Court astronomers looked up one morning and saw something that had no business being in the sky.

In the Song dynasty, court astronomers in China monitored the celestial area that is currently known as Taurus and observed an unexpected occurrence. A star appeared in a location where a star was not previously located and it was bright enough for people to see during the day. As they did not possess telescopes or a theory for this event, they used eyes that were trained over many years to identify changes in the sky.

On official documents, they noted this guest star because it appeared without warning plus was visible during the day for 23 days. At night the star was bright enough for observers to view it for over one year and it disappeared from sight after approximately two years. And because astronomers in Japan and the Islamic world also wrote about the object, historians have multiple separate records to verify the date of the observation.

With those records, it is clear that the observers saw a star as it became inactive. A star with large mass reached its end but also fell inward because of its gravity, which caused a supernova explosion. Due to the power of the explosion, the object was brighter for a short time than any other star. By this process the star released a large quantity of energy and produced light that people on Earth were not able to explain at that time - but their decision to record the data provided scientists with a specific date for a significant supernova.

After multiple hundred years passed, astronomers directed telescopes toward the same region of space and observed that material continued to emit light there. Gas and dust from the initial event moved outward to form an irregular cloud which people call the Crab Nebula. It received this name because it looks similar to a crab when people use early telescopes to see it. At the center of the nebula is a neutron star that rotates approximately 30 times each second - this object is the dense material that remains from the original star and it emits regular pulses of radiation into space.

As time passes modern astronomers continue to observe the Crab Nebula with care. They use it as a location where they can learn how stars cease to exist and what materials remain after that process. For this research to be significant, scientists require the records that court officials wrote one thousand years ago - those authors did not have the concepts of neutron stars or supernovae but they attempted to describe an object that was present in an unexpected location. When they watched a star end its life, they did not understand the nature of the event. Due to their detailed observations, modern science possesses a specific date to connect a large physical event in the galaxy to one particular morning.