The outside gas of the sun goes far beyond the photosphere. Since they are transparent to most visible radiation & emit small amount of light, these outer layers are difficult to see. The area of the solar atmosphere directly above the photosphere is called the chromosphere. Until this century, the chromosphere was only visible when the photosphere was obscured by the moon during a total solar eclipse.
In the 17th century, various observers described what appeared to them as a narrow red “streak” or “fringe” around the edge of the moon over a period of time. Shortly after the sun’s photosphere was covered. This red line was given the name chromosphere, derived from the Greek “colored sphere”.
Observations during eclipses show that the chromosphere is approximately 2,000-3,000 kilometers thick and its spectrum consists of bright emission lines, suggesting that this layer is made up of hot gases that emit light at discrete wavelengths. The reddish color of the chromosphere arises from one of the strongest. Emission lines in the visible part of its spectrum, the bright red line caused by hydrogen, the element that, as we have already seen, dominates the composition of the sun.
In 1868, observations of the chromospheric spectrum showed a yellow emission line that did not correspond to any previously known element on Earth. The scientists quickly realized that they had found a new element and named it helium (after Helios, the Greek word for “sun”). It was not until 1895 that we discovered helium on our planet.. Students today probably know it best as the light gas used to inflate balloons, although it turns out to be the second most abundant element in the universe.
The chromosphere temperature is around 10,000 K. This means that the chromosphere is hotter than the photosphere, which should seem surprising. In all situations we know of, temperatures drop as one moves away from the heat source and the chromosphere is further away. from center of the sun than photosphere is.