Spectral lines
About 98 percent of the cosmos is composed of the two simplest elements, hydrogen and helium, mostly in the form of giant gas clouds and stars.
The smarter kids who come to our programs at the “O” sometimes fold their arms and belligerently ask, “How do you know that?”
Ah, yes. A skeptic. Good. That’s the way scientists are made.
That simple discovery resulted from the work of many scientists over hundreds of years.
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. 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. It is not just a torture device used to flunk students in beginning 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 those colors 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 as they would have emitted if they had been burned.
These 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 this 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 its surface probably produce the light from the sun. 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 three-fourths 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.
Knowledge of the universe doesn’t come from a single moment of insight. It is built 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.
Planets
Venus still shines brightly in the west just after sunset. During late April and May, the closest planet to Earth will be making its death dive toward the western horizon, sinking lower each day as the month progresses. Ironically, each day Venus also shrinks to a thinner and thinner crescent, making it even more attractive in a small telescope. In fact, by the end of May, the crescent will be easily visible in binoculars.
The sharpest eyes among you may be able to see the crescent with the binoculars you were born with — your own two eyeballs. Children are far more capable than old folks like me because their eyes aren’t so worn out. Such a view can be a life-changing experience. Take your kids out and look.
Mars is high in the south just after dark hovering on the belly of Leo, the Lion.
Saturn still sits to the left of the bright star Spica in the constellation Virgo, the Virgin.
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