Never Seen Before: Indian Scientists Find Germanium In Ultra-Rare Helium Star
Indian scientists identified Extreme Helium star A980 with unprecedented germanium signatures, challenging current theories on stellar nucleosynthesis and element formation.
By Anubha Jain
Published : July 7, 2025 at 12:29 PM IST
Bengaluru: In a landmark discovery, scientists at the Indian Institute of Astrophysics (IIA), Bengaluru, have identified a star with a chemical signature never seen before, shaking up long-held ideas about stellar evolution and element formation. Located 25,800 light-years away in the Ophiuchus constellation, this enigmatic object—named A980—belongs to the ultra-rare class of Extreme Helium (EHe) stars and has shown, for the first time ever, the presence of singly-ionised germanium (Ge II) in its optical spectrum.
A Peculiar Star with a Rare Identity
EHe stars are fascinating cosmic relics thought to be the result of mergers between two white dwarfs—one rich in carbon-oxygen and the other in helium. These stars are almost entirely made of helium, with hardly any hydrogen, making them astrophysically rare and chemically unique. Initially, A980 was suspected to be a hydrogen-deficient carbon star, but observations using the Hanle Echelle Spectrograph (HESP) on the 2-m Himalayan Chandra Telescope in Hanle, Ladakh, revealed otherwise.
"We compared A980's spectrum with known hydrogen-deficient carbon stars," explained Gajendra Pandey, senior professor at IIA.
During an interview with ETV Bharat, Pandey discussed the key spectral features that confirmed A980 as an Extreme Helium (EHe) star rather than a hydrogen-deficient carbon star. "To our surprise, key carbon features like C₂ molecular bands were absent, and instead, we saw significantly enhanced neutral helium lines, indicating a much hotter star." The spectral match with a known EHe star confirmed A980's identity.
Faint, and Hard to Find
Extreme Helium stars are exceedingly rare—fewer than 25 are known to date. A980 is now a valuable addition to this elusive population. "There are certainly more such stars out there, but detecting them is tough," said Pandey. "These stars are faint, and their spectral signatures are subtle. One can't identify an EHe star just by its light—it requires careful inspection of the star's spectrum."
What truly stunned the astronomers was the detection of four distinct Ge II lines, marking the first-ever observation of germanium in an Extreme Helium star. "The strength of the Ge II lines indicated that germanium is about eight times more abundant than in our Sun," said lead author and PhD scholar Ajay Saini.
Explaining the process of spectral identification, Pandey said that the team used atomic line databases and theoretical models to match observed transition strengths with predictions. This involved comparing laboratory-measured wavelengths, transition probabilities, and the required abundance of germanium to reproduce the observed spectral features.
He added that the spectrum was first corrected for radial velocity, after which the energies (wavelengths) of the Ge II lines aligned perfectly with laboratory values, including those of well-known transitions from other elements. The corrected spectrum was then compared with that of an extreme helium star of similar temperature, known to contain no germanium.
“In this comparison, we clearly saw strong Ge II transitions—completely absent in the reference star,” Pandey said. “That’s how we confirmed the presence of germanium beyond doubt. Identifying such lines is highly challenging in helium-rich, hydrogen-deficient stars, where thousands of spectral lines overlap, Pandey further said, adding that it is laborious and time-consuming work as two elements can have similar energy transitions, so every line must be verified to rule out blending.
Abundance Analysis and Modelling
To estimate germanium's abundance in A980, the researchers used detailed model atmospheres, radiative transfer codes, and atomic data. "We increased the germanium abundance until the strength of the predicted Ge II lines matched the observed spectrum," said Pandey.
The professor explained, "We carried out a detailed analysis of the star's chemical makeup. We compared its spectrum with that of a well-studied star, where the germanium line, temperature, and gravity are already known. The spectra matched closely, but the germanium line in our star was much stronger.
At first glance, the stronger germanium line suggested that this star has more germanium. To confirm, we calculated the abundance based on how strong the line appeared, which depends on the amount of germanium, the transition probability, and the star's temperature and gravity. We adjusted the germanium levels in our model until it matched the observed spectrum. That's how we confirmed the higher germanium abundance."
The entire analysis, including the time taken for proposing and conducting the observations, from discovery to confirmation, spanned nearly two years.
Unravelling the Origins: S-process and Beyond
Germanium is a heavy element typically synthesised via the s-process (slow neutron capture) during a star's Asymptotic Giant Branch (AGB) phase. In this process, heavy atomic nuclei capture neutrons and become unstable, leading to beta decay, which ultimately forms heavier elements.
Pandey explained, "Neutrons don't carry any electric charge, so they can easily enter the nucleus of heavy atoms. This makes them much easier to absorb compared to protons or electrons, which are charged and face repulsion from the atom." He said that when we try to create heavier elements, the atoms we start with already have more electrons and protons. So, adding more electrons is difficult due to repulsion. Protons, which are positively charged, also face resistance from the protons in the nucleus. But since neutrons have no charge, they are more readily accepted.
"However," Pandey added, "When a heavy atom captures a neutron, it often becomes unstable. To regain stability, it undergoes a process called beta decay, where it may release an electron or a positron (subatomic particles with the same mass as an electron but with a positive charge). Essentially, a neutron can turn into a proton, or a proton into a neutron, while releasing a tiny particle like an electron in the process."
He further explained that in nature, this process occurs slowly through what is known as the slow neutron capture process, or s-process. "Here, the atom has time to stabilise after each neutron it absorbs, before taking in the next one. This is how many heavier elements are gradually built up in stars over time."
The broader implications of the discovery
A980's formation might not be limited to s-process nucleosynthesis. The team is also exploring more exotic origins, suggesting that A980 may share properties with Thorne–Żytkow Objects (TŻOs)—a rare theoretical class of stars with a neutron star core. These objects could synthesise heavy elements like germanium via alternate, more extreme nuclear processes, including rapid proton or neutron capture, possibly under intense conditions found in compact star cores.
Pandey explained that this discovery helps us understand both how elements are synthesised in stars and how stars themselves are formed. Germanium, for example, is usually created through the slow neutron capture process (s process), but it can also form through rapid proton capture under certain extreme conditions, like when proton-rich material bombards a dense star core. In such cases, especially around compact objects like neutron stars, which are denser than white dwarfs, germanium can be produced in greater amounts.
Future Prospects and Instruments
Pandey emphasised the importance of advanced instrumentation and observational facilities in pushing the boundaries of stellar astrophysics. "High-resolution spectrographs like HESP are essential, but we now need larger telescopes and space-based observatories to probe these stars more deeply, especially in the infrared and ultraviolet regions."
He praised the Himalayan Chandra Telescope's site at Hanle, Ladakh, as ideal for such work. "It's a dry, cloud-free location offering clear skies almost year-round—perfect for high-precision optical and near-infrared astronomy."
A Milestone for Indian Astronomy
This discovery, now published in The Astrophysical Journal, not only marks a breakthrough in understanding stellar nucleosynthesis but also showcases the capabilities of India's ground-based observatories. It opens new avenues in the study of exotic stars, element formation, and cosmic evolution, bringing us one step closer to decoding the complex alchemy of the universe.
