Scientists discover ‘cradle’ for newborn stars at the Milky Way’s centre

Astronomers have discovered a ‘cradle’ for potential new stars in the centre of our galaxy – the Milky Way.

In the midst of turbulent and supersonic gas movements, they describe a ‘calm island’ which may represent a site of future star formation, where gravity can shape solid formations. 

Rojita Buddhacharya, a PhD researcher at the Center for Astrophysics at Harvard & Smithsonian and Liverpool John Moores University, used the ALMA telescope in Chile as part of the ALMA CMZ Exploration Survey (ACES), which maps gas in our Milky Way’s Central Molecular Zone - a thick ring of dense gas in the inner region of our Galaxy.

“Our results show that even in the extreme environment of the Galactic Centre, gas can reach a state similar to what we see in nearby star-forming clouds,” said Buddhacharya. “This suggests that calm gas structures are a fundamental part of star formation, even in the most turbulent regions of our Galaxy. The gas that formed the Sun likely experienced a similar calm phase before collapsing into stars.”

Oasis of calm allows gases to clump

Gas in the CMZ is typically highly turbulent; in most of this region, gas motions are supersonic, or faster than the speed of sound. However, the scientists discovered a small area they call the subsonic region, where gas is much calmer than its surroundings.

To study these motions, the team looked at light from a molecule called isocyanic acid, or HNCO, which ALMA can measure in very fine detail, both in position on the sky and in speed along the line of sight. Using data analysis tools, they separated the many overlapping gas signals along each line of sight, then grouped pieces of gas that appeared to move together into larger structures.

To find this cradle is extremely exciting and crucial to our understanding of where we came from

Steven Longmore, Professor of Astrophysics, LJMU

“Using ALMA, we were able to measure the motions of dense gas in the Galactic Center with very fine resolution,” said Buddhacharya. “This lets us see, in remarkable detail, how gas motions transition from a highly turbulent state to calm on very small scales.”

Inside the subsonic region, they found a thread-like structure, or filament of gas, along which the gas moves in small random motions. At the edges of the subsonic region, the gas becomes more turbulent again and moves faster. Thus, Buddhacharya and her colleagues showed that the change from supersonic to subsonic happens over a short distance.

Gravity pulls material together

The team also measured gravity in this region and found that the subsonic region is dense enough that its gravity is strong enough to hold the gas together. Since areas of gas with this density and gravity are candidates for forming new stars, the area could be a site of future star formation, the scientists noted.  

Until now, scientists discovered such calm gas within star forming regions near the Sun at the present day. However, most stars, including our own Sun, formed 4.5 billion years ago when the universe was much younger and conditions were more extreme.

Professor Steven Longmore, of LJMU’s Astrophysics Research Institute, said: “Until now we have only been able to observe the cradles of stars and planets which form in the present day. But most stars in the Universe, including our own solar system, formed when the Universe was much younger and the conditions were much more extreme. To find a cradle like that, similar to where we think our own solar system and others were forged, is extremely exciting and crucial to our understanding of how we came to be here.”

The discovery shows that even in extreme environments, gas can become calm and cool enough for gravity to take over, resembling the star-forming clouds in the Milky Way’s disk. The result suggests that the earliest steps of star formation may work in a similar way across diverse environments of our galaxy. And, the gas that formed the Sun likely experienced a similar calm phase before collapsing into stars.

Collaborators include Qizhou Zhang, Jonathan Henshaw, Daniel Walker, Rebecca Houghton, Ashley Barnes and Adam Ginsburg as a part of the ALMA Exploration CMZ Survey (ACES) team.