We have a pretty good idea of how stars form in general. You start with a big cloud of gas, a nebula, and let a part of it collapse (possibly triggered by a collision with another cloud, or a nearby supernova). The gas contracts, falling toward the center of the collapsing zone. That’s where the star begins to form, first as a protostar, a hot ball of gas but not quite massive enough to ignite nuclear fusion in its core. The infalling gas has angular momentum, so it forms a disk directly around the star, which is the last step before it falls into the growing protostar.

That’s the very general idea, but we do see this happening in nearby star-forming nebulae, so we’re pretty sure this is how it works.

Except, maybe not always. Astronomers have found a star that appears to be getting fed directly from the nebula around it via a long streamer of infalling gas, with no disk to be seen [link to journal paper].

That image was taken using ALMA, the Atacama Large Millimeter/submillimeter Array, which detects light in the millimeter wavelength range (so, longer than infrared but shorter than radio). It shows what astronomers call a core, which is a forming star (indicated by the star symbol in the image) surrounded by gas — although the gas is mostly composed of hydrogen, helium, and more, the observations were tuned to look for methanol, which is a common molecule in such situations, and can be detected by ALMA. The core is called G336.018-00.827 and has about 10 times the mass of the Sun, so when it’s done it will be a massive star. It’s roughly 10,000 light-years from us, a decent fraction of the way across the galaxy.

The image shows material (dust and gas) falling in a long streamer toward the core. The astronomers could measure the Doppler shift in the light, which indicates which gas is moving toward us (the blue arrow) and which is moving away (red arrow). The material is clearly spiraling around the core, feeding it.

What’s weird is that there’s no disk to be seen in this image, and for stars this massive the disk should be pretty obvious. The streamers are bringing in gas from about 300 billion kilometers from the star (2,000 times the distance from Earth to the Sun) down to about 60 billion km (400 Earth-Sun distances, or about 15 times the distance of Neptune from the Sun). At that distance, the gravity of the star is balanced by the centrifugal force the gas feels as it spins around, so it stalls there, and the flow is reduced to a trickle. Well, a trickle on a cosmic scale; it’s still about the mass of Earth falling into the star every day, which is a mind-crushingly colossal amount on human scales. The streamer can be traced even farther down from there to only about 9 billion km, where it may finally form a very small disk (at least, small for such a massive star), though if it’s there it’s not detectable.

Interestingly, if you look closely at the image you can see a bit of gas forming a vertical line from the protostar. That’s an outflow, a jet of material moving away from the star. That part is aimed toward us, more or less, and there is some indication of another jet on the opposite side of the protostar moving away from us. Those jets can be created when there’s a disk of material around the star; the magnetic field in the disk can pull material away, focus it into a beam, and blast it away. So that’s indirect evidence there is a disk. The jets look weak to me — in many protostars, the jets are super obvious — so that also might imply the disk isn’t very big.

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