Breakthrough Study Accurately Measures Helium in Sun’s Photosphere for First Time

This limitation has led to long-standing assumptions and uncertainties regarding the precise ratio of Helium to Hydrogen (He/H) in the Sun's surface layer.

In a remarkable scientific breakthrough, researchers from the Indian Institute of Astrophysics (IIA), Bengaluru, have successfully derived the most accurate estimate to date of the abundance of Helium in the Sun’s photosphere—marking the first time such a direct and precise measurement has been achieved. This development holds the potential to revolutionize our understanding of stellar compositions, especially in Sun-like stars, and fine-tune models of solar opacity and radiation transfer.

The Challenge of Measuring Helium in the Photosphere

Helium, the second most abundant element in the Sun after Hydrogen, poses a unique observational challenge. Unlike many elements, Helium does not produce visible absorption spectral lines from the Sun's photosphere, the outermost layer of the solar atmosphere visible to the naked eye. Traditional methods for estimating Helium content have thus relied on indirect techniques such as solar seismology, solar wind analysis, or extrapolation from hotter stars—none of which provide a direct measure from the Sun's photosphere itself.

This limitation has led to long-standing assumptions and uncertainties regarding the precise ratio of Helium to Hydrogen (He/H) in the Sun's surface layer. Typically, astronomers have assumed a He/H ratio of 0.1, but until now, this value lacked confirmation based on direct observations.

A Novel Spectroscopic Approach

In their study published in the Astrophysical Journal, researchers Satyajeet Moharana, B.P. Hema, and Gajendra Pandey from IIA present a pioneering technique that circumvents this observational limitation. The researchers analyzed high-resolution solar spectra by focusing on elements that do produce visible spectral lines—namely, neutral Magnesium (Mg I) and Carbon (C I), as well as their hydrogenated molecular forms (MgH, CH, and C₂).

"Using a novel and consistent technique, whereby the spectral lines of neutral Magnesium and Carbon atoms in conjunction with the lines from the Hydrogenated molecules of these two elements are carefully modelled, we are able to constrain the relative abundance of Helium in the Sun’s photosphere now," said Moharana, who is currently pursuing a Ph.D. at the Korea Astronomy and Space Science Institute (KASI), South Korea.

How the Method Works

The team performed detailed calculations of line formation physics and subjected their data to Equivalent Width (EW) analyses and spectrum synthesis. Their approach hinges on the principle that the atomic and molecular features of Magnesium and Carbon must independently yield the same elemental abundance—assuming a specific Hydrogen abundance.

Since Helium affects the overall Hydrogen content by virtue of being the second most abundant element, any deviation in the assumed Helium abundance will affect molecular formation and, consequently, the derived elemental abundances of Mg and C from both atomic and molecular lines.

"For example," Moharana explained, "if Helium was assumed to be slightly more abundant, this would proportionately decrease the abundance of Hydrogen, which will decrease the opacity of the Sun’s photosphere and reduce the formation of Hydrogenated molecules with Magnesium and Carbon."

In such cases, the molecular absorption features weaken, and to match observed spectra, an increase in elemental abundance is required. By testing different He/H ratios and checking for consistency in Magnesium and Carbon abundances across atomic and molecular lines, the researchers could pin down the ratio that best fit all observations.

Findings and Significance

The team found that their derived Helium-to-Hydrogen ratio (He/H) was consistent with the traditionally assumed value of 0.1, aligning closely with results from independent helioseismological studies.

"Our derived He/H ratios are in fair agreement with the results obtained through various helioseismological studies, signifying the reliability and accuracy of our novel technique in determining the solar helium-to-hydrogen ratio," said co-author B.P. Hema.

By providing the first observationally grounded estimate of Helium in the Sun’s photosphere, this study not only validates a long-held scientific assumption but also opens new avenues for improving stellar atmosphere models, understanding energy transport in stars, and even refining the inputs for simulations of solar evolution.

Broader Implications and Future Work

This method could now be extended to other Sun-like stars, potentially revising estimates of elemental abundances across stellar populations and helping astronomers better model stellar formation and evolution. It also offers a more accurate basis for calibrating radiative opacity models, which are crucial for interpreting stellar light curves and internal structures.

The success of this study showcases the power of innovative spectroscopic techniques and highlights the crucial role of precise modelling in unlocking the mysteries of the cosmos.