Key Insights
Over the last 100 years, our understanding of massive stellar objects has continued to evolve.
For many of the objects that remain invisible, researchers need to refine their theoretical understanding of the object’s stellar evolution.
Using signatures of elements, researchers are looking for elusive, invisible objects, uncovering some surprising findings about stars and their evolution along the way.
Black holes are famously invisible, which is why astronomers took a long time to directly detect what had long been theorized. In September 2015, researchers first sensed the chirp of a gravitational wave caused by the merger of two black holes over 1.3 billion light-years away.
But scientists have other ways to detect black holes and other dim celestial bodies: not by their radiation or by gravitational waves but by the chemical trails left by the stars that interact with them.
With advancements in computational science, high-resolution spectroscopy, and sophisticated sky surveys, astronomers are now observing new puzzles and correcting the older diagnoses about the mysterious and invisible objects of the sky.
With these techniques, they are challenging long-standing viewpoints about stellar evolution by closely studying the unique chemical and thermal signatures of these bodies. This better understanding of elemental fingerprints is now revealing the true nature and evolution of the most extreme and fascinating objects in the sky—from stripped stars to the supermassive black holes thought to sit at the center of every galaxy.
A star is an element factory
“Stars are actually a type of laboratory you cannot create on Earth,” says Ylva Götberg, an astrophysicist at the Institute of Science and Technology Austria (ISTA). “When you study stars, basically you’re studying factories of elements. . . . This is why the star is alive, why it shines.”
All stars begin their lives from dust clouds or nebulae, where gravity gathers enough matter to begin nuclear fusion and hydrogen starts converting into helium. This stage, called the main sequence phase, is 90% of a star’s life.
Small stars, like our sun, will remain in the main sequence phase for about 10 billion years, while a massive star might last only a few million.
After they’ve exhausted the hydrogen fuel in their core, low-mass stars, like our sun, eventually cool to become white dwarfs. But massive stars, more than eight times the mass of the sun, behave differently.
Toward the end of a massive star’s life, it forms a structure like an onion. The outermost layer of the star is too cool and diffuse for nuclear fusion to occur. The first active fusion is found a little deeper inside, in a shell where hydrogen fuses into helium. Progressively hotter layers lie closer to the core, and in each layer, heavier elements—from helium to carbon, neon, oxygen, and silicon—fuse in succession, until iron forms at the very center.
When the core has nothing but iron, the star collapses into a supernova, spewing heavy elements out into space before becoming a super-fast spinning neutron star or crumbling under the pressure of gravity and becoming an invisible black hole—so dense that no wavelength of light is allowed to escape.
Everything we know about the stars and the universe comes from the analysis of light that the celestial bodies emit. But when bodies do not emit light or are not visible, scientists use a number of methods to study them, including analyzing the neighborhood.
Learning about celestial objects from their location
In the 1960s, scientists observed a strong X-ray source in the Cygnus constellation, about 6,070 light-years away from Earth. The X-rays were coming from an obscure object they named Cygnus X-1. Using subsequent observations and studying the surroundings in the 1970s, they found a massive blue supergiant star orbiting this unseen companion and confirmed the companion to be a black hole.
At Heidelberg University, astrophysicist Varsha Ramachandran studies this same supergiant star to find out more about the nearby Cygnus X-1 black hole. By studying the light from the companion star and using advanced atmosphere models, Ramachandran can estimate the mass and other properties of the black hole. “We get a multiwavelength picture (UV [ultraviolet], optical, and X-ray) of the atmosphere of the orbiting star and try to estimate parameters like temperature, gravity, mass, and chemical composition,” she says.
In her latest paper, published in May, Ramachandran and her team found that the mass of both the black hole and the companion star in the Cygnus X-1 system are “lower than what people assumed before.” The new corrected mass of the black hole is 14 solar masses, and that of the companion star is 29 solar masses, compared with previous estimates of 21 and 41 solar masses, respectively.
But Ramachandran did more than just measure the masses; she also measured the elemental signature of the companion star. Since the young star is in our own galaxy, researchers expected it to have a similar composition to our sun. But Ramachandran and her team found that the star is unexpectedly rich in heavy elements like iron, magnesium, and silicon, up to 30–80% more than the sun.
These high iron, magnesium, and silicon abundances found in the Cygnus X-1 system are puzzling because they are not consistent with the composition of the neighboring stars, Ramachandran says. She says they suggest that Cygnus X-1 had a more complex formation history or origin, perhaps originating from an enriched interstellar medium.
Studying systems where a star orbits an invisible black hole can reveal crucial details about the black hole’s properties. But to understand why such a system exists—and how it evolved in the first place—scientists must look at the stars’ earlier interactions, long before one of them collapsed to become a black hole.
When two massive stars orbit close to each other, their evolutionary paths are no longer straightforward. Such systems, where each star influences the other’s orbit, properties, and lifetime, are known as binary systems. And just as the sun’s light hides the stars during the day, in these close pairs, a bright star can outshine its companion, making its companion seem invisible.
Against a dark backdrop with distant, tiny white stars, a smaller round body has wisps of brightness around it, which it appears to be pulling from a larger body nearby.
Visualization of a binary star experiencing mass transfer, by Ylva Götberg
Credit:
Ylva Götberg
Not everything that appears invisible is a black hole. “Many invisible objects in space have been wrongly tagged as black holes,” Ramachandran says. “Some of them were later discovered to be stripped stars.”
Astronomers spot the elusive: Stripped stars
For millennia, stars were perceived as solitary, independent objects—each following its own isolated path and life in the cosmos. The prevalent view held that every star was born alone, lived alone, and died alone.
That perception started to shift in the late 1700s and early 1800s when astronomer William Herschel spotted stars that orbited a common center of gravity and named them double stars, later known as binary stars.
Today, astronomers think binary systems are quite common. “Observational studies show that approximately 50% of solar‑type stars reside in binary systems,” says Zhanwen Han, an astronomer at Yunnan Observatories at the Chinese Academy of Sciences who studies low- and intermediate-mass binary star systems. “But for massive stars (O/B types), the fraction is far higher—well over 70% of them interact with or are influenced by companions.”
In a sense, Han says, studying binary systems is essential to understand the phases that massive stars might go through—including ending up as supernovae, neutron stars, or black holes. According to the theory of the evolution of stars in close binary systems that was developed in the 1960s and ’70s, both stars are usually born from the same molecular cloud. But as they mature, one of them can start pulling mass from its companion. As the companion’s outer blanket of hydrogen is stripped away, its hot helium core becomes exposed—creating what astronomers call a “stripped star.”
Though astronomers have found a lot of stripped stars in low-mass binary systems, only one such observation for a high-mass star was mentioned in a 2008 paper. But with subsequent research, that supposed massive stripped star became a subject of debate, and eventually its stripped nature was credited to a strong magnetic field rather than gravitational interactions.
When astrophysicist Götberg realized during her PhD research that the theory of massive binary stellar evolution had existed for over half a century without any observation of one of its intermediary stages, she got curious. Throughout her PhD program and thereafter, Götberg worked to understand the formation of massive stripped stars. She presented her hypotheses at conferences, suggesting how researchers might look for signs of the elusive stripped stars.
Four women smile at the camera as they stand in front of a metal railing with a backdrop of bright sunlight over mountains.
From left: Bethany Ludwig, Anna O’Grady, Maria Drout, and Ylva Götberg at the Magellan telescopes at Las Campanas Observatory in Chile
Credit:
Ylva Götberg
At one such conference in 2016, Götberg met Maria Drout, an observational astronomer at the University of Toronto, and the pair decided to use Götberg’s theory to hunt for the missing stripped stars. Meanwhile, Ramachandran and her colleagues from Heidelberg University approached Götberg about something they had just “accidentally discovered.”
While Ramachandran’s team was scrutinizing archival spectroscopic data of massive stars in the Small Magellanic Cloud (SMC), one star in particular caught their attention. SMCSGS-FS 69 had previously been cataloged as a massive B star, but on careful examination, the researchers observed that the star’s absorption lines were not static. They wiggled.
That wiggling was a telltale sign that the massive star might be locked with an unseen partner. Furthermore, the unseen object’s spectral lines showed enhanced chemical fingerprints of hydrogen, along with carbon, nitrogen, and oxygen—elements that are generally found in the inner layers of massive stars.
These chemical signatures of heavier elements along with the presence of hydrogen meant that it was a star that wasn’t fully stripped of its outer layers.
SMCSGS-FS 69 became the first observational evidence of a star caught in a predicted but never-seen-before evolutionary phase. A partially stripped star is less hot than the fully stripped ones but more luminous and is another transitional phase in stellar evolution.
The problem with finding a massive stripped star has been that such a star appears to be “very faint” in the company of its brighter companion, Götberg says. And other challenges also exist; for example, the gas clouds of neutral hydrogen and helium that surround our solar system obstruct the UV rays coming from far-away objects.
Astronomers look for the right stellar signatures
While developing her own theory, Götberg looked up to the “incredible” work of female astronomers like Cecilia Payne-Gaposchkin, whose doctoral thesis of 1925 changed how stars were seen and understood.
One hundred years ago, researchers thought stars were made of the same matter as Earth’s crust because the spectral lines of the stars showed strong traces of heavier elements that are abundant on Earth, like iron, carbon, and sodium. But by studying a variety of stars, Payne-Gaposchkin established that stronger lines did not mean an abundance of heavier elements. She emphasized that, in fact, the universe and the stars in it are more abundant in hydrogen and helium than any other element.
Like Payne-Gaposchkin’s work, Götberg’s work is also based on the behavior of ionized elements, specifically helium and nitrogen, at super-high temperatures: ionized nitrogen with three of its electrons removed (N3+) and ionized helium with one or two electrons removed (He+ or He2+). Such ionized states cannot be seen in visible light, so Götberg turned to the UV spectrum.
Götberg theorized that “a star of 15 solar masses will have a temperature about 30,000 K. Once stripped, it may reduce to about 5 solar masses, but its temperature is going to be over 85,000 K. A 15-solar-mass star on its own would never reach this temperature.” At such high temperatures, ionized gases—like N3+ and He2+—should be visible.
To look for that evidence, Götberg and Drout went to the Las Campanas Observatory in Chile.
The pair looked for stars with very high temperatures, over 50,000 K, and with helium and nitrogen on their surfaces. “This is a signature that tells us that the surface once was the center or core of a star. Seeing both helium and nitrogen in enhanced levels is a good sign that you are looking at the core of a massive star,” Götberg says.
Using this model and scanning years of data, they narrowed down their search to 25 candidates in the SMC that were highly likely to be stripped stars. In December 2023, Götberg and Drout published their findings.
“Many stellar astronomers tend to specialize in either theoretical or observational work,” Yunnan Observatories’ Han says. “While some do bridge both, it’s relatively rare for a researcher to lead both theory development and observational campaigns within the same study. Ylva’s work is exceptional.”
The pair continued to work and test their observations. They ultimately confirmed 10 of their candidates to be stripped stars, with masses between 8 and 25 solar masses.
Research continues on massive bodies
Since Götberg’s work identifying stripped stars, she has collaborated with other researchers to look for and understand other stellar activities. One of those collaborations is with Brenna Mockler, an astrophysicist at Carnegie Observatories and the University of California, Davis, who studies what happens when supermassive black holes—black holes a million or billion times the mass of the sun, which are thought to sit at the center of every galaxy—chew up a star, known as tidal disruption events (TDEs).
When they studied these TDEs, which rip the star apart and release huge amounts of energy in the process, the two found something unusual.
In 2024, Mockler stumbled upon a surprisingly high degree of nitrogen enrichment in a TDE she was studying. She approached Götberg, and they worked to find an explanation.
Mockler’s findings indicated that the disrupted star was more massive than typically expected for TDEs. Perhaps it was a stripped star, a product of binary evolution, which naturally has a high nitrogen-to-carbon abundance ratio in its outer layers. This finding is significant because theories of star formation suggest that many more small stars exist than massive stars, but it’s difficult to measure this directly in the center of galaxies.
Mockler suspects that the universe may have more massive stars near the centers of galaxies than previously imagined. But she acknowledges that the lack of data is a problem. TDEs are rare, “with an average rate of about one every 10,000 years per galaxy,” she says.
Ramachandran is also concerned about the lack of targeted data for her future research on massive stars. “There’s a severe shortage of data in crucial wavelengths, such as the ultraviolet,” she says. “Particularly after the time of the Hubble Space Telescope, there are no planned UV telescopes.”
Yet some new telescopes are coming online.
“I am excited to study the observations from Vera C. Rubin Observatory in Chile, which is expected to find thousands of TDEs over the survey’s lifetime,” Mockler says. The observatory is equipped with a digital camera that will take time-lapse images of the sky for 10 years to capture stellar events that researchers can then study with their different methods.
Götberg also has lots of hope for the future, especially with respect to elusive objects like stripped stars. She suggests that, as well as making new observations, researchers not forget the data that already exist. “There are a lot of observations already,” she says. “Perhaps the problem is rather that we haven’t yet developed the most efficient way to search.”
Monika Mondal is freelance journalist whose work focuses on agriculture, sustainability, and environmental injustice. She partially wrote this story while receiving funding from the Institute of Science and Technology Austria through its Journalist in Residence program. ISTA did not influence any editorial decisions.
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