It is well known that as a massive cloud of gas collapses under its own gravity, baby stars may form. The intense gravitational collapse kicks off fusion processes that begin the coalescence of more matter that feeds into a newborn star. Though the general process is fairly well understood, the details are not.

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For example, a stellar embryo growing inside a gas cloud isn’t “fed” directly from that cloud; matter from the cloud spirals toward the baby star, creating a rapidly-swirling, hot disk. The star is therefore fed by the disk, which is itself fed by gas from the surrounding cloud. This disk acts almost like a mother’s placenta; it’s the placenta that provides nutrients for the developing embryo, not the mother herself.

But astronomers have not been able to precisely observe where the disk around a newborn star ends (the “placenta”) and where the inner boundary of the gas cloud (the “mother”) begins. Now, astronomers using the stunningly powerful Atacama Large Millimeter/submillimeter Array (ALMA) have seen this boundary, a direct observation that will undoubtedly improve star (and planetary) formation models.

“The disks around young stars are the places where planets will be formed,” said Yusuke Aso, of the University of Tokyo and lead author of a paper published in the Astrophysical Journal. “To understand the formation mechanism of a disk, we need to differentiate the disk from the outer envelope precisely and pinpoint the location of its boundary.”

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Zooming in on a protostar named TMC-1A, which is located around 450 light-years away in the constellation Taurus, Aso’s team were able to see its spinning inner disk (the protoplanetary disk) and differentiate it from the cloud feeding it. ALMA’s extreme precision at measuring velocity distributions was key to this endeavor.

In the case of TMC-1A, the transition boundary from spinning disk to surrounding gas cloud envelope was measured to extend 90 AU (astronomical units; where 1 AU is the average distance the Earth orbits the sun) from the central baby star, a distance 3-times bigger than the orbit of Neptune. What’s more, the ALMA observations revealed the protostar’s disk obeys Keplarian motion; the material closest to the star orbits faster, whereas the material further out orbits slower.

This is important: using the rotation speed of the disk’s gas, the researchers could calculate the mass of the baby star. This stellar infant “weighs in” at a healthy 0.68 times (68 percent, or roughly two-thirds) the mass of our sun. They were also able to deduce the rate matter was falling from the disk onto the star — one-millionth of the mass of our sun is falling into TMC-1A every year at a speed of 1 kilometer per second.

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Interestingly, this mass in-fall speed is much less than what would be expected if the gas was falling at freefall speed (i.e. if nothing was impeding its flow). This slower than expected in-fall velocity could be down to the protostar’s young magnetic field buffering the in-falling material, throttling the quantity of matter that feeds it.

“We expect that as the baby star grows, the boundary between the disk and the infall region moves outward,” said Aso. “We are sure that future ALMA observations will reveal such evolution.”

In summary, astronomers have taken an interstellar ultrasound of a star that is in the process of growing inside its stellar nursery, revealing unparalleled detail in how protostars form. And that’s just awesome.