“In the first period Chih-Ho, the fifth moon, the day chi-ch’ou, a guest star appeared. After more than a year it became invisible”. So wrote an ancient Chinese observer on the fourth of July, 1054. This guest star was seen not far from the moon and near what we now call the constellation Taurus. This guest we now call the Crab nebula.
The Crab nebula is the remnants of an exploding star, or supernova, which was first observed in 1054 AD by Chinese astronomers. It is located near the southern horn of the constellation Taurus and was seen to be four times brighter than Venus. The records show that it was visible during daylight hours for 23 days and visible in the night sky for 653 days. There is also a possibility that the Crab nebula was recorded by Anasazi Indians in what is now Arizona and New Mexico as drawings from that period suggest.
The astronomer John Bevis rediscovered this supernova remnant in 1731 and Charles Messier also located it in 1758 as he was looking for comets. The name “Crab” was given to the nebula by Lord Rosse in about 1850 and was based on a drawing that he made of the supernova remnant. It was discovered to be still expanding in 1921 and even today it is expanding at a phenomenal rate of 700 miles per second.
It has become the most studied nebula due to its range of radiation emission over the entire electromagnetic spectrum and in the 1960s a pulsar at the core of the nebula was discovered. This pulsar, thought to be the collapsed remnants of the supernova, flashes about 30 times a second in radio waves as well as visible, x-ray and gamma frequencies. This pulsing is the force that allows the supernova to keep on expanding at such a startling rate. The pulsar is a neutron start that has collapsed upon itself and contains the mass of the Sun yet is only a few kilometers in diameter.
It is amazing to think that the Crab nebula is ten light years across yet has this small driving pulsar, the size of a city, at its core. Also the fact that the explosion was seen during the daylight hours tells us of the magnitude of this event. The waves emitted from the nebula reach across vast expanses of the universe and can be used to give us information about our own solar system. The corona of the Sun passes in front of the Crab nebula every June and from the difference in waves received from the Crab, astronomers can gain more insight into the density of the corona. Very rarely Saturn will pass in front of the nebula and the shadow of Titan, Saturn’s moon, formed from the emitted waves allow astronomers to measure the thickness of Titan’s atmosphere.
So we can see that the Crab nebula has become a vital tool in measuring and understanding our own solar system and beyond. Yet, even with all our technology there is still so much mystery about space and about the Crab nebula itself, first discovered by the Chinese without any of this technology.
