In a flash! Science

Bright lines on a dark background

View of a total solar eclipse is seen in Longyearbyen on Svalbard on March 20, 2015.   | Photo Credit: Nesvold, Jon Olav

Have you seen a total solar eclipse? Even if you haven’t experienced it first hand, you would have at least seen a lot of pictures of the same. Though the moments of totality experienced through centuries add up only to a small amount of time, the research and progress in this field has been phenomenal.

Eclipse spectroscopy has been the reason behind the remarkable rise of this branch of astronomy. Spectroscopes are optical instruments that are used to measure the properties of light over a portion of the electromagnetic spectrum. Even though it was still a new invention, astronomers took to it and started employing them in their trade.

Spectroscopy in astronomy

They began using it to observe the solar corona in 1868. Prominences (large, bright feature extending outwards from the sun’s surface) gave a bright line spectrum, proving that they are gaseous in nature.

Helium was discovered in the sun in 1869 and a green line was observed in the spectrum, which proved to be unaccounted for in the decades that followed. It was only after nearly 70 years that the green line and others found in the coronal spectrum were proved to indicate temperature of the order of 1 to 2 million degrees.

It was during this time that Charles Augustus Young, who had changed his plans to become a missionary and later became a professor of mathematics and natural philosophy in 1856, accepted the Appleton professorship at Dartmouth in 1866. It was here that he started his serious research in astronomy, along with a powerful ally in the form of a spectroscope, made possible owing to the Appleton fund.

Flash of brilliance

Young undertook many eclipse expeditions, including the one that took him to Jerez, Spain on December 22, 1870. And while observing the total eclipse through his spectroscope, Young made his discovery.

Bright lines on a dark background
At the instant when the photosphere was completely covered up by the moon, Young observed bright spectrum lines on a dark background, wherein a moment ago there had been dark lines on a bright background. The swiftness of the change, which happened once at the start and once again at the end of totality and lasted perhaps for a second or two, prompted Young to name the bright line spectrum as the flash spectrum.

A rare phenomena in observational astronomy, it was this research that also led Young to announce the “reversing layer” of the sun. A layer of relatively cool gas (when compared to the very hot photosphere) forming the lower part of sun’s chromosphere, the reversing layer gives rise to the dark absorption lines in the sun’s spectrum.

Studying the flash spectrum over the years has enabled us to gain valuable insight about the physical state of the solar chromosphere, the lowest layer of the sun’s atmosphere.

Reach the writer at ganesh.a.s@thehindu.co.in


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Printable version | Oct 19, 2021 12:17:03 AM | https://www.thehindu.com/in-school/sh-science/bright-lines-on-a-dark-background/article8013759.ece

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