What the sun’s spectrum tells us


Psst, I have a confession to make. Don’t tell anybody, but sometimes it’s hard to see the value of what astronomers do.

For example, astronomers know with some certainty that the sun and all the other stars are thermonuclear furnaces, gigantic hydrogen bombs erupting with the power of trillions of our hydrogen bombs every second.

We depend on the sun for the energy that keeps us alive every second of our lives, but here’s the thing. The sun will keep producing its energy whether we know how it functions or not. Why bother?

The sun had been merrily percolating along for 4.6 billion years when in 1859, the German physicist Gustav Kirchhoff pointed his spectroscope at a Bunsen-burner flame. He was trying to find out what the sun was made of.

Come now, Herr Kirchhoff! Does it matter what the sun is made out of or exactly how it produces its energy?

Just four years earlier, French philosopher Auguste Compte had scoffed at the suggestion that anyone could possibly discover the chemical composition of stars such as the sun. They were, after all, very far away. What were scientists going to do — scoop up a cup of stars for analysis?

Some people’s objections were more practical. Even Kirchhoff’s banker got into the act.

“Suppose you do discover gold on the sun,” he said. “Of what use is that gold if it cannot be brought down to Earth?”

Kirchhoff knew that light can be broken down into its component colors, its spectrum, with a prism. Practically, everyone has seen the spectrum of the sun. When sunlight passes through moisture in the air in just the right way, a rainbow results. The rainbow is the spectrum of the sun.

Kirchhoff used a fancy prism called a spectroscope, which breaks down the spectrum into many discrete lines of color. By heating individual elements over a flame, he found out that each one had a spectral signature, a unique set of color lines that could be used to identify that element.

For example, the spectral signature of sodium consists of two bright lines in the yellow part of its spectrum. (Streetlights often have a yellow cast to them because the glowing gas inside them is sodium-based.)

When Kirchhoff looked at the spectrum of the sun, he was surprised to see two dark lines where the sodium lines were supposed to be.

Did that mean that the sun had no sodium in it? No, he reasoned. The heat and light from the sun probably came from deep inside it. The hot part of the sun was thus surrounded by an atmosphere of cooler gases. As the sunlight passed through sodium vapors in the sun’s outer atmosphere, the cool sodium gas absorbed the sodium part of the light, leaving the dark lines.

Further experiments verified his theory. Amazingly, he had discovered a way of determining the chemical composition of any object that glowed and had a cooler atmosphere around it.

It turns out that the sun is made up of 75 percent hydrogen and 25 percent helium with scant traces of other elements like sodium.

Somehow that hydrogen was being used as fuel to produce energy. It couldn’t be burning. The hydrogen would have been used up after only 50,000 years or so.

And thus it was that less than 100 years later, the explosion of the first hydrogen bomb annihilated an anonymous atoll in the South Pacific.

We have lived under the threat of thermonuclear annihilation ever since. No one said that pure scientific knowledge didn’t have unintended and even catastrophic consequences.

Even at that moment in history, Kirchhoff’s discovery was world-shattering. It was a true scientific revolution. Hidden in all light is its spectrum, and its spectrum tells us much about what the universe was made of. And it didn’t matter how far away an astronomical object was as long as it put out enough light to see with a telescope.

Buried below the surface of things is much information that we cannot hear, taste or see. Kirchhoff had reached up with his spectroscope and touched the stars.

However, it took a clever application of technology to make it bear fruit.

Over time, the simple prism has evolved into the more complex spectroscope. Telescopes have gotten larger and far more capable of gathering light in wavelengths beyond what we can see with our eyes: the infrared, the ultraviolet, microwaves, x-rays and gamma rays. And our knowledge about how our universe has exploded like a hydrogen bomb.

In the last few decades, we have come to appreciate the effects of spiraling advances in technology. In just my lifetime, communication has grown from a clunky dial-based contraption screwed to the wall to communication by regular humans at great distance. And the contraption that does it fits into the palm of your hand, but it is rife with unintended social consequences.

Kirchhoff’s discovery in 1859 was a first, giant step that has spiraled in the same way to our enormous knowledge about the universe.

In honor of his work, Great Britain awarded Kirchhoff a medal and some cash, paid to him in gold coins.

Don’t get me wrong. Kirchhoff knew what all scientists ought to know — that the value of knowledge is not in any monetary reward, that the struggle for knowledge about the universe is deeply a part of the human psyche. We can’t help ourselves. We must know these things.

Even so, it’s hard to blame him for what he did next. When he visited his banker to deposit his handful of golden coins, he couldn’t help saying with ironic pride, “Here is gold from the sun!”


By Tom Burns


Tom Burns is director of the Perkins Observatory in Delaware.

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