Stars are born in dense molecular clouds, but did they always form this way?
Recent research suggests that in the early universe, some stars emerged from “fluffy” molecular clouds rather than filamentary ones. By studying the Small Magellanic Cloud, a galaxy with conditions similar to the early cosmos, scientists found that cloud structure and temperature impact star formation. This discovery hints that heavy elements may play a crucial role in shaping the stars and planets that form in a galaxy.
How Do Stars Form, and Has It Always Been This Way?
Stars are born in vast regions of space known as stellar nurseries, where dense clouds of gas and dust come together to create new stars. These nurseries, also called molecular clouds, can stretch across hundreds of light-years and give rise to thousands of stars. While scientists have a solid understanding of the star formation process thanks to modern telescopes and technology, many details remain unclear. One key question is whether stars formed the same way in the early universe as they do today.
A new study published today (February 20) in The Astrophysical Journal by researchers from Kyushu University and Osaka Metropolitan University suggests that star formation in the early universe may have been different. Their findings, based on observations of the Small Magellanic Cloud (SMC) — a dwarf galaxy near the ” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>Milky Way — indicate that some early stars formed in “fluffy” molecular clouds, a departure from the structured clouds seen in our galaxy today.
Molecular Clouds and Star Formation
In the Milky Way, star-forming molecular clouds typically have a long, thread-like “filamentary” shape, about 0.3 light-years wide. Scientists believe that our Solar System originated in a similar environment, where a large filamentary molecular cloud fragmented, creating dense cores, also known as molecular cloud cores. Over hundreds of thousands of years, gravity pulled gas and dust into these cores, eventually igniting nuclear fusion and forming new stars.
“Even today our understanding of star formation is still developing, comprehending how stars formed in the earlier universe is even more challenging,” explains Kazuki Tokuda, a Post‐doctoral Fellow at Kyushu University’s Faculty of Science and first author of the study. “The early universe was quite different from today, mostly populated by hydrogen and helium. Heavier elements formed later in high-mass stars. We can’t go back in time to study star formation in the early universe, but we can observe parts of the universe with environments similar to the early universe.”
The Small Magellanic Cloud as a Window to the Past
The team set their sights on the Small Magellanic Cloud (SMC), a dwarf galaxy near the Milky Way about 20,000 light-years from Earth. The SMC contains only about one-fifth of the heavy elements of the Milky Way, making it very close to the cosmic environment of the early universe, about 10 billion years ago. However, the spatial resolution for observing the molecular clouds in the SMC was often insufficient, and it was unclear whether the same filamentary structure could be seen at all.
Fortunately, the ALMA radio telescope in Chile was powerful enough to capture higher-resolution images of the SMC and determine the presence or absence of filamentary molecular clouds.
Fluffy vs. Filamentary Molecular Clouds
“In total, we collected and analyzed data from 17 molecular clouds. Each of these molecular clouds had growing baby stars 20 times the mass of our Sun,” continues Tokuda. “We found that about 60% of the molecular clouds we observed had a filamentary structure with a width of about 0.3 light-years, but the remaining 40% had a ‘fluffy’ shape. Furthermore, the temperature inside the filamentary molecular clouds was higher than that of the fluffy molecular clouds.”
This temperature difference between filamentary and fluffy clouds is likely due to how long ago the cloud was formed. Initially, all clouds were filamentary with high temperatures due to the clouds colliding with each other. When the temperature is high, the turbulence in the molecular cloud is weak. But as the temperature of the cloud drops, the kinetic energy of the incoming gas causes more turbulence and smoothens the filamentary structure, resulting in the fluffy cloud.
If the molecular cloud retains its filamentary shape, it is more likely to break up along its long “string” and form many stars like our Sun, a low-mass star with planetary systems. On the other hand, if the filamentary structure cannot be maintained, it may be difficult for such stars to emerge.
The Role of Heavy Elements in Star Birth
“This study indicates that the environment, such as an adequate supply of heavy elements, is crucial for maintaining a filamentary structure and may play an important role in the formation of planetary systems,” concludes Tokuda. “In the future, it will be important to compare our results with observations of molecular clouds in heavy-element-rich environments, including the Milky Way galaxy. Such studies should provide new insights into the formation and temporal evolution of molecular clouds and the universe.”
Reference: “ALMA 0.1 pc View of Molecular Clouds Associated with High-Mass Protostellar Systems in the Small Magellanic Cloud: Are Low-Metallicity Clouds Filamentary or Not?” by Kazuki Tokuda, Yuri Kunitoshi, Sarolta Zahorecz, Kei E. I. Tanaka, Itsuki Murakoso, Naoto Harada, Masato I. N. Kobayashi, Tsuyoshi Inoue, Marta Sewilo, Ayu Konishi, Takashi Shimonishi, Yichen Zhang, Yasuo Fukui, Akiko Kawamura, Toshikazu Onishi and Masahiro N. Machida, 20 February 2025, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ada5f8
