On 4 July 1054, astronomers in Song dynasty China recorded a new point of light near the southern horn of the constellation we call Taurus. They called it a guest star, the term they used for a star that appeared where none had been. It was bright enough to be seen in full daylight for 23 days, and it remained visible at night for around 21 months before fading. The expanding debris from that explosion is still spreading across the sky, and we now know it as the Crab Nebula.

The user-facing version of this story is mostly right. A few details are worth getting exactly so.

What the records actually say

The Chinese records are the most precise, and they survive in official histories such as the Song Shi and the Song Huiyao. They place the guest star near Tianguan, the star we label Zeta Tauri, give its first appearance in July 1054, and note that it stayed visible for more than a year before vanishing. You can read translations of the surviving Song dynasty passages, which are brief and matter-of-fact in the way of administrative astronomy.

The Chinese were not the only ones. Japanese records describe the same event, differing slightly on the dates. A Middle Eastern account is attributed to the physician Ibn Butlan, who placed a new star in Taurus that year. Native American rock art in the American southwest has been suggested as a possible record, though that reading is speculative. What stands out is the near-silence in Europe: contemporary European notices of the event are sparse and disputed, despite an object that anyone looking up would have seen.

One detail often gets inflated. The guest star is sometimes said to have been as bright as the full Moon. The better estimate, drawn from the recorded colour and visibility, is that it peaked at roughly four times the brightness of Venus, brilliant enough to show in daylight but well short of the Moon. The careful reconstructions of the event settle on that lower figure; the full-Moon version seems to have come from trying to force the brightness to fit older light-curve models.

When the star actually exploded

It is worth being precise about “a star exploded in 1054.” What happened in 1054 is that the light reached Earth. The Crab Nebula sits about 6,500 light-years away, so the explosion itself occurred roughly 6,500 years before that, and the light had been travelling ever since. The date in the Chinese records is the date of arrival, not the date of the event.

This is not a quibble for its own sake. It is the difference between describing what was seen and describing what occurred, and the records can only ever tell us the first.

How we know the guest star is the Crab

The link between the 1054 record and the Crab Nebula was not known to anyone at the time. It was reconstructed in the twentieth century.

The nebula itself was catalogued long before its origin was understood, and became the first entry in Charles Messier’s catalogue of objects that were not comets. The crucial step came in the early 1900s, when photographs of the nebula taken several years apart showed that it was visibly expanding. Carl Lampland reported this expansion in 1921. Running the expansion backwards gave an age of around nine centuries, which pointed to an origin in the eleventh century, and the recorded guest star of 1054 sat in the right part of the sky. By 1939, the astronomer Nicholas Mayall argued that the Crab was the remnant of a supernova. Later positional work, including a 1983 analysis in Nature by Clark, Stephenson, and Crawford, tightened the match between the historical record and the nebula’s location.

So the identification rests on three things lining up: the position in the sky, the rewound age, and the historical date. None alone would be decisive. Together they are convincing.

The expansion that sped up

Here the reconstruction reveals something the simple story misses. When you trace the nebula’s current expansion straight back, it does not land on 1054. It lands a few decades later. That gap is not an error in the records. It means the nebula has been expanding faster over time, not coasting outward at a steady rate.

The cause sits at the centre. The Crab holds a pulsar, a rapidly spinning neutron star left behind by the collapse, turning about thirty times a second. It pumps energy into the surrounding gas and magnetic field, and that input has been pushing the filaments outward, accelerating an expansion that would otherwise be slowing. The nebula is now spreading at roughly 1,500 kilometres per second and measures about 11 light-years across.

The Crab remains one of the most closely watched objects in the sky, observed across radio, optical, X-ray, and gamma-ray wavelengths, in part because it is a rare case where a remnant can be tied to a dated event in the historical record. The wreckage is still moving, slightly faster each year, almost a thousand years after the light first reached a group of court astronomers who wrote down what they saw and moved on.