Wonders of the Universe

WHAT ARE STARS MADE OF?

Over a simple campfire I recreated the experiments of Gustav Kirchhoff and Robert Bunsen that made such a major impact in the development of quantum theory. Just as they discovered 150 years ago, when I threw the copper into the fire it burned with a spectacular blue flame.

The Sun, the burning star at the heart of our solar system, is 150 million kilometres (93 million miles) away from Earth. Beyond that, the nearest known star, the red dwarf Proxima Centauri, requires a journey of over four light years or forty thousand billion kilometres (twenty-five thousand billion miles). We have learnt a lot about Proxima Centauri since it was discovered by Robert Innes at the Cape Observatory, in South Africa, in 1915. It is thought that Proxima Centauri is part of a triple star system with its neighbouring binary star system, Alpha Centauri A and B, and although it cannot be seen with the naked eye, we have been able to measure its mass and diameter and chart its brightness across the last 100 years. Despite the fact that our only contact with these neighbouring stars, and with any star other than our Sun, is the light that has crossed the Universe to reach us, we have been able to go much further than simply cataloguing their vital statistics. We can measure the precise constituents of any and every visible star in the sky, because encoded in the light that rains down on Earth is the key to understanding what they are made of. It is all made possible by a particularly beautiful property of the elements.

The tale of how we learnt to read the history of the stars in their light began with the work of Isaac Newton in 1670. In his ‘Theory of Colour’, Newton demonstrated that light is made up of a spectrum of colours, and that with nothing more complicated than a glass prism you can split the white light of the Sun into its colourful components. Almost 150 years later, the German scientist Joseph von Fraunhofer made a startling discovery about the solar spectrum whilst calibrating some of his state-of-the-art telescopic lenses and prisms. Lying within the solar spectrum, Fraunhofer documented the existence of 574 dark lines; there were literally hundreds of gaps – missing colours in the Sun’s light. Unaware of the significance of this discovery at the time, Fraunhofer carefully mapped their positions in great detail. He went on to discover black lines in the light from the Moon and planets, and from other stars. These are now known as Fraunhofer lines.

Further work by two more of the great German scientists of the nineteenth century, Gustav Kirchhoff and Robert Bunsen (perhaps best known to schoolchildren everywhere as the inventor of the Bunsen burner), finally gave meaning to these lines. They surmised correctly that these black spectral lines were the fingerprints of the chemical elements in the atmosphere of the Sun itself. Across 150 million kilometres (93 million miles) of space, the light of our star had carried the signature of its constituents to us.

Kirchhoff and Bunsen’s discovery was purely empirical – they had observed that when gases are heated on Earth they do not simply glow like a piece of hot metal, they give off light of very specific colours – and interestingly those colours depend only on the chemical composition of the gas and not on the temperature. In particular, each chemical element gives off its own unique set of colours. The element strontium, for example, burns with a beautiful red colour, sodium with a deep yellow, and copper is a haunting emerald green.

The two German scientists also noticed that the missing black lines in the solar spectrum corresponded exactly to the glowing colours of the elements. There are, for example, two black lines in the yellow part of the Sun’s light that correspond exactly to the two distinct yellow emission lines of hot sodium vapour. You will be familiar with this mixture of two very slightly different yellows – it is the colour of sodium streetlights.

Interestingly, Kirchhoff and Bunsen had no idea why the elements behaved in this way, but this didn’t matter if all you wanted to do was to match the signatures of elements observed on Earth with the signatures in the light from the Sun and stars. It wasn’t until the turn of the twentieth century that an explanation for this strange behaviour of the elements was discovered. The answer lies in quantum mechanics, and the spectrographic work of physicists and chemists such as Kirchhoff and Bunsen was a major motivating factor in the development of the quantum theory. Elements emit and absorb light when the electrons surrounding their atomic nuclei jump around. The key insight that led to quantum theory was that electrons can’t exist anywhere around a nucleus like planets around a star, but they are instead placed in specific, very restrictive ‘orbits’. The deep reason for this is that electrons do not always behave as point-like particles of matter. They also exhibit wave-like properties, and this severely restricts the ways in which they can be confined around the atomic nucleus. What happens at a microscopic level when an atom absorbs some light is that an electron jumps to a different, more energetic, orbit and it emits light when the electron falls back from a higher to a lower energy orbit. The difference in energy between the lower orbit and the higher orbit must correspond exactly to the energy of the light absorbed or emitted.

In the early nineteenth century, German scientist Joseph von Fraunhofer documented the existence of 574 dark lines within the solar spectrum. This diagram is a visual representation of these Fraunhofer lines.

Spectographic investigations have revealed that Sirius, the dog star, is metal-heavy, with an iron content three times that of the Sun.

Isn’t it simply wonderful that just by looking at the light from those twinkling stars we can tell what those fiery worlds, so far away, are made of?

Although Polaris, the pole star (top and middle), is 430 light years away, we know by looking that is has about the same heavy element abundance as our sun, but markedly less carbon and a lot more nitrogen. Vega (bottom), meanwhile, as the second-brightest star in the northern sky, consists of only about a third of the amount of metals as our sun.

However, quantum theory also stipulates that light should not always be thought of as a wave. Just like electrons, light can behave as both a wave and a stream of particles. These particles are called photons. Now, here is the key point: photons of a particular energy correspond to a particular colour of light, so red photons have a lower energy than yellow photons, which have a lower energy than blue photons. Since each element has electrons in unique orbits around the nucleus, this means that each element will only be able to absorb particular photons in order to move its electrons around into higher energy orbits. Conversely, when the electrons drop from higher to lower energy orbits, they will only emit photons of a particular energy and therefore a very particular colour. This is what we see when we observe the elements emitting or absorbing particular colours of light. We are in a very real sense seeing the structure of the atoms themselves.

When looking at a spectrum of light from our sun you can see hundreds of Fraunhofer lines, and each and every one of those corresponds to a different element in the solar atmosphere which absorbs light as it passes through. From sodium in the yellow, through iron, magnesium, and all the way across to the so-called hydrogen alpha line in the red, the signatures of each of the elements are encrypted in the solar code.

So by looking at these lines in precise detail you can work out exactly which elements are present in the Sun. This turns out to be roughly 70 per cent hydrogen, 28 per cent helium, and the remaining 2 per cent is made up of the other elements.

It is worth repeating here that you can apply this theory not only to the Sun, but for any of the stars you can see in the sky – which allows us to measure the constituents of their atmospheres with extraordinary accuracy. Isn’t it simply wonderful that just by looking at the light from those twinkling stars we can tell what those fiery worlds, so far away, are made of?

These spectrographic investigations of the light from the cosmos have confirmed what our scientific intuition suggested to us: wherever we look, we only ever see the signatures of the set of ninety-four naturally occurring elements that we have collected and identified here on Earth.

So it is clear that we are connected in a very real sense to the whole of the Universe – with its hundreds of billions of stars across billions of galaxies – because we are all intrinsically made of the same stuff. And, as we will explain, there is one very simple reason for that: everything in the Universe shares the same origin