Star-forming theory thrown into question

New observations suggest ideas about the mass of stars are wide of the mark. Lauren Fuge reports.

Studying distant nebulae is providing more information about how stars form.
Studying distant nebulae is providing more information about how stars form.
Mike Agliolo / Getty Images

One of the most powerful radio telescopes in the world has made a discovery that challenges current theories of star formation.

Understanding why stars are born with different masses is a key question in stellar astrophysics, since mass is closely linked to colour, brightness and evolution. Stars form from massive clouds of gas and dust called nebulae, where material clumps together into dense pockets that become hotter and more pressurised until nuclear fusion is triggered and a star is born. These clumpy regions are known as star-forming cores.

Astronomers previously thought that the number of stars born with different masses (known as the initial mass function, or IMF) is directly linked to how mass is distributed among the star-forming cores (known as the core mass function, or CMF). But this conclusion was reached by studying nebulae next door to our solar system, which aren’t representative of the diverse range of star-formation regions across the galaxy.

An international team of astronomers has now studied star-forming cores in a denser, more distant nebula: the active star-formation region W43-MM1, about 18,000 light-years from Earth. To the team’s surprise, W43-MM1 has an overabundance of massive cores and fewer smaller cores than expected. The results are published in Nature Astronomy.

“Our finding seriously challenges the relation between the CMF and the IMF, which our community thought was direct — roughly ‘one core gives one binary star’,” says lead author Frédérique Motte from the Université Grenoble Alpes in France.

The structure of W43-MM1 is far more typical of nebulae in our galaxy than those previously observed, so these new observations also question whether the IMF itself is universal. This means that the mass distribution of young stars may not be the same everywhere in the galaxy.

“The number of massive stars with respect to solar-type stars may vary with galactic environments,” Motte explains. “We are at the dawn of a paradigm shift for star formation, which suggests that environmental effects do matter in the definition of star properties, and especially their mass, across galaxies.”

The observations were made using the Atacama Large Millimetre/submillimetre Array (ALMA) in Chile. Its 66 high-precision antennas work together to produce exquisitely high-resolution images, allowing the team to observe the distribution of star-forming cores over a range of stellar masses, from stars like our sun to those more than 100 times more massive.

Motte and his team aim to study a further 15 regions to see whether their results can be generalised.

If so, then Motte says, “the astrophysical community at large will be forced to re-examine its calculations about star formation in the Milky Way, nearby galaxies, as well as across cosmic times.

“It will also eventually force us to correct any estimates that depend on the number of massive stars, such as the chemical enrichment of the interstellar medium and the numbers of black holes and supernovae.”

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