Alchemical Furnaces of the Cosmos

by Nicholas Mee on September 21, 2017

Arthur Eddington was raised in a Quaker family in northern England. After studying in Manchester, he won a scholarship to Cambridge University and by the 1920s he established himself as the world’s leading astrophysicist. Eddington developed models of the structure and evolution of stars that form the foundations of the subject even today.

The key to these models is the vast amount of thermal energy generated within a star that supports it against its tendency to collapse under gravity. But the source of all this energy was a complete mystery.

Feeding the Stellar Furnace

Einstein and Eddington

Eddington was an enthusiastic advocate of Einstein’s theory of relativity. One of Einstein’s most profound insights was the equivalence of mass and energy, as expressed in his famous equation:

E = mc2 ,

where E represents energy, m represents mass and c is a constant, the speed of light. Eddington realised this equation implies that converting mass into energy would provide an unprecedented power source and this might explain what keeps the stars shining. In 1920, Eddington wrote prophetically: ‘If, indeed, the subatomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfillment our dream of controlling this latent power for the well-being of the human race—or for its suicide.’

Eddington had an idea for how this energy might be released in stars. Accurate new measurements showed that the total mass of four atoms of the lightest element hydrogen is slightly greater than an atom of the second lightest element helium. Eddington reasoned that if there were some way to transform hydrogen into helium, then the difference in mass would be released as energy. This was a remarkable proposal, as nuclear physics was in its infancy, it was just ten years since the discovery of the nucleus and three year since the discovery of the proton. Nevertheless, Eddington suggested that the energy of the stars might be due to the conversion of hydrogen into helium.

But there was a serious problem with the idea. Kirchhoff and Bunsen’s spectroscopic methods had been used for decades to detect elements in the sun and stars. Astronomers assumed that elements with prominent lines in the solar spectrum, such as calcium and iron, were the major constituents of the sun, so it appeared that the sun was composed of similar elements to the Earth. Indeed, the eminent astronomer Henry Norris Russell stated that if the Earth’s crust were heated to the temperature of the sun, its spectrum would look almost the same. The composition of the Earth’s crust by mass is: oxygen (46%), silicon (28%), aluminium (8%), iron (6%), calcium (4%), with lesser quantities of the other elements. The proportion of hydrogen is less than 0.15%. If Russell and his colleagues were correct, then Eddington’s proposal looked rather improbable, as there would be comparatively little hydrogen in the sun. This issue would be resolved by an astronomer who was inspired by Eddington, but was severely hampered throughout her career by the institutional sexism of the time.

Undoubtedly Brilliant

Times report on Eddington’s eclipse expedition of 1919.

Cecilia Payne won a Cambridge University scholarship in 1919 to study botany, physics and chemistry. Eddington had recently returned triumphant from an eclipse expedition to Principe in West Africa organised to test whether light bends in a gravitational field as predicted by Einstein’s theory of general relativity. The expedition found good evidence that Einstein was correct and Eddington was keen to promote his theory as the greatest scientific breakthrough since Newton. This publicity propelled Einstein towards the superstar status that he would enjoy for the rest of his life. When Payne heard Eddington’s captivating account of the expedition she was spellbound. She said of the lecture, ‘The result was a complete transformation of my world picture. My world had been so shaken that I experienced something very like a nervous breakdown.’

Payne later nervously approached Eddington and told him she wanted to be an astronomer. He encouraged her and invited her to use Cambridge Observatory’s library where she could read the latest astronomy journals. She completed her studies at the university, but was not awarded a degree, as the university would not confer degrees on women until 1948. Payne realised that she would be unable to pursue an academic career in England, so she left for the United States where she obtained a fellowship that would fund her graduate studies in astronomy at Harvard University.

Cecilia Payne.

Payne studied the spectral lines of the sun and showed how to determine its composition from its spectrum. She demonstrated that, although the relative abundance of elements such as carbon, silicon, iron and other heavy elements is similar in the sun and the Earth, this is not the case for the two lightest elements hydrogen and helium which are vastly more abundant in the sun. All the evidence suggested that the sun and other stars were essentially huge balls of hydrogen and helium and the other elements were present in tiny proportions. Payne submitted her doctoral dissertation describing this research in 1925. It has been described as ‘undoubtedly the most brilliant PhD thesis ever written in astronomy’.

Initially, some astronomers expressed their doubts and Henry Norris Russell even persuaded Payne to include a warning suggesting there might be some mistake in the measurements. Within four years, however, he was convinced and the astronomy community followed his lead. Modern values for the elemental composition of the sun by mass are as follows: hydrogen (71%), helium (27%), with just 2% shared out between all the other elements, leading with oxygen (1%) and carbon (0.4%), and with much smaller quantities of others elements. Payne’s discovery of the composition of the sun and stars was fundamental to understanding the physics of the stellar furnace.

Payne made many important contributions to astronomy throughout her career, but struggled to receive the status that should have been her due. It was not until 1956 that she was appointed to a full professorship at Harvard.

Keeping the Lights On

We now know that Eddington was correct. The vast energy output of the sun is produced by the gradual conversion of hydrogen into helium. The precise nuclear mechanisms by which this happens were worked out by Hans Bethe and Charles Critchfield in the late 1930s. Hydrogen fusion is the process that powers the stars for at least the first 90% of their lives, a period known as the Main Sequence.

The rate at which stars burn their fuel depends on their mass. The greater the mass of a star the hotter its core and the faster the nuclear reactions occur. It will take the sun around ten billion years to burn the hydrogen fuel in its core.

Stars of less than half the sun’s mass are known as red dwarfs. A star with a mass of one tenth of the sun will fizzle away for 10 trillion years, which is one thousand times longer than the age of the universe. Most of the stars in the galaxy are rather feeble red dwarfs.

A star with twenty times the mass of the sun will use up its nuclear fuel in just ten million years. Twenty times as much fuel is burnt in one thousandth of the time, so energy is released at 20,000 times the rate of the sun and the star will shine 20,000 times as bright as the sun.

Cosmic Destiny

When the hydrogen nuclear fuel in the core has been converted into helium, the core contracts and if the star’s mass is great enough its temperature rises until helium fusion begins. Helium is converted into carbon and oxygen in the core, and meanwhile the outer layers of the star swell up to form a bloated red giant. One such star is Betelgeuse, the shoulder of Orion the Hunter. In five billion years time the Earth will be engulfed by the outer layers of the sun as it approaches the end of its life.

In a star such as the sun these outer layers will eventually disperse into space to reveal the star’s core as an extremely dense glowing ember about the size of the Earth. The nuclear reactions in the core will have ceased and the core will gradually cool as it radiates its heat into the depths of space. These stellar remnants are known as white dwarfs. They are extremely hot, but very faint as they are so small.

The diameter of Betelgeuse is about 1000 times that of the sun, so it would stretch as far as the orbit of Jupiter. It is so large that astronomers have even imaged its face. Betelgeuse is quite unstable and varies in luminosity quite noticeably. It is belching large amounts of gas into space, as can be seen in the photograph below.

Zoom in to Betelgeuse. Credit: ESO, P.Kervella, Digitized Sky Survey 2 and A. Fujii.

Stellar Alchemy

In very massive stars, like Betelgeuse, further fusion processes will forge heavier atoms, such as neon, sulphur, silicon and iron. But eventually no new nuclear fusion reactions are possible and the final collapse begins. At this point so much energy is released that the star blasts itself apart in a supernova explosion which may be as bright as an entire galaxy of 100 billion stars. In this inferno even heavier atoms are created and dispersed into interstellar space to form gas clouds that eventually coalesce into the next generation of stars. Our bodies and all else around us are formed from atoms forged in the alchemical furnaces of the cosmos.

Further Information

A video version of this post is now available on The Cosmic Mystery Tour YouTube Channel here: The Cosmic Mystery Tour. Please don’t forget to subscribe to the video channel.

There is a lot more information about stellar evolution and supernovae in my book Higgs Force: Cosmic Symmetry Shattered.

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