Finding the edge of a protostar disk

Finding the edge of a protostar disk

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A baby is already growing before it is born, inside the belly of its mother. The same is true for a star: even before it starts to shine, it is growing inside a large cloud of gas and dust. Japanese astronomers have now discovered new details about the early growth of such a protostar.

Stars grow from collapsing gas clouds. The gas in the cloud falls in on itself because of gravity. But at some stage, the gas begins to accumulate in a flattened disk, with the protostar at its center. Eventually, planets may be born from this disk.

So when does the disk starts to form? And where is the boundary between the flattened, rotating disk and the infalling gas from the original cloud? Using ALMA, the Japanese team has found out – at least in the case of one protostar, known as TMC-1A.

Thanks to its sharp vision and sensitivity, ALMA could pinpoint the transition between the disk and the infalling gas, at some 13.5 billion kilometers from the growing baby star. In other words: the diameter of the disk is about three times the size of the orbit of the planet Neptune in our own Solar System.

The astronomers also measured how fast the disk is rotating. From this, they could calculate the mass of TMC-1A: about two-thirds of the mass of the Sun.

And they found a surprise. The ALMA measurements revealed that the gas from the original cloud is falling in at a velocity of about 1 kilometer per second. That may seem very fast, but in fact it’s much slower than what you would expect if gravity would be the only force at work.

The astronomers think that the infalling gas may be slowed down by magnetic fields. They also expect the disk to grow as the protostar grows older.

Unfortunately, it will probably take tens of thousands of years before TMC-1A is really ‘born’ as a real star, so ALMA will never witness its birth.


TMC-1A is a protostar: a star-in-the making. It glows with the heat of infalling gas, but it’s not yet producing energy from nuclear reactions in its core. TMC is an abbreviation of Taurs Molecular Cloud: a large cloud of cold gas and dust in the constellation Taurus the Bull, at a distance of approximately 450 light-years The protostar cannot be seen by optical telescopes: its light is absorbed by the surrounding dust. However, ALMA can peer through the dust and reveal details about the growth of TMC-1A.


The ALMA observations of TMC-1A were carried out by Yusuke Aso and his colleges. Yusuke is a graduate student at the University of Tokyo in Japan. He works in a team led by Nagayoshi Ohashi, a professor of astronomy at the Japanese Subaru Telescope in Hawaii. Together with ten other Japanese astronomers, Yusuke and professor Ohashi wrote an article on their new results for The Astrophysical Journal, a professional astronomy magazine.