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.<\/p>\n
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.<\/p>\n
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\u2014from stripped stars to the supermassive black holes thought to sit at the center of every galaxy.<\/p>\n
A star is an element factory<\/p>\n
\u201cStars are actually a type of laboratory you cannot create on Earth,\u201d says Ylva G\u00f6tberg, an astrophysicist at the Institute of Science and Technology Austria (ISTA). \u201cWhen you study stars, basically you’re studying factories of elements. . . . This is why the star is alive, why it shines.\u201d<\/p>\n
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\u2019s life.<\/p>\n
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.<\/p>\n
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.<\/p>\n
Toward the end of a massive star\u2019s 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\u2014from helium to carbon, neon, oxygen, and silicon\u2014fuse in succession, until iron forms at the very center.<\/p>\n
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\u2014so dense that no wavelength of light is allowed to escape.<\/p>\n
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.<\/p>\n
Learning about celestial objects from their location<\/p>\n
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.<\/p>\n
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. \u201cWe 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,\u201d she says.<\/p>\n
In her latest paper, published in May, Ramachandran and her team found that the mass of both the black hole and the companion star<\/a> in the Cygnus X-1 system are \u201clower than what people assumed before.\u201d 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.<\/p>\n 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\u201380% more than the sun.<\/p>\n 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.<\/p>\n Studying systems where a star orbits an invisible black hole can reveal crucial details about the black hole\u2019s properties. But to understand why such a system exists\u2014and how it evolved in the first place\u2014scientists must look at the stars\u2019 earlier interactions, long before one of them collapsed to become a black hole.<\/p>\n When two massive stars orbit close to each other, their evolutionary paths are no longer straightforward. Such systems, where each star influences the other\u2019s orbit, properties, and lifetime, are known as binary systems. And just as the sun\u2019s light hides the stars during the day, in these close pairs, a bright star can outshine its companion, making its companion seem invisible.<\/p>\n Visualization of a binary star experiencing mass transfer, by Ylva G\u00f6tberg<\/p>\n Credit: Not everything that appears invisible is a black hole. \u201cMany invisible objects in space have been wrongly tagged as black holes,\u201d Ramachandran says. \u201cSome of them were later discovered to be stripped stars.\u201d<\/p>\n Astronomers spot the elusive: Stripped stars<\/p>\n For millennia, stars were perceived as solitary, independent objects\u2014each 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.<\/p>\n 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.<\/p>\n Today, astronomers think binary systems are quite common. \u201cObservational studies show that approximately 50% of solar\u2011type stars reside in binary systems,\u201d says Zhanwen Han, an astronomer at Yunnan Observatories at the Chinese Academy of Sciences who studies low- and intermediate-mass binary star systems. \u201cBut for massive stars (O\/B types), the fraction is far higher\u2014well over 70% of them interact with or are influenced by companions.\u201d<\/p>\n In a sense, Han says, studying binary systems is essential to understand the phases that massive stars might go through\u2014including 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 \u201970s, 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\u2019s outer blanket of hydrogen is stripped away, its hot helium core becomes exposed\u2014creating what astronomers call a \u201cstripped star.\u201d<\/p>\n 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<\/a>. 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<\/a> rather than gravitational interactions.<\/p>\n When astrophysicist G\u00f6tberg 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, G\u00f6tberg 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.<\/p>\n From left: Bethany Ludwig, Anna O\u2019Grady, Maria Drout, and Ylva G\u00f6tberg at the Magellan telescopes at Las Campanas Observatory in Chile<\/p>\n Credit: At one such conference in 2016, G\u00f6tberg met Maria Drout, an observational astronomer at the University of Toronto, and the pair decided to use G\u00f6tberg\u2019s theory to hunt for the missing stripped stars. Meanwhile, Ramachandran and her colleagues from Heidelberg University approached G\u00f6tberg about something they had just \u201caccidentally discovered.\u201d<\/p>\n While Ramachandran\u2019s 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\u2019s absorption lines were not static. They wiggled.<\/p>\n That wiggling was a telltale sign that the massive star might be locked with an unseen partner. Furthermore, the unseen object\u2019s spectral lines showed enhanced chemical fingerprints of hydrogen, along with carbon, nitrogen, and oxygen\u2014elements that are generally found in the inner layers of massive stars.<\/p>\n 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.<\/p>\n SMCSGS-FS 69 became the first observational evidence<\/a> 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.<\/p>\n The problem with finding a massive stripped star has been that such a star appears to be \u201cvery faint\u201d in the company of its brighter companion, G\u00f6tberg 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.<\/p>\n Astronomers look for the right stellar signatures<\/p>\n While developing her own theory, G\u00f6tberg looked up to the \u201cincredible\u201d work of female astronomers like Cecilia Payne-Gaposchkin, whose doctoral thesis of 1925 changed how stars were seen and understood.<\/p>\n One hundred years ago, researchers thought stars were made of the same matter as Earth\u2019s 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.<\/p>\n Like Payne-Gaposchkin\u2019s work, G\u00f6tberg\u2019s 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 G\u00f6tberg turned to the UV spectrum.<\/p>\n G\u00f6tberg theorized that \u201ca 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.\u201d At such high temperatures, ionized gases\u2014like N3+ and He2+\u2014should be visible.<\/p>\n To look for that evidence, G\u00f6tberg and Drout went to the Las Campanas Observatory in Chile.<\/p>\n The pair looked for stars with very high temperatures, over 50,000 K, and with helium and nitrogen on their surfaces. \u201cThis 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,\u201d G\u00f6tberg says.<\/p>\n 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, G\u00f6tberg and Drout published their findings<\/a>.<\/p>\n \u201cMany stellar astronomers tend to specialize in either theoretical or observational work,\u201d Yunnan Observatories\u2019 Han says. \u201cWhile some do bridge both, it’s relatively rare for a researcher to lead both theory development and observational campaigns within the same study. Ylva\u2019s work is exceptional.\u201d<\/p>\n The pair continued to work and test their observations. They ultimately confirmed 10 of their candidates to be stripped stars<\/a>, with masses between 8 and 25 solar masses.<\/p>\n Research continues on massive bodies<\/p>\n Since G\u00f6tberg\u2019s 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<\/a>\u2014black holes a million or billion times the mass of the sun, which are thought to sit at the center of every galaxy\u2014chew up a star, known as tidal disruption events (TDEs).<\/p>\n When they studied these TDEs, which rip the star apart and release huge amounts of energy in the process, the two found something unusual.<\/p>\n In 2024, Mockler stumbled upon a surprisingly high degree of nitrogen enrichment in a TDE she was studying. She approached G\u00f6tberg, and they worked to find an explanation.<\/p>\n
\n 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.<\/p>\n
\n Ylva G\u00f6tberg<\/p>\n
\n Four women smile at the camera as they stand in front of a metal railing with a backdrop of bright sunlight over mountains.<\/p>\n
\n Ylva G\u00f6tberg<\/p>\n