubble image captures the beating heart of the Crab Nebula – a rapidly spinning pulsar
 ??  ?? Above: When SN 2018zd was observed, scientists were able to examine archive images taken by Hubble to detect the likely progenitor
Above: When SN 2018zd was observed, scientists were able to examine archive images taken by Hubble to detect the likely progenitor
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 ??  ?? David Crookes Science and technology journalist
David has been reporting on space, science and technology for many years, has contribute­d to many books and is a producer for BBC Radio 5 Live.
David Crookes Science and technology journalist David has been reporting on space, science and technology for many years, has contribute­d to many books and is a producer for BBC Radio 5 Live.

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