Stars like our Sun are born in stellar nurseries – cosmic clouds of dust and gas that churn out thousands of astral progeny in their lifetimes.
In two new papers due to be presented this week at the 238th meeting of the American Astronomical Society, scientists have for the first time charted the stellar nurseries in the nearby Universe, challenging the prevailing notion that all clouds look and act the same.
For six years between 2013 and 2019, the international team of astronomers used the Atacama Large Millimeter/submillimetre Array (ALMA) in the Atacama desert of northern Chile to survey 100,000 stellar nurseries across 90 galaxies, with the aim of understanding how they connect to their parent galaxies. This was part of the PHANGS (Physics at High Angular Resolution in Nearby GalaxieS) project.
“To understand how stars form, we need to link the birth of a single star back to its place in the Universe,” says Eva Schinnerer, an astronomer at the Max Planck Institute for Astronomy (MPIA), Germany, and principal investigator of PHANGS.
“It’s like linking a person to their home, neighbourhood, city, and region. If a galaxy represents a city, then the neighbourhood is the spiral arm, the house the star-forming unit, and nearby galaxies are neighbouring cities in the region.
These observations have taught us that the ‘neighbourhood’ has small but pronounced effects on where and how many stars are born.”
The team compared the molecular properties and star formation processes at different galactic regions, including galaxy discs, stellar bars, spiral arms and galaxy centres, and confirmed that location plays a key role in star formation.
Annie Hughes, an astronomer at L’Institut de Recherche en Astrophysique et Planétologie (IRAP), France, says that this is the first time scientists have a snapshot of what star-forming clouds are really like across such a broad range of different galaxies.
“We found that the properties of star-forming clouds depend on where they are located: clouds in the dense central regions of galaxies tend to be more massive, denser, and more turbulent than clouds that reside in the quiet outskirts of a galaxy.
“The lifecycle of clouds also depends on their environment. How fast a cloud forms stars and the process that ultimately destroys the cloud both seem to depend on where the cloud lives.”
Co-author Erik Rosolowsky, a physicist at the University of Alberta, Canada, says this complex mapping would not have been possible without ALMA.
“We are finally seeing the diversity of star-forming gas across many galaxies and are able to understand how they are changing over time. It was impossible to make these detailed maps before ALMA,” says Rosolowsky. “This new atlas contains 90 of the best maps ever made that reveal where the next generation of stars is going to form.”
This epic cosmic chart is just one of the crowning achievements of ALMA. Another paper, also due for presentation at the AAS meeting, details findings from radio astronomy observations of organic molecules in planetary nebulae.
A planetary nebula is created when certain stars reach the end of their life cycle: as the dying star sheds its mass into space and becomes a white dwarf, it emits strong UV radiation, which was traditionally believed to break up any molecules into their constituent atoms.
The team behind the paper, led by Lucy Ziurys at the University of Arizona, used ALMA to observe radio emissions from hydrogen cyanide (HCN), formyl ion (HCO+) and carbon monoxide (CO) in five planetary nebulae: M2-48, M1-7, M3-28, K3-45 and K3-58. They found that organic molecules manage to escape being torn apart, and these nebulae may in fact seed space with the molecules key for the formation of new stars and planets.
“It was thought that molecular clouds which would give rise to new stellar systems would have to start from scratch and form these molecules from atoms,” says Ziurys. “But if the process starts with molecules instead, it could dramatically accelerate chemical evolution in nascent star systems.”