A Strange Occurrence

People have been looking up at the night sky for thousands of years. There are records from ancient Chinese and Arabic scholars in the Middle East. People have found and translated these records, discovering clues to modern-day mysteries. One of these accounts was of a bright object suddenly appearing in the sky.

Chinese Astronomers recorded that a ‘guest star’ about as bright as the full moon suddenly appeared on 4th July 1054 (Left an impression of what it may have looked like). In fact, it was so bright that it could be seen during the day for several weeks and was then visible at night for about two years before disappearing. It wasn't just the Chinese that saw this. There are records from Japan through to the Middle East. Some people even think that a piece of rock art in North America depicts the event.

What was going on? What could cause an object to suddenly appear in the sky and then disappear? Was it a bad omen? (The Chinese didn’t think so, but the appearance coincided with the plague in the Middle East, so the Arabs were a little more annoyed). Perhaps aliens were watching us!

We're going to move on to the 18th century. By then, the astronomical telescope had been developed, and people were using them to study the night sky. One of these was Charles Messier, who was interested in comets and was getting frustrated. He kept observing other objects which looked like comets but were nothing of the sort! He kept track of these objects by cataloguing them, hoping to prevent astronomers from confusing them. The first entry was a fuzzy patch in the constellation of Taurus, which he named M1. He eventually compiled a catalogue of over a hundred Messier objects, which is ironic considering he always said he didn't want anything named after him.

As time passed, telescopes got better. In the 19th century, William Parsons could sketch the nebula (left), showing some of the details of its structure. As people thought it looked like a crab, M1 became known as the Crab Nebula. Do you think it looks like a crab? Personally, I’m getting hints of a flea!

The field of astrophotography developed when astronomers devised ways to attach cameras to telescopes. They realised it was expanding by taking photos of the Crab Nebula over time (below). 

Could they calculate when this object appeared by working backwards? Their estimate was sometime during the 11th Century. Interestingly, the position of the nebula was the same as where the Chinese saw the guest star.  Was the bright object a supernova? Could the nebula be what remains of an exploding star? Exciting stuff, but this isn't the end of the story! In 1968 astronomers in South America were using a radio telescope to study the sky. They found a radio signal that 'flashed' 30 times a second, a pulsar. The first of these had been discovered just the year before in the UK, but what are pulsars? 

Pulsars belong to a group of objects called neutron stars. These are the remains of massive stars whose atoms have collapsed, turning them into a ball of neutrons. They are so dense that a teaspoon of this material weighs billions of tonnes (that’s a lot of elephants). ‘Massive’ in science means ‘containing lots of mass’, not ‘huge’. Would it surprise you that the Crab Pulsar is only about 20 km across? Pulsars have a strong magnetic field and spin incredibly quickly, emitting electromagnetic radiation from their magnetic poles.  

Pulsars appear to flash because as it spins, the beams point towards the earth, like the light of a lighthouse. Why was the discovery of this pulsar exciting? The radio signal was coming from the centre of the Crab Nebula. Were there the remains of the star that exploded in 1053? Absolutely!

Since then, many more pulsars have been discovered, all in the centre of nebulae, which adds to the evidence that stars can be born and die. Some spin hundreds of times a second, so we must slow recordings down to hear their pulses. We have discovered flashes at a different rate for each pulsar, though how the beams are produced is still a mystery. 

 The Crab Nebula today.

In the 1970s, two space probes, Pioneer 10 and Pioneer 11, were launched to study the outer solar system. They were designed to fly past some outer planets and take measurements. The mission designers knew that the probes would eventually leave the solar system. Heading into interstellar space. Where would they end up? Would someone find them? Could we use them to send a message? How do we design a message that aliens would understand? It was decided to place two plaques on the spacecraft containing information about the origin of the probes. (Right)

Images were used to represent scientific ideas in the hope that these could be interpreted by anyone.  One image is a star-like diagram showing pulsars' relative distance and pulse rate.  These distances and rates aren’t going to change anytime soon; the Crab Pulsar is so reliable that it has been used to calibrate instruments on telescopes. Our solar system is in the centre of this ‘star’ diagram, with the pulsars acting as beacons to show the way. Other diagrams on the plaque showing where the earth is and what we look like. Perhaps we will be visited by Little Green Men after all.

To find out more about the life cycle of stars, click here.