Massive stars -- those at least 8 times the mass of our Sun -- present an intriguing¹（有趣的） mystery: how do they grow so large when the vast majority of stars in the Milky² Way are considerably³ smaller? To find the answer, astronomers⁴ used the Atacama Large Millimeter/submillimeter Array (ALMA) telescope to survey the cores of some of the darkest, coldest, and densest⁶ clouds in our Galaxy⁷ to search for the telltale（迹象） signs of star formation.

 

These objects, known as Infrared⁸ Dark Clouds, were observed approximately 10,000 light-years away in the direction of the constellations⁹ of Aquila and Scutum.

 

Since these cloud cores are so massive and dense⁵, gravity should have already overwhelmed their supporting gas pressure, allowing them to collapse¹⁰ to form new, Sun-mass stars. If a star had not yet begun to shine, that would be a hint that something extra was supporting the cloud.

 

"A starless core would indicate that some force was balancing out the pull of gravity, regulating star formation, and allowing vast amounts of material to accumulate in a scaled-up version of the way our own Sun formed," remarked Jonathan Tan, an astrophysicist（天体物理学家） at the University of Florida, Gainesville, and lead author of a paper published today in the Astrophysical Journal. "This suggests that massive stars and Sun-like stars follow a universal mechanism¹¹ for star formation. The only difference is the size of their parent clouds."

 

Average stars like our Sun begin life as dense, but relatively¹² low-mass concentrations of hydrogen, helium, and other trace elements inside large molecular¹³ clouds. After the initial kernel¹⁴（核心） emerges from the surrounding gas, material collapses¹⁵ under gravity into the central region in a relatively ordered fashion via a swirling¹⁶ accretion¹⁷ disk, where eventually planets can form. After enough mass accumulates, nuclear fusion¹⁸ begins at the core and a star is born.

 

While this model of star formation can account for the vast majority of stars in our Milky Way, something extra is needed to explain the formation of more massive stars. "Some additional force is needed to balance out the normal process of collapse, otherwise our Galaxy would have a fairly uniform stellar population," said Tan. "Alternatively, there has been speculation¹⁹ that two separate models of star formation are needed: one for Sun-like stars and one for these massive stars."

 

The key to teasing out the answer is to find examples of massive starless cores -- to witness the very beginnings of massive star birth.

 

The team of astronomers from the United States, the United Kingdom, and Italy used ALMA to look inside these cores for a unique chemical signature involving the isotope²⁰ deuterium to essentially²¹ take the temperatures of these clouds to see if stars had formed. Deuterium is important because it tends to bond with certain molecules²² in cold conditions. Once stars turn on and heat the surrounding gas, the deuterium is quickly lost and replaced with the more common isotope of hydrogen.

 

The ALMA observations detected copious²³ amounts of deuterium, suggesting that the cloud is cold and starless. This would indicate that some counter force is forestalling²⁴ core collapse and buying enough time to form a massive star. The researchers speculate that strong magnetic fields may be propping²⁵ up the cloud, preventing it from collapsing²⁶ quickly.

 

"These new ALMA observations reveal objects that are quite similar to the nurseries of Sun-like stars, but simply scaled-up by tens or a hundred times. This may mean that nature is more important than nurture²⁷ when it comes to determining a star's size," concludes Tan.

 

These observations were conducted during ALMA's early science campaign. Future studies with ALMA's full array of 66 antennas²⁸ will uncover even more details about these star-forming regions.

 

ALMA, an international astronomy facility, is a partnership²⁹ of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory³⁰ (NRAO), and on behalf of East Asia by the National Astronomical³¹ Observatory of Japan (NAOJ). The Joint³² ALMA Observatory (JAO) provides the unified³³ leadership and management of the construction, commissioning and operation of ALMA.

 

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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