On paper, the two most spectacular astronomical discoveries of the last few weeks couldn’t possibly have less in common.
One is an image captured by the James Webb Space Telescope of a newborn star in our Galaxy, the Milky Way, about 450 light-years away. It’s an amazing picture of the birth of a solar system in a thin disc of dust in which planets are, as we speak, slowly starting to form.
The other is a combination of optical and radio data showing us a giant astrophysical system, larger than the Milky Way, and so far away that its light has taken more than 12 billion years to reach us. It gives us a glimpse of the incredible, intergalactic violence wrought by a supermassive black hole actively devouring its surroundings.
Still, a glance at the images shows a striking similarity, challenging any sense of scale. Both objects seem to be shooting light or material out into space in long, straight jets, stretching far out into the distance, like double-sided lightsabers.
Astrophysical jets are extremely common in the Universe and while the details vary, the physics that drive them are all based on the same basic features: gravity, rotation and magnetic fields.
The first thing you have to know is why so many things in space make discs. There are the spiral-armed, disc-shaped galaxies (like the Milky Way we live in); there are the proto-planetary discs that coalesce into planets orbiting more or less in a flat plane; and there are accretion discs, where gas and dust swirl into black holes and other massive objects with disc-y whirlpools around them.
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In all these cases, the formation of a disc happens in a few simple steps. First, gravity attracts material toward an object from all directions.
You might think that would be the end of the story and it would just fall right in, but since everything in space is moving all the time, the chance that the path is an absolute straight shot is very small. Generally, the material has some momentum to one side or another and falls in an arc, possibly settling into an orbit around the object.
But when there’s a lot of falling material – bits of gas and dust, all on different trajectories – it tends to get knocked around en route. If two specks of dust are coming from the same direction, they’ll start to orbit together, but if they’re coming from opposite sides, they’ll crash into each other, lose momentum and fall toward the centre.
Over time, this means that whatever the average rotation of the collapsing cloud was in the beginning, the whole thing will flatten out to a rotating disc in the same direction.
So gravity and rotation can account for all the discs. But how do jets happen? This is where magnetic fields come into the picture.
Magnetic fields are pretty much everywhere in the cosmos. They show up inside stars and planets due to the movement of the stuff inside them (like the fluid metal around Earth’s inner core or the roiling plasma inside the Sun), they trace through the spiral arms of galaxies and can even be found in intergalactic space.
Magnetic fields are produced whenever charged particles are moving and there are a lot of charged particles in space, because collisions and radiation can ionise otherwise neutral atoms, separating them into charged particles.
When gas and dust collect in discs, magnetic fields tend to be along for the ride. Those magnetic fields can form complicated shapes, but often it’s a kind of puffy doughnut on the disc’s plane with long streamers of magnetic field lines stretching out vertically at the poles.
When material swirls in along the disc, pulled in by gravity, some of those charged particles get caught up in the long, straight polar fields and spiral outward along them into space, forming a jet. How strong and narrow that jet is depends on the magnetic field and rotation that launches it.
In the case of a protostar, the jet can be bright, but with such a small amount of material and a relatively weak magnetic field, it tends not to extend much beyond the disc in the plane. A supermassive black hole at the centre of a galaxy, on the other hand, can launch jets that stretch hundreds of thousands, or even millions, of light-years into space.
In that case, the supermassive black hole is providing a powerful engine for the jet: the extreme gravity pulls material into the disc, and the black hole’s spin can tighten the existing magnetic field and launch the particles out with tremendous strength.
We see astrophysical jets elsewhere, too, like in stellar-mass black holes devouring their orbiting companion stars, and rapidly rotating pulsars beaming lighthouse flashes across the cosmos.
It’s best not to get too close to the source of the beams, but from a distance, jets can give us spectacular – and beautiful – insights into the gravitational and magnetic engines powering the most extreme wonders of the cosmos.
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