This is the mess that is left when a star explodes.
– Astronomy Picture of the Day: 17 February, 2008
'Beauty is in the eye of the beholder' could have been written to highlight the difference between a professional astronomer's 'mess' and the amateur astronomer's gaze of awesome wonder and appreciation of the astonishing magnificence that is the Crab Nebula.
The Guest Star
On 4 July, 1054, Chinese astronomers observed a fantastic new object in the night sky: they called it 'the guest star'. It was 20 times brighter than any other star, around −6 magnitude, and was visible even during daylight hours for a few weeks. This 'new star' was actually a supernova, the massive explosive death of a gigantic star, now catalogued SN 1054. We now know that it occurred 6,300 light years1 away, but ancient astronomers had no way of knowing that.
Although this new star would have been visible to everyone on the Earth, only the Chinese and Arabs made meticulous records at that time. There is some evidence that the Anasazi2, who lived in what is now north-west New Mexico, painted a representation of (what could be) the supernova at Chaco Canyon. The artwork is a handprint just above a crescent moon, with a ten-ray star to the left of the moon. Beneath this is what could be Halley's Comet, which followed the supernova 12 years later, or it could be another comet. This is speculation as there are no written records, but it is fascinating nevertheless.
William Parsons, 3rd Earl of Rosse
Today we call the supernova remnant the 'Crab Nebula' after the drawing sketched by William Parsons, 3rd Earl of Rosse, at Birr Castle around the year 1844, although it more resembles a pineapple than a crustacean (unless you compare the sketch to a South Pacific 'yeti crab')!
The York-born Earl Rosse was President of the Royal Society from 1848 to 1854. Unfortunately his son Laurence, the 4th Earl, rated the 1855 sketch by RJ Mitchell 'the best representation' of the mysterious object, but the nebula carries the moniker coined by his father.
The Crab Pulsar
We know that the original star in Taurus which exploded to create the Crab Nebula was supermassive because only these sized stars go supernova. Smaller stars don't have such a glorious bowing-out stage, they usually just run out of gas (astronomers think this is what will happen to our Sun). What remains of the exploded star itself is what is termed a neutron star of super-dense3 material which is rapidly rotating. Such remains are called 'pulsars': they generate beams of X-rays and gamma radiation. The cosmic lighthouse Crab Pulsar, an active source of radio waves, spins 30 times a second, creating the energy required to cause the surrounding gas to glow. Voila, one stunning vista; and thanks to modern technology like space telescopes, we get to ogle the eye-candy that is the Crab Nebula.
Dr John Bevis
English amateur astronomer Dr John Bevis (1695 - 1771) discovered the supernova remnant in 1731 and wrote it up as a nebula for his Uranographia Britannica (English Celestial Atlas). Although the star atlas was being prepared for publishing from 1748, Dr Bevis never saw his work in print because the publisher went bankrupt. The printing plates were used to create the posthumous catalogue in 1786.
The Messier Connection
French astronomer Charles Messier (1730 - 1817), the 'comet ferret', identified the 'elongated nebula' independently on 12 September, 1758, while he was searching for the return of Halley's Comet (which duly appeared as predicted in 1759). He listed it as M1 in his famous catalogue of 'non-comets', Mémoires de l'Academie:
Nebula above the southern horn of Taurus, it doesn't contain any star; it is a whitish light, elongated in the shape of a flame of a candle, discovered while observing the comet of 1758. See the chart of that comet, Mem. Acad. of the year 1759, page 188.
Dr Bevis wrote to Messier around 1760 to claim the earlier discovery, which Messier acknowledged in a later publication of his own catalogue:
M1 observed by Dr Bevis in about 1731. It is reported on the English Celestial Atlas.
Dr Bevis and Messier exchanged letters until 1771 when a fall from his telescope fatally injured Bevis just four days short of his 76th birthday.
The New General Catalogue was compiled by John Louis Emil Dreyer, the director of the Armagh Observatory from 1882 to 1916. Dreyer listed M1 as NGC 1952. Other designations carried by the Crab Nebula are CM Tauri and Taurus A.
Recorded for Posterity
The Crab Nebula was first photographed in 1892, and further observations led it to be classified as a planetary nebula in 1918. However, the correct classification of supernova remnant was registered in 1933. It has expanded somewhat since the original explosion, at the extremely high velocity of 1,800km/sec. At this rate, the roadrunner would circle the Earth every 22 seconds.
Speaking of animation, in 1973, four decades after its origin was correctly identified, a photograph was taken from the Kitt Peak Observatory on the Tohono O'odham Reservation, Arizona, USA, and 28 years after that another corresponding image was made. Those two images were superimposed and uploaded as an animated picture by the Astronomy Picture of the Day website owners. The rate of expansion during the last quarter of the 20th Century can clearly be seen. Current estimates of its overall size vary between nine and ten light years.
How and Where to View
The Crab Nebula can be viewed with a suitably-strong optical aid like a telescope. Without a telescope the Crab Nebula is about 8th magnitude and can be seen with binoculars, although at best it is just a slight blur of white light, it is possible if you give yourself time for your eyes to adapt to the darkness. The galactic co-ordinates are Right Ascension: 5h 34m, Declination: +22° – or, if you stargaze via recognised constellations, you'll need to be looking in the direction of Taurus, the Bull.
If you find the bright reddish-coloured star Aldebaran (alpha Tauri, the famed 'eye of the bull') south of the Pleiades, then follow a line towards the feet of Gemini, the next brightest star is zeta Tauri. The Crab Nebula is within the same binocular field just one degree (the width of two full moons) to its north. You may have to exclude zeta Tauri out of the field of view to get the best condition to see it.
In April 2011 the Crab Nebula was the source of a gamma-ray emission – the highest-energy form of light. It was only registered due to the advance technology of the Fermi space observatory, and scientists are perplexed by the activity. At the Kavli Institute for Particle Astrophysics and Cosmology, USA, researcher Rolf Buehler said that such things had not been recorded before 'because there was not an instrument like Fermi sensitive enough to capture it'. Some ideas are being bandied about, including a magnetic field theory, but it will take a lot of study to reach an accurate conclusion.
The find is a testament to the power of the Fermi telescope to elucidate new physics in the cosmos.
– Fermi project scientist Julie McEnery
Image courtesy of NASA