960 years ago a brilliant new object appeared in the constellation, Taurus. This "guest star," as Chinese astronomers called it, was four times brighter than Venus and was visible both and night and during the day.

All of humanity with a view of the Northern Sky likely saw it - the brightest thing in the sky aside from the sun and the moon. Astronomers from China, Japan, Iraq, Europe, and what is now the US southwest all recorded the seemingly new star which gradually faded after its conspicuous appearance on July 4th, 1054 until its disappearance in late spring of 1056.

(0:28) But it didn't disappear from the human view forever. With the advent of telescopes, we were able to see it again by the mid 18th century - but by then, it looked very different. Today, we know that the celestial phenomena of 1054 was a supernova, and its blast created a massive cloud of debris and gas that contains some of the most studied astronomical objects in history: the Crab Nebula and its pulsar, M1.

We've come a long way in the last 900 years, but there's a still a lot we don't know about the supernova that lit up the daytime sky. 

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(0:58) The supernova of 1054 is actually about 7,500 years old; at 6,500 light years away, the light from the star (and therefore the news of its explosion) just didn't reach us until the 11th century. 

Supernovae in the observable universe are actually very common, but within our galaxy the Milky Way, they only happen about every 40-80 years, and even these are too dim or distant to see with the naked eye; 1054's was the brightest of only eight historically observed supernovae.

(1:25) And, you can still see its nebula, the brightest such observable object, with binoculars at night. This is what's left of the ex-star's atmosphere, a cloud of ionized helium, hydrogen, and other simple gases.

And it centers the neutron star M1. Because it's spinning while it's emitting a beam of electromagnetic radiation, it's called a pulsar. This is the tiny, extremely dense core left over from the massive star. 

In its previous incarnation, M1 was about 10x the size of the sun, when its enormous gravitation caused all of its mass and energy to collapse into an area only 20km in diameter. Then, the force of all that compressed energy pushed back, exploding the star's atmosphere into an area that became the Crab Nebula, leaving the tiny, dense pulsar behind. 

(2:01) The Crab Nebula is bright and huge; about 10 light years in diameter. To put that in perspective, the entire solar system is 1-2 light years in diameter, depending on where you draw the edge. And, the Crab Nebula is still expanding, at a rate of about 1500km/second - this is actually how we know it's from the supernova of 1054.

20th century astronomers - including Edwin Hubble and Jan Oort - compared pictures of the Nebula taken at different times to calculate how quickly it was expanding; then, they worked backwards to the date of the explosion: 1054.

(2:30) That math and the fact that the nebula is in the same spot as the legendary "guest star" implied a connection. But some astronomers later called that connection into question, arguing that the nebula seemed to be expanding too fast to be associated with the 1054 date. 

As it turns out, in 2007, Dartmouth College astronomers realized that the Crab Nebula's rate of expansion was actually accelerating. Experts now think that a growing bubble of high-energy particles coming from the pulsar is what's driving the acceleration - which was making the supernova harder to date. 

(3:00) But one more riddle about the remnants of the 1054 supernova remains: the combined mass of its pulsar and nebula is only a fraction of what it should be, adding up to significantly less than the predicted mass of their parent star. 

To account for this missing mass some scientists theorize that the star's own stellar wind (a non-stop torrent of particles that stars emit) blew away its outer layers before the star even exploded. It may sound strange, but astronomers have recently confirmed the phenomenon in rare stars known as Wolf-Rayet, super-massive stars that generate winds so powerful that they rip their own atmospheres to shreds.

So a lot has changed in 960 years, but in many ways the supernova of 1054 is as fascinating and strange to modern astronomers as it was to the ancient ones. 

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