Spectral lines
Go out and look at any star in the nighttime sky, and you are looking at an incomprehensibly huge hydrogen bomb. Go ahead. Look. I’ll wait.
That tiny speck of light is a thermonuclear inferno. It’s apparent puniness is wrought purely by the enormity of its distance from Earth. The sun, our daystar, is much closer. The energy streaming along its 93-million-mile distance will damage your vision permanently if you give it more than a glance.
Hydrogen bombs are, of course, made up primarily of hydrogen, which the high temperatures and densities at the star’s core convert to helium. They are indeed the primary components of the sun and all its brother and sister stars. You should be amazed that humans know the chemical elements of the sun to a certainty. After all, we cannot go out and scoop up a cup of star to study its chemical properties. The discovery of the method by which we find out is one of the great triumphs of science. That profound process resulted from the work of many scientists over hundreds of years. Let’s set the wayback machine for almost 350 years ago and find out what happened.
The first clue came in 1665 when Sir Isaac Newton took a triangular piece of glass called a prism and split the sun’s light into colored bands. Apparently, the way we see light is an illusion. It is really composed of different bands, or wavelengths, of color.
In 1802, British chemist William Wollaston noticed that these color bands are not continuous. Dark gaps existed between some of the colors. That observation was perhaps the most important of all time for astronomical research, but Wollaston did not realize its significance.
Using better equipment than Wollaston’s, German physicist Joseph von Fraunhofer discovered that not only is the spectrum of light not continuous, but that it is composed of hundreds of discrete bands. He rediscovered the mysterious dark bands, which eventually were named after him — the Fraunhofer lines.
In 1857, a German chemist, Robert Bunsen invented a gas burner that produced a nearly colorless flame. (Perhaps you used a Bunsen burner in high school chemistry.) The advantage of the device was that when a scientist burned a chemical with Bunsen’s device, the light produced could not be confused with the light from the chemical. Scientists could now see what colors chemicals produce when they are heated.
A co-worker of Bunsen’s, Gustav Kirchhoff, studied the colored bands produced by such burning chemicals and found that their spectra are not continuous — in spades. Each pure substance produced only a few colored bands. Each chemical now had a color fingerprint, and scientists could analyze that color pattern to find out what anything was made out of.
But Kirchhoff went further in his study. He set up containers of various pure substances and shone light through them. He discovered that the substances absorb exactly the same color bands that they would have emitted if they had been burned. Those absorbed colors show up as black lines in the spectrum. Wollaston’s mysterious dark bands had finally been explained.
It didn’t take long for astronomers to exploit that technique. Swedish physicist Anders Angstrom noticed that the dark spectral lines produced by the sun exactly matched those of light that shines through a container of hydrogen. Thus, explosions beneath the sun’s surface probably produce its light. As the light shines through the non-burning outer part of the sun, hydrogen absorbs some of the color bands. Hydrogen must be present in the sun.
Over the years, astronomers have refined their equipment and given us a pretty clear notion of what the sun is made out of. About 3/4’s of it is hydrogen, and most of the rest is helium. It also has trace amounts of oxygen, carbon, neon, nitrogen, magnesium, iron, and silicon. Most of the rest of the elements that make up the universe are also present in very tiny amounts.
Similar results have been obtained for most of the rest of the stars and galaxies of stars that have been studied. And it all started when Newton decided to play with a prism.
Knowledge of the universe doesn’t come from a single moment of insight. Understanding builds slowly over time. As Isaac Newton wrote, “If I have seen further, it is by standing upon the shoulders of giants.” How lucky we are to be standing on his broad shoulders.
Tom Burns is the director of Ohio Wesleyan University’s Perkins Observatory. tlburns@owu.edu.
