Special lines in the spectrum

¡SkyCaramba! Weekly astronomy blog for the week ending August 22, 2015

Just as science learned to make bigger and better magnifying lenses for telescopes, science also learned to make bigger and better prisms and diffracting glasses for breaking light into its component colors. And just as better telescopes revealed more detail in the things above, better prisms and diffracting glasses revealed more too.

When you see a rainbow, you see all the colors present in sunlight. For convenience, we say all the colors that can exist are present: red, orange, yellow, green, blue, and purple. But in the 1850s, scientists using the best diffracting glasses and prisms of the time noticed some dark lines in it. The spectrum also had dark lines when they shined light through various gases. The gases were absorbing those wavelengths. Scientists found that they could identify gases by where the dark lines were.

Scientists also found that light from the atmosphere around the sun has an opposite appearance. The light from it appears in narrow wavelengths, and the rest of the spectrum is dark. Noting the placement of these emission lines also turned out to be a very useful way of identifying gases.

Soon, scientists knew that the sun contains hydrogen, calcium, and sodium. They began to identify those and other gases in the dark lines of other stars’ spectra. But two emission lines from the sun’s corona didn’t correspond to any gases they had found on Earth.

Pierre Janssen, a French astronomer, discovered the first of the new gases while observing a solar eclipse in 1868. Since scientists couldn’t find the gas with the same yellow emission anywhere but in the sun, they used the Greek word for sun to name it. From helios came the word helium.

The following year, Charles Augustus Young and William Harkness observed another total solar eclipse. They found a green emission line that had never been seen before. Because it occurred in the sun’s corona, it was named coronium.

Scientists eventually found helium on Earth. Small amounts of it are trapped in mineral deposits. Helium is lighter than every element except hydrogen. And helium atoms don’t combine with other atoms to weigh them down. So when helium escapes into the air, it rises to the very top of the atmosphere. Industry has learned to gather enough helium to harness its lighter-than-air property in balloons.

Scientists also learned something about coronium. It wasn’t a new material at all. Rather, it was something familiar rendered in an unfamiliar way. Temperatures in the sun’s corona are so high, iron ionizes in ways scientists hadn’t been able to reproduce on Earth until the 1930s. When it does, it emits green light at exactly the wavelength Young and Harkness noted.

This is one of many stories to demonstrate that in science, what you see can be as important as what you don’t. In the early part of the 1800s, scientists learned that sunlight is more than just the visible spectrum. In the middle part of the century, they learned that it’s also a little less.