What element was first discovered in the sun's spectrum?

The element first discovered in the Sun's spectrum was helium. Astronomers detected its signature as an unfamiliar bright yellow spectral line during a total solar eclipse on August 18, 1868, before helium had been found on Earth.

French astronomer Jules Janssen observed the line while studying the Sun's chromosphere from Guntur, India. Later that year, English astronomer Norman Lockyer observed the same feature and concluded that it belonged to an element unknown on Earth. Lockyer named it helium, from helios, the Greek word for Sun.

The discovery was unusual because spectroscopy identified the element remotely, through the wavelengths of light it emitted. The yellow line was near sodium's well-known spectral lines but did not match them. The new element's existence on Earth was confirmed in 1895, when William Ramsay extracted helium from the uranium mineral cleveite; Swedish chemists Per Teodor Cleve and Abraham Langlet independently isolated it that year.

Helium is now recognized as the second-lightest element and a noble gas. It is chemically unreactive and is used in cryogenics, leak detection, scientific instruments, and some lighter-than-air applications. Hydrogen is more abundant in the Sun, but helium was the first element identified there before its terrestrial discovery.