Tag Archives: 21 cm hydrogen line

The whole flipping universe in 1951 …

The whole flipping universe in 1951 

A brand new window opened up on the universe in 1951. And it was all thanks to the Hydrogen 21- centimetre line.

Yes folks – you heard it here first. Well, strictly speaking you didn’t. But I think you know what I mean.

You are probably wondering what  the Hydrogen 21-centimetre line actually is. There is really no easy way to explain it but I’ll have a go. You may prefer to walk away now. I wouldn’t blame you. Go and order a curry perhaps. Descale the kettle maybe. Clean out the fluff filter on your tumble drier. Walk the dog. That sort of thing.

Still here. Well done you!

All matter emits radiation in the form of waves. This radiation or energy is transmitted on different frequencies or wavelengths on the electromagnetic spectrum. (see diagram below) A small part of this spectrum is made up of visible light which we can detect with our eyes. But most radiation is invisible and requires other means to tune into it. This includes radio waves, microwaves and infrared. Some waves are just a few billionths of a metre in length – X-rays for example. At the other end of the scale are long-waves which can exceed 1000 metres.

All the elements emit radiation and the most common of these is Atomic hydrogen. It makes up about 75% of the observable matter in the Universe.

A hydrogen atom is made up of a proton with an electron spinning round it. It will radiate a small amount of energy when the electron ‘flips’. Atomic hydrogen atoms only do this once every 10 million years. But there are just so many of them out there that it seems they’re flipping at it all the time.

During the 1930s, it was noticed that there was a radio ‘hiss’ that varied on a daily cycle and appeared to be extraterrestrial in origin. After initial suggestions that this was due to the Sun, it was observed that the radio waves seemed to propagate from the centre of the Galaxy. These discoveries were published in 1940 and were noted by Jan Oort who knew that significant advances could be made in astronomy if there were emission lines in the radio part of the spectrum. He referred this to Hendrik van de Hulst who, in 1944, predicted that neutral hydrogen could produce radiation at a frequency of 1420.4058 MHz or on a 21cm wavelength which is in the microwave region of the spectrum due to two closely spaced energy levels in the ground state of the hydrogen atom. At the time there was no device capable of detecting this happening.

Receiver and mixer of the Hydrogen 21-cm line in 1951

The 21 cm line (1420.4 MHz) was first detected in 1951 by Ewen and Purcell at Harvard University, and published after their data was corroborated by Dutch astronomers Muller and Oort, and by Christiansen and Hindman in Australia. After 1952 the first maps of the neutral hydrogen in the Galaxy were made, and revealed for the first time the spiral structure of the Milky Way and put Radio Astronomy on the map.

The horn antenna first used to detect radiation from the 21 cm hydrogen line in 1951

Ewen and Purcell built their detecting equipment, which consisted of a horn antenna and a mixer and receiver, in their spare time with a grant of $500. The whole project, from receipt of the $500 to detection of the line, took one year. Since the work was done on the weekends, the total time spent working on the project was (2/7)x12 = 3.4 months. 

Interestingly, the Large Hadron Collider took 10 years to build with the combined efforts of 10,000 scientists and engineers from over 100 countries at a cost of 4.5 billion dollars.

So congratulations if you are reading this final bit. You made it through. I applaud you. The Hydrogen 21 cm line applauds you. The entire flipping universe applauds you. Drinks all round then.