The discovery of the Double Helix

By playing with pieces of cardboard cut into the shapes of the 4 chemical components that make up DNA, 2 scientists called Francis Crick and James Watson were able to work out how the pieces fitted together in pairs. From this discovery in 1953 it only took them a day or two to make a Meccano like model, perhaps the most famous piece of work in modern science, the model was made up of metal plates bolted together in a spiral. It was without question a brilliant piece of detective work, for it they were awarded the nobel prize.

Crick and Watson decided that if you could find out the shape of a DNA molecule you'd be able to see how it did what it did. The shape is rather like a spiral staircase:the famous double helix:

Double helix DNA is actually very simple. It has just 4 basic components- which is like having a alphabet of only 4 letters. 

The components pair up in  particular ways to form rungs and the order in which they do this as you move up or down the ladder forms the DNA code. Because you can combine them in different ways, like you do with the dots and dashes of morse code, you end up with 3.2 billion letters of coding, enough to provide a number of possible combinations that is almost impossible to imagine (10 x 10 1,920,000,000 times if you really want to know).

DNA is not itself alive, no molecule is, but DNA is especially 'unalive'. Thats why it can be recovered from patches of long-dried blood in murder investigations, and coaxed from ancient bones to date prehistoric people.

DNA exists for just one reason: to create more DNA-and you have a lot of it inside you, nearly 2 metres squeezed into almost every cell. In fact you may have as much as 20 billion kilometres of DNA.

Age of the earth

By the 1940's scientists were at last getting close to dating the earth. One scientist, Willard Libby was busily inventing radiocarbon dating, a process which would allow scientists to get a accurate dating of the age of bones and other organic remains, something scientists had never been able to do before.

Libby's idea was based on the realisation that all living things contain a special kind of radioactive carbon called carbon-14, which begins to decay at a steady rate as its atoms start to die. Since half of the atoms in carbon 14 decay over a period of 5,600 years, this is known as its half life. So Libby was able to work out how much carbon-14 was left in any dead organism and get a good idea on its age. But there was a fault, Libby could only this for objects up to about 40,000 years old. 

In fact there were lots of problems with carbon dating, and with every other technique that followed it to find the final definite age of the earth.  Even the best of these  couldn't date anything older than about 200,000 years. But most of all, these techniques couldn't date inorganic materials like rocks, which is, of course, what you need to do to if you want to determine the age of your planet.

So it was left to a man called Clair Patterson to come up with the solution. He began work on the project in 1948, making very precise measurements of the lead-uranium ratios in specially selected rocks. These had to be rocks that were extremely ancient and contained lead- and uranium bearing crystals that were about as old as the planet - anything much younger would misleadingly youthful dates. But the problem was that rocks that old were rarely found on earth. 

finally it occurred to him that he could get around the rock shortage by using rocks from beyond earth. He turned to meteorites. The assumption he made- rather a large one, but correct was that many meteorites are left over building materials from the early days of the solar system, and thus more or less in their original state. Measure the age of these rocks and you would also have the age of the earth.

It took Patterson 7 years of patient work just to find and measure suitable samples for final testing. By this time, he had his specimens, containing minute quantities of uranium and lead locked up in ancient crystals- and he was able to tell the world that the definite age of the earth was 4,550 million years ( plus or minus 70 million years) in 1953. The earth finally has a age.

Pulsar Stars

In 1976 Jocelyn Bell and Antony Hewish discovered the first pulsar star, a type of neutron star. Pulsars are rapidly rotating neutron stars which  have very strong magnetic fields. Beams of radiation are emitted from their magnetic-polar regions, and we can detect these if the earth is within the path of the beam.


 pulsar stars were discovered by accident, when Bell and Hewish were looking for twinkling sources of radio radiation. The explanations from the radio pulses proved the existence of neutron stars, incredibly dense remains of massively collapsed stars. Bell and Hewish were tracking the twinkling sources of radiation using a radio telescope. The telescope produced around 30 meters of chart paper each day, the data was then analysed by Jocelyn Bell. She quickly learned to recognise scintillating sources. But two months into her observations Bell became aware of a bit of scruff on the records. The new signal didn't look like it came from any scintillating source or a man made source. When Bell checked back on the records she found the scruff had occurred before, but always from the same patch of sky. This strange new signal deserved more attention so Bell starting making faster chart recordings on the area the signal had appeared. For weeks she found nothing, then finally at the end of november the signal appeared on the fast recording. the scruff was a series of pulses equally spaced and 1.3 seconds apart, 1.3 seconds is far to fast for a pulsation rate for something as large as stars so this regular signal must be man made. But when Hewish checked the records he found the position of the source meant it must be coming from outer space. Hewish and Bell considered various explanations, it wasn't radar reflected from the moon, also it wasn't a strange satellite of course. It wash't equipment failure because another telescope had also picked up the signal. Calculations showed it was well out of the solar system as well. The team measured the duration of the pulses and they found the time was only 16 milliseconds. The short duration suggested the source could be no larger than a small planet. If the radio waves were coming from another planet their frequency would shift as the planet orbited the alien sun. The frequency would be higher when it would be coming towards us and lower when it moves away. Hewish made measurements to see if the signal could come from another planet, but the measured changes were only there due to the movement of our own planet around the sun. The radio astronomy group were not sure as to how to publish their discovery. It seemed highly unlikely that the signals came from aliens but no one had a better answer. If the team had discovered alien life, who should they tell first. Bell was quite annoyed that the supposed aliens had chosen his telescope, his frequency. But when bell  returned to his analysis he found another scruff in a completely different part of the sky. Bell went out to the telescope that night and found it was another pulsing signal, slightly faster with pulses every 1.2 seconds. Now she was reassured that the pulses had some natural explanation, its very unlikely that two lots of aliens would both choose the same telescope at the same time trying to signal the same planet. By January Bell and Hewish had discovered 4 of these pulsing sources . They wrote a paper describing the source and sent it to journal nature, where it was  published on the 24th February, 1968. Thus pulsar stars were discovered. 

   

 when a star collapses on itself it ether produces a neutron star, a black hole, a dwarf star or a pulsar star

Black holes

Sadly black holes are not portals to other universes. Anyway you can't really see black holes because they are so powerful they suck in light itself, but scientists can record them by studying the effect they have on things around them. Black holes are incredibly dense points in space where the gravity is so strong all the laws of physics you have learned or will learn at school will get thrown out of the window. 

After some stars have been through a supernova, their cores collapse in on themselves so much that they form a single, incredibly dense point in space called a singularity. The space immediately surrounding the singularity is called a black hole. most scientists believe that gravity would crush anything that entered a black hole out of existence.

A cloud of gas or a star passing a black hole within the event horizon (boundary where the black hole's force reaches) will be torn apart by the black holes gravity. As matter is pulled sharply towards the black hole, it speeds up and rubs against other matter, generating friction. This creates incredibly hot temperatures as high as 1 billion degrees . This gives of lots of x rays which can be detected as they travel through space. This is another way to detect black holes.

Finally, and coming to the point, the first black hole to be discovered was Cygnus X-I in 1971. It is thought to be 30 to 60 kilometres in diameter, yet has the mass of as many as 10 suns.