On August 18, 1868, French astronomer Pierre Janssen (1824–1907) observed a total solar eclipse in Guntur, India. Two months later on October 20, 1868, English astronomer Joseph Norman Lockyer (1836–1920) observed the solar chromosphere in daylight using a specialized spectroscope.

Both scientists independently discovered a brilliant yellow emission line ($\lambda = 587.49\,\text{nm}$) in the solar spectrum that belonged to no known terrestrial element. Lockyer named the new element Helium (from the Greek Helios, meaning “Sun”).

Discovery of the $D_3$ Spectral Line

Prior to 1868, the yellow doublet in the solar spectrum was attributed to sodium ($D_1 = 589.6\,\text{nm}, D_2 = 589.0\,\text{nm}$).

The $D_3$ Line Discovery

Janssen and Lockyer observed a bright yellow line near the sodium doublet at $\lambda = 587.49\,\text{nm}$, designated $D_3$.

Because $D_3$ matched no element in terrestrial chemical laboratories, Lockyer and chemist Edward Frankland concluded that $D_3$ represented a unknown element existing in the solar atmosphere:

$$\text{Helium } (\text{He}, Z=2)$$

Helium remained an “extraterrestrial element” until 1895, when William Ramsay isolated helium gas on Earth from the uranium mineral cleveite.

Quantum Significance & Cosmic Abundance

Helium is the second most abundant element in the Universe ($24\%$ of cosmic baryonic mass).

In quantum mechanics, Helium ($\text{He}, 1\text{s}^2$) is the lightest two-electron quantum system, exhibiting superfluidity ($^4\text{He}$ below $2.17\,\text{K}$) and forming the alpha particles emitted in radioactive quantum tunneling.

Key Takeaways

  • Year: 1868
  • Key Figures: Pierre Janssen & Norman Lockyer
  • Core Discovery: Discovered Helium ($Z=2$) by identifying the $D_3$ solar spectral line ($587.49\,\text{nm}$).
  • First Extraterrestrial Element: Discovered in the Sun 27 years before terrestrial isolation.
  • Quantum Relevance: Second light element in primordial Big Bang nucleosynthesis and quantum superfluidity ($^4\text{He}$).