This is issue #701, so, in just 334 more weeks — 6 years, but more importantly in 1,000 more issues — I’ll have to write a special Star Trek version of this newsletter.

Yesterday, astronomers released a pretty cool image of Sgr A*, the supermassive black hole in the center of the Milky Way (the galaxy of which our Sun is a member). But the image is also weird, scientifically complicated, and (especially on social media where descriptions are needfully brief) in some cases explained in a somewhat misleading way. So let’s take a look!

The image was taken by the Event Horizon Telescope, a collection of several radio telescopes scattered across our planet. When their data are combined, their resolution — the ability to see small details — is the same as if it were from a telescope the size of their separations, so it’s kinda like having a radio telescope as big as Earth! This allows it to tease out incredibly fine details in distant objects, even when they’re small (cosmically speaking).

Sgr A* (short for Sagittarius A*, usually said out loud as “Saj A star”) is a black hole in the exact center of the Milky Way, with a mass of over 4 million times that of the Sun. That makes its event horizon (the classical black hole Point of No Return) about 25 million kilometers across. That can’t be seen directly — there’s a reason we call them black holes; light cannot escape once inside the event horizon. But their gravitational grasp is wide, and gaseous material falls in to the galactic center. It doesn’t plunge directly into the black hole, though: It has a little bit of sideways velocity leftover from orbiting the Milky Way center. As it gets closer that causes it to spin around the black hole at mad speed, and in fact right before it falls in it’s moving at nearly the speed of light!

It forms what’s called an accretion disk, with the center swirling rapidly, but material farther out moving more slowly. This creates immense friction, which in turn heats the material up a lot. It glows, and some of these disks can be so bright we can see them clear across the observable Universe.

Sgr A* is relatively quiet, so it doesn’t have one of those hellishly luminous disks. But there is some material falling in, so it does have a weak accretion disk. In 2022 astronomers released the first image of that disk. It was tough to create, because the material moves so rapidly its motion blurred out the image!

The image released yesterday is similar — it shows the accretion disk — but has new info in it: It also shows polarization. This is a property of light that tells us a lot about the magnetic field of the material emitting that light. I explained this in detail back on The Old Blog, back when the Event Horizon Telescope image of polarized light from the black hole in the galaxy M87 was released.

This is pretty important, because the gas in an accretion disk is so hot it’s electrons are stripped away from atoms, giving all that stuff an electric charge, and the motion of charged material can be strongly affected by magnetic fields. Some black holes have magnetic fields that power truly soul-freezing blasts of matter focused into narrow beams that scream away from the black hole — we call these phenomena jets. There are usually two, one moving up and the other down, away from the plane of the accretion disk.

Many galaxies have jets in their cores, and they get so bright that the jets are pretty much all we can see from truly distant galaxies. Understanding them helps us understand what’s going on near the black holes, so any info we can get on the black hole’s magnetic field is also critical.

That’s what the new image is showing us: The strength, structure, and direction of the magnetic field embedded in Sgr A*’s accretion disk.

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