To answer the question, what are stars made of, we firstly need to look at the Sun, the great burning star that lays in the heart of our solar system and sits just 150 million kilometres (93 million miles) away from Earth. After that, the nearest star that is known is the red dwarf Proxima Centauri. This requires a journey of over four light years or forty thousand billion kilometres (twenty-five thousand billion miles).
Nearest star after the Sun
Since Proxima Centauri was discovered back in 1915 by Robert Innes at the Cap Observatory, South Africa, we have come to learn a great deal about the star. It is believed to be part of a triple star system with its neighbouring binary star system, Alpha Centauri A and B, and even though it can not actually be seen with the naked eye, we have still been able to measure its mass and diameter and chart its brightness across the last 100 years. Despite this limited contact with these neighbouring stars, and any other star than our own (the sun), is just their light that has travelled over the universe to reach us. The precise constituents of every star visible to us in the sky can be determined because of what’s encoded in the light that travels to earth. All made possible by a particularly beautiful property of elements.
Theory of Colour
This tale of how we learnt how to read the history of stars in there light all began with the work of Isaac Newton in 1670. In his “Theory of Colour”, Newton demonstrated that light consists of a whole spectrum of colours, and with something as simple as a glass prism you can split the white light of the Sun into its colourful components. Nearly 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.
Defining the lines
Further work by two more of the great German scientists of the nineteenth century, Gustav Kirchoff 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.
Chemicals and colours
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 correspond 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, Kirchoff 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 signature of elements observed on Earth with signatures in the light from 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 physicist and chemists such as Kirchoff 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.
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, which have 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 move its electrons around into higher orbit. 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.
Elements of the Sun
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 in made up of the other elements.
Any star you see
You can apply this theory not only to the Sun, but for any of the stars you can see – 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?
Re-occurrence of the elements
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.
We have a clear connection
So it is clear that we are connected in a very real sense to the whole 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 extremely simple reason for that, everything in the universe shares the same origin.