In the young Universe, some stars likely formed within "fluffy" clouds.

Thanks to modern astronomical tools, scientists understand how stars are formed today. They are born in dense and cold "cradles" of space — molecular clouds.

These regions consist of giant clusters of gas and dust, where temperatures can drop to several tens of degrees above absolute zero (down to minus 260 degrees Celsius). They are called molecular because they primarily consist of molecular hydrogen, although they can contain a variety of different compounds. Hydrogen atoms come together, bond through covalent connections, and form hydrogen molecules.

Under the influence of gravity, gas and dust compress. The compressed material gathers into dense clumps — protostars, which then heat up. Once the temperature in their centers becomes sufficiently high, nuclear fusion begins: hydrogen is converted into helium, and the protostar ignites like a real star.

When gas in a molecular cloud starts to compress due to gravity, it already contains not only hydrogen but also some heavy elements (or "metals"). These elements, formed in previous generations of stars, contribute to cooling the gas and help form molecules that effectively radiate heat.

Thousands of celestial bodies can form in molecular clouds, which are also very massive: they stretch across hundreds of light-years. In the Milky Way, these clouds resemble long filamentous structures about 0.3 light-years wide. Scientists believe that the Sun was born in a similar "filamentous" cloud.

But how did celestial bodies form in the young Universe, where there were almost no heavy elements? A team of Japanese astronomers led by Kazuki Tokuda (Kazuki Tokuda) from Kyushu University sought to answer this question. To do this, the scientists studied a neighboring galaxy with conditions similar to those of the early Universe.

Tokuda and his colleagues observed the Small Magellanic Cloud — a dwarf galaxy located 20,000 light-years from Earth. It contains five times fewer heavy elements than the Milky Way, making it very similar to the cosmic environment of the Universe about 10 billion years old. Using the ALMA radio telescope in Chile, the scientists obtained detailed images of 17 molecular clouds where young stars were forming, each with a mass 20 times greater than that of the Sun.

The results were surprising: 60 percent of the clouds have a filamentous structure 0.3 light-years wide, similar to those in our Galaxy. However, the remaining 40 percent looked different — resembling "fluffy" clumps of gas without a distinct shape. At the same time, the temperature inside the filamentous clouds was found to be higher than in the "fluffy" ones. According to the study's authors, this difference can be explained by the age of the structures.

Tokuda explained that initially, all molecular clouds were filamentous and very hot, as particles within them constantly collided and heated up. When a cloud is hot, it remains calm, with less mixing occurring. At high temperatures, gas movements (turbulence) are weak.

However, over time, the cloud cools. When cold gas enters this structure, it starts to move faster (increasing turbulence) and mixes the cloud as if it were being "whipped." This causes the filamentous structure to blur, and the molecular cloud takes on a "fluffy" shape.

“The shape of the cloud affects the types of stars that form within it. If a molecular cloud remains filamentous, it can split into many small pieces along its length. In each of these pieces, a star similar to our Sun, with surrounding planets, may emerge. If the cloud becomes fluffy, meaning the filamentous structure disappears, it will be more challenging for stars like our Sun to form,” — explained Tokuda.

The authors of the study emphasized that the surrounding environment of a molecular cloud influences its shape. For instance, a sufficient amount of heavy elements allows the filamentous structure to be maintained and can consequently play a crucial role in the formation of planetary systems.

In the future, Tokuda and his colleagues plan to compare data on molecular clouds in the Small Magellanic Cloud with data on similar structures in other galaxies. This will help understand how the evolution of the chemical composition of the Universe has changed the "rules" of star formation.

The scientific work has been published in The Astrophysical Journal.