How the dance of death stars puts a new spin on the way suns are born

The Star

01-07-2025

Some stars nearing the end of their lives slow their spin in a 'retirement solo' that not only shows the end is nigh but also how others are born.
Chinese astronomers have used this cosmic dance to look into the Milky Way's past and find that stars born today spin much faster than those formed billions of years ago.
Using data from Europe's Gaia space telescope and other sky surveys, researchers at the Changchun Observatory in northeastern China studied thousands of stars, each with a mass roughly 1½ times that of the sun.
They found that as the Milky Way evolved, newly formed stars began spinning up to 10 times faster than their much older counterparts. That extra spin causes gas clouds to break into smaller fragments, making it harder for massive stars to take shape.
The study, accepted for publication in The Astrophysical Journal, presents the first direct evidence that the galaxy's star-forming clouds have grown more turbulent over time.
'Our study shows that the angular momentum of stars in a certain mass range holds key clues to the Milky Way's history,' the team said in a statement on the observatory's website. 'It offers a new way to study how the galaxy has changed over time.'
This shift in the star-forming environment could also reshape our galaxy, gradually favouring the birth of smaller, long-lived stars, while massive stars – and the explosive supernovae they trigger – could become rarer.
Scientists have long relied on a star's chemical make-up, age, and motion to see back into the Milky Way's past.
Stars are born spinning, and how fast they spin depends on the conditions in the clouds that formed them.
Many stars slow down as they age. For instance, when the sun formed about 4.6 billion years ago, it likely took less than 10 days to complete a rotation.
Today, it rotates once every 25 days at the equator. By the time it runs out of fuel and expands into a red giant, it will turn thousands of times more slowly and possibly take centuries to complete one spin.
In the new study, the researchers from Changchun focused on stars between 1.4 and 1.8 times the sun's mass. Unlike the sun, these heavier stars hold on to much of their original spin for billions of years.
Near the end of their lives, these stars expand dramatically. As their size increases, their spin slows.
'It's much like a ballerina extending her arms to slow down as she spins,' the team said, dubbing this final slowdown the star's 'retirement solo'.
By using precise measurements from the Gaia mission, the team compared the angular momentum of thousands of such stars of various ages and found a striking pattern: younger stars spin much faster than older ones – not because the older stars slow down, but because they were born that way.
The finding suggests that the Milky Way's star-forming environment has steadily grown more turbulent and energetic over the past 6 billion years, spinning up new stars with each generation.
The team hopes future studies will refine models of how young stars develop just before they ignite.
They also call for more data on stars with a wider range of chemical compositions, which could help reveal how different types of stars gain or lose spin during formation. - SOUTH CHINA MORNING POST