We know a lot about how stars form. For example, gas clouds call nebulae collapse — sometimes on their own, or when they collide with another cloud — and if the material is dense enough clumps will start to collapse under their own gravity. As it falls to the center that matter heats up, and material around it will form a flattened disk due to conservation of angular momentum. That accretion disk feeds the growing protostars, which eventually has enough mass to create pressure and heat in its core sufficient to trigger nuclear fusion. Voilà! A star is born! 

OK, great! But there’s a bit of handwaving there, and some steps missing. Of course, there are tons of details, and lots of different kinds of situations that can change this scenario (low-mass versus high-mass stars, single versus binary stars, and so on). Those details are difficult to ascertain, partially because the scales are small, astronomically speaking. A lot of action takes place with a few tens of billions of kilometers of the star, and that’s hard to resolve even with big telescopes since the distances to forming stars can be large.

One big question is how magnetic fields play into this. Are they a minor factor, or can they drive a lot of the action? We know the material falling in is hot enough to be ionized — the atoms have lost one or more electrons — and when those charged particles move around they create a magnetic field. But what role does it play?

New observations using the Atacama Large Millimeter/submillimeter Array (or ALMA) have given us a big clue on that.

In 2000, a binary system of two protostars was found in the Perseus molecular cloud, a huge complex of cold gas and dust about 1,000 light-years from Earth. The protostars, collectively called SVS 13A (with two component stars called VLA 4A and VLA 4B) are about 13 billion kilometers apart, and were seen to have a circumbinary disk — a disk of material surrounding both stars — that is clearly feeding them from a nebula outside them. A spiral arm was also seen in the disk, which is interesting: streamers like that can more efficiently feed material to the stars, helping them grow. The spiral runs for about 75 billion kilometers, a long way (the distance from the Sun to Neptune is about 4 billion km).

But what sculpts the arm? How does it keep its shape?

The new observations answer that question [link to journal paper]. The astronomers used ALMA to look at the stars in polarized light, which traces magnetism.

Let’s take a sec to talk about that. Light is a wave, a 2D oscillation of an electric field (there’s also a magnetic field at right angles to it, so this is really an electromagnetic wave). When you see light reflected off an object, those waves are oriented in all different directions; some are up-and-down, some left-and-right, and everything in between.

However, for some types of material, the reflection mechanism orients a lot of the waves in one direction. This is called polarization. So instead of all the waves coming at you with all different rotations, some larger than usual fraction is coming, say, up-and-down. This is why polarized sunglasses work; they filter out those waves, blocking them, reducing glare. The molecules of air in the sky polarize light, so sunglasses help a lot there (that also depends on the angle between the Sun, the air you’re looking at, and your eye, so if you look at different parts of the sky you’ll see more or less glare).

Here’s a fun thing: tiny grains of dust in space also reflect light. These grains, made of rocky material, are elongated a bit, and can be affected by magnetic fields, lining themselves along the field lines (think of iron filings aligning themselves along a magnetic field from a bar magnet and you’ll get the idea).

When this happens, the dust can polarize the light reflecting off of it. ALMA can detect that polarization, so it can use that to measure the strength of the magnetic field (the stronger the field, in general, the more polarized the light is). They did this for SVS 13A and found that indeed, the dust was polarizing the light strongly, and the direction of polarization follows the arm!

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