Hidden Nurseries of Stars Unveiled Thanks to a Sweet-Smelling Cosmic Clue

Astronomers have uncovered a treasure trove of young stars in the Milky Way galaxy. Revealed not by their light, these stars were discovered due to the telltale glow of a molecule better known on Earth for its sweet, fruity scent: methyl formate (CH₃OCHO).

The discovery, led by an international team of astronomers in the ALMA-IMF collaboration, was published in Astronomy & Astrophysics in July 2024. Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the team surveyed 15 active star-forming regions across our galaxy (see Figure 1). They identified nearly 80 protostars -baby stars still forming - by detecting their “hot cores,” the warm, compact cocoons of gas and dust where stars are born. These hot cores were revealed through the radio emission of methyl formate (see Figure 2), a molecule known on Earth as a colorless liquid that is used in the flavor and fragrance industries, though it is toxic in high concentrations. In space, gas-phase methyl formate and other complex organic molecules-carbon-based compounds with six or more atoms-are fantastic tools for locating and studying the early stages of star formation.

Figure 1: A 360-degree false-color view of the Milky Way. The purple stars mark locations of the 15 star-forming regions studied by the ALMA-IMF program.

Among the newly identified protostars, 22 will form massive stars more than eight times the mass of our Sun. These high-mass stars are rare, but they play an outsized role in shaping the structure and evolution of galaxies. Because they form deep within dense clouds of gas and dust, they are often invisible in optical light; however, methyl formate glows in radio wavelengths, which allows astronomers to pinpoint these deeply embedded stars. By tracing the subtle signals of complex organic molecules, scientists are opening a window into the hidden birthplaces of massive stars and the rich chemistry that surrounds them.

Figure 2: ALMA map of the methyl formate emission observed toward the star-forming region G338.93. The brighter areas on the map (numbered 1 to 5) reveal spots where large amounts of methyl formate are detected. These bright regions mark hot, dense zones where new stars are forming.

The ALMA-IMF survey more than tripled the number of known hot cores in the 15 observed star-forming regions. This large sample of hot cores provides a valuable foundation for studying the conditions under which stars form. Follow-up studies of their chemically rich environments are underway to identify additional molecules beyond methyl formate. These studies will also enable a better understanding of the origins of complex organic molecules in space, some of which might be linked to the building blocks of life.