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A star is born

Updated Wednesday, 26th May 2010

Dr Alan Cooper tells the story of the pioneering discoveries in physics and astrophysics 100 years ago, from Newton and Einstein, to the birth of cosmology.

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Breaking down walls

There are times in physics and astrophysics when much of the scientific edifice has to be allowed to fall apart in order for a far stronger structure to appear in its place. Perhaps the most important such period was in the second half of the 17th century, when Aristotelian doctrine was discarded, and an immensely fruitful period of experiment-based science took its place.

This revolutionary direction was led by English natural philosopher Robert Hooke and Anglo-Irish natural philosopher, chemist and physicist Robert Boyle. This era also marked the arrival of The Royal Society, a learned society for science and a place for research and discussion. This fertile ground paved the way for Sir Isaac Newton’s mathematical treatment of gravity, but he was important in a rather different way. He invented (in modern terms) mathematical modelling to extend the application of the rules revealed experimentally. The next such leap forward came at the start of the 20th century, with the beginnings of cosmology and quantum mechanics.

The story of cosmology

Discoveries rarely occur in the best order for telling the story, so I will pretend I am a ‘supreme ultimate editor’ who can make the important research papers appear at the optimum time.

We can take the state of understanding in 1890 as the beginning of the story.
An aging William Gladstone was due to become Prime Minister for the fourth time, and a flamboyant Edward VII succeeded his still grieving mother Victoria at the turn of the century. The blaze of Victorian engineering brilliance had faded. Nevertheless it was a good time for science. But science was still largely a personal quest, the first international physics conference was still 20 years in the future. Europe just about held the lead for ideas, but the money was in the USA.

Heroic efforts had been made by the Herschel family, notably Sir Frederick William Herschel, in the construction of larger and better telescopes, and new types of objects had been seen. But the time and distance scales of the Universe were now in disarray. Disagreement is usually constructive in science; it means there are discoveries to be made.

Tackling the problem of time

Assuming the common belief was that the energy of the sun came from burning, its lifetime could not be longer than hundreds of thousands of years. But this meant that it was impossible to fit geological evolution into such a short time. Cosmology was stuck and some walls had to crumble. The ‘wall’ was Newton’s Laws, respected and revered, and very useful for 200 years. For the solution I need to bring in Einstein’s paper on Special Relativity, which reconstructs Newton's calculations for moving bodies so that the speed of light is always the same. This leads to the famous equation, E=mc2, which as a fastidious editor, I will insist that he rewrites as:

∆E=∆mc2 (∆ meaning change in)

So that by giving up a small mass (∆m), I can exchange it for a huge amount of energy (E), because the exchange rate is a whopping c2 = 9 x 1016 joule/kilogram (30,000 million kilowatt-hours per kilogram), where c2 is the square of the speed of light.

With this much energy available, the sun will shine far longer than had been thought possible – for thousands of millions of years in fact. This is plenty of time for geological evolution and, as we now know, a substantial fraction of the age of the universe. This is the energy that we all rely on, whether from the sun or from nuclear reactors. Einstein’s paper does another, more subtle thing, it upgrades c (the speed of light) to the status of a ‘physical constant’, the same everywhere and at all times. We could say that c governs kinematics. Only if such constants exist can we hope to understand the universe, and it seems, fortunately, as if only a few are needed.

Far, far away

Next I must bring in a paper for the distance scale. To advance cosmology observationally, we need to see the universe in 3D and not just its projection on the sky. Distances as far as the nearest few stars can be estimated by triangulation methods similar to those of a terrestrial surveyor. But the following step, to the galaxies, needs to be completely different. The method is to recognise an individual star in another galaxy as a type studied in a known galaxy, the Milky Way. Clearly it needs to be a very bright type of star to be seen well at such large distances.

This is an opportune time for the Hertzsprung-Russell diagram. The observed classification of stars had progressed well. Studies of a bright, variable type of star called Cepheids, notably by American astronomer Henrietta Leavitt, showed that the period of the variation was very simply related to the energy output (absolute luminosity). This was perfect; if the astronomers could have created stars to use in cosmology, they would not have come up with anything better. Once a Cepheid is found, the period (easily measured) gives its luminosity, and the much weaker luminosity as seen from Earth (apparent luminosity) tells you its distance.

An example of a Hertzsprung-Russell diagram Copyrighted  image Icon Copyright: Nature (93) (7 May 1914) p.252
An example of a Hertzsprung-Russell diagram
[Nature (93) 7 May 1914, p.252]

To exploit this elegant method needs a large telescope. Before that time the biggest telescope was Herschel’s 1790 telescope, 40ft long and with a 4ft diameter mirror. He paid for it himself, made most of it in wood, and the operating staff were him and his sister, Caroline. This is admirable, but future advances would need to be on a bigger scale and funded by generous sponsors and government grants. The arrival of the 100-inch Hooker Telescope and the skill of American astronomer Edwin Hubble found the distance to the Andromeda galaxy using a Cepheid variable. This marked the beginning of the long task of setting cosmological distance scales.

100-inch Hooker telescope Copyrighted  image Icon Copyright: AndrewDunn via Wikimedia
100-inch Hooker telescope
[Image copyright: Andrew Dunn – CC-BY-SA licence]

However, cosmology is about the evolution of the universe. If it is evolving by expanding, the movement causes the frequency of the light received to be lower than at emission (this has no effect on the speed of light). Hubble found and measured these frequency shifts (called Doppler shifts). Of course he had to know the frequency of emission, but fortunately we can recognise spectra in a familiar galaxy as those of substances known and measured on Earth (usually hydrogen).

All the galaxies Hubble used were redshifted, receding from us, at speeds proportional to their distances. The cosmology they were looking for was that of an expanding universe. The speeds gave a rough but direct estimate of the age of the universe and it was compatible with the lifetimes of stars, estimated from the time to use their fuel. The contradictions in time and distance scales had been removed.

That ends my imagined job as editor; we have reached the 1930s and observational cosmology had been created. Today, the search has narrowed to a very special type of expansion, and the nature of the 'big bang' which might have started it.

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