New Study Links Helium Enrichment to Lithium-Rich Cool Giants, Revealing Stellar Evolution Clues

Helium, the second most abundant element in the universe after hydrogen, plays a central role in determining the physical and chemical characteristics of stars.

A groundbreaking study by astronomers at the Indian Institute of Astrophysics (IIA), an autonomous institute under the Department of Science and Technology (DST), has uncovered a compelling link between lithium-rich red giant stars and enhanced helium abundance in their outer layers. Using high-resolution observations from the Himalayan Chandra Telescope (HCT) in Hanle, Ladakh, along with archival data from major observatories around the world, the researchers present new evidence that could reshape current understanding of how cool giant stars evolve during the red giant phase.

Why Helium Matters in Stellar Evolution

Helium, the second most abundant element in the universe after hydrogen, plays a central role in determining the physical and chemical characteristics of stars. However, measuring helium abundance directly in cool stars remains a major challenge. In stars like the Sun or cooler red giants, the surface temperatures are not high enough to excite helium atoms into producing detectable spectral lines. As a result, astronomers rely on indirect techniques—evaluating how helium affects stellar structure, internal mixing, atmospheric composition, and the abundances of other elements.

One of the key scientific principles used in such analyses is the interlinked nature of hydrogen and helium abundances. Any reduction in the hydrogen content of a star’s atmosphere typically implies a proportionate increase in helium. This delicate interplay becomes especially important when astronomers attempt to decode a star’s chemical fingerprints.

A Novel Method to Estimate Helium in Cool Stars

In the new study, the IIA team refined a previously developed technique for estimating helium abundance by measuring hydrogen depletion. The method relies on comparing magnesium levels derived from two different spectral sources: atomic magnesium lines (Mg I) and molecular magnesium hydride bands (MgH). If these two measurements diverge, it points toward a hydrogen deficit—and consequently, a helium enhancement.

This innovative technique had earlier been used by the same research group to estimate the Sun’s helium abundance. For the present work, they applied the method to 20 cool giant stars—18 red giants and 2 supergiants—using sophisticated stellar atmosphere models with varying helium-to-hydrogen (He/H) ratios.

Key Observations From Himalayan Chandra Telescope and Global Archives The team analyzed high-resolution spectra obtained primarily from the 2-meter Himalayan Chandra Telescope. Supplementary observations from international astronomical archives provided additional spectral coverage and comparative data. Each star was examined for its effective temperature, surface gravity, and abundances of 23 different chemical elements, using both atomic and molecular spectral diagnostics.

Out of the 20 stars studied, six displayed higher-than-standard helium abundance (standard He/H ≈ 0.1). Among these six, five were red giants, while one was classified as a supergiant.

Lithium and Helium: A Surprising Correlation One of the most intriguing findings is the strong correlation between helium enrichment and lithium abundance. Lithium is a fragile element easily destroyed in stellar interiors, yet a small fraction of red giants exhibit unusually high lithium levels—an enduring puzzle in astrophysics.

According to lead author B. P. Hema, the helium-enhanced red giants in their sample were all found to be “super-lithium-rich,” except for the supergiant star, which did not show comparable lithium enrichment. This suggests that, in red giants, increases in helium may be linked to internal processes that also boost lithium—a dual enrichment phenomenon not universally present in all giants but significant when it occurs.

Co-author and IIA professor Gajendra Pandey highlights that, while all helium-rich giants were lithium-rich, the reverse was not always true. Some lithium-rich red giants retained normal helium abundance, indicating that different physical mechanisms might be responsible for lithium production and helium enhancement, or that the timing and efficiency of these processes vary across the red giant evolutionary path.

A Milestone in Stellar Chemistry Research

This study marks the first spectroscopic measurement of photospheric helium abundance in normal and lithium-rich field giants, providing empirical evidence that complements existing theoretical models. That helium-enhanced, lithium-rich giants are spread across various evolutionary stages further underscores the complexity of stellar internal mixing, mass loss, and nucleosynthesis processes.

By shedding new light on how elements behave and evolve within cool giant stars, this work opens fresh avenues for exploring stellar life cycles, chemical evolution of the galaxy, and the mechanisms driving exotic abundance patterns.