Here’s a view of the twisted 5,000 light-year-long relativistic jet blasting into space from the supermassive black hole at the heart of the elliptical galaxy Messier 87 (M87), located 53 million light-years away in the constellation Virgo. The data comprising this image were acquired by the Hubble Space Telescope in January 2017 in optical and near-infrared wavelengths.
If you recall, the supermassive black hole in M87 is also the one whose “shadow” was famously revealed to the world in April 2019 by the Event Horizon Telescope team. This cosmic behemoth is estimated to contain the equivalent mass of 6.5 billion of our Suns.
What’s a relativistic jet, you ask, and how can it be coming out of a black hole if nothing—not even light—is supposed to be able to do that? Read on…
One of the questions I often get is “if a black hole is supposed to swallow up everything and not even light escapes, then how can jets of anything come out of it?” The thing is, the material in jets from black holes isn’t coming from inside the event horizon, and that makes all the difference.
Here’s a very basic idea of what’s going on:
“Feeding” supermassive black holes (SMBHs)—sometimes called active galactic nuclei (AGNs)—are surrounded by an orbiting accretion disk of super-hot material made up of the stuff of basically anything that got to close to them—gas, dust, planets, even stars. When the innermost material falls in past the event horizon (the point past which the gravitational pull of the black hole is stronger than the escape velocity of the speed of light) of the SMBH, it’s gone.
But SMBHs have, in addition to an incredible gravitational pull, very powerful magnetic fields due to their rapid rotation. These fields are like those on Earth or the Sun, but much stronger and complex. Earth’s magnetic field is a raindrop compared to a SMBH’s Niagara Falls.
The magnetic field lines from a SMBH can and do extend past the event horizon—they’re not limited by that. So plasma (all that super hot ex-star material) in the accretion disk can get caught up in the magnetic field lines and twisted, accelerated, and carried up toward the rotational pole of the SMBH.
Around the poles of the SMBH the field lines get very complex and unstable, and if they’re loaded with all that superheated material they can end up breaking, firing the plasma out into space in concentrated “astrophysical” jets sometimes at velocities approaching very close to the speed of light (as in the jets seen in M87.) It’s kind of like a railgun of star-stuff.*
Sometimes, again as in M87, the jet can “fool” us into appearing like it’s moving even faster than light. We know this to be impossible, for light and especially for massive stuff like plasma in a jet, and the reason for this is that if if the jet happens to be aimed toward us the material in it can look like it’s traveling faster than the light that’s coming from it. It’s a phenomenon called superluminal motion, but the jets aren’t really traveling at warp speed—just amazingly close to light speed itself (which in the vacuum of space is 670,616,629 mph.)
The jet appears globby in its shape because the SMBH doesn’t fire out material evenly but rather in bursts—probably caused by the SMBH’s irregular eating pattern. But also faster material behind can push against slowing material ahead and create turbulence forms inside the jet.
Image credit: NASA/ESA, STScI, Jason Major. Observation PI John A. Biretta.
*This is one running hypothesis of the mechanism behind astrophysical jets. There are others and really the exact process is still not precisely known for sure.