As some of you know, I’ve had some issues with billing here on Beehiiv. So, a note:

When you subscribe to a newsletter on Beehiiv it’s set to automatically renew, so if you get it by the month or year, once the term is over it automatically bills you again and you get access to the next cycle. If you want to turn that off, then go to the footer of the newsletter email (the part in blue at the bottom). There is a link that says, “Update your email preferences or unsubscribe here”; click that and scroll down to “Account Actions”. Click “Downgrade”. That will end your subscription when the cycle is up. So if you do this, say, 3 months into an annual subscription, you’ll get the next 9 months, and then it will automatically downgrade you to a free subscription (just Mondays). If you want to unsubscribe after that, just go to that same link and click “Unsubscribe”.

My apologies for any problems! The interface for me here is a little more complex than it was at Substack so I’m still working through stuff. Thanks.

I’ve written many times about Fomalhaut (like BAN 723 and BAN 562). It’s one of the brightest stars in the night sky, and it’s young, only about 400 million years old. It’s also surrounded by an incredible series of disks and rings of debris left over from its formation, material that is likely forming planets.

It’s also part of a trinary star system! This part is weird to me: The other two stars are pretty widely separated from the bright primary star (called Fomalhaut A), way farther apart than I usually think of for stars that are orbiting each other. Fomalhaut B (also called TW Piscis Austrini) is nearly a light-year away from the primary, and Fomalhaut C (LP 876-10) is about 2.5 light-years out. That’s a helluva long way, but they are still all gravitationally bound into a single star system. 

Fomalhaut A is a bright A-type star, more massive than the Sun. B is a K-type, cooler and with a lower mass. C is an M4 star, a red dwarf, very faint and cool indeed. Even though the system is about 25 light-years from us, Fomalhaut C is 12^th magnitude, meaning so faint you’d need a decent telescope to see it. Red dwarfs are dim.

All three stars are the same age, as you’d expect for them all being in the same stellar system and likely born together. In 2013, a ring of material was discovered around star C. It was detected by ALMA, which can see cold gas and dust; anything above absolute zero emits light, with the wavelength depending on the temperature (you, as a living human, emit thermal infrared light with a wavelength that peaks around 9 microns).

Now, astronomers have used JWST to detect that same debris disk around star C in scattered light, which means light from the star reflected off of material in the disk [link to journal paper]. This makes Fomalhaut C only the fifth red dwarf to have its disk seen this way, and the coolest star in that cohort.

We’ve seen lots of disks around stars, but they’ve always been with much bigger, brighter stars (like Fomalhaut A, for that matter). For a while astronomers wondered if red dwarf disks were just less common than around more massive stars, but now it’s thought the disks are simply so faint they’re just hard to see — there’s less material in them than around bigger stars, and the red dwarfs themselves are so faint there’s not as much light illuminating the disks. That’s one reason the detection of the star C disk is so important.

Another reason is that seeing stuff in different kinds of light can tell us more about it. The light a disk scatters from the star depends on the size of the grains of material in the disk, for example, which can tell us about how old the disk is, how material is distributed in it, what that material might be (rock, ice, whatever), and more. 

Red dwarfs are hard to study because they’re so danged faint. Even with big telescopes they can make life difficult for astronomers. Being able to spot the leftover construction material from which they’re made is a big boon, especially when so few examples are currently known. Not only will this help us understand how these stars form, it will help us understand how planets form around them.

Mind you, red dwarfs are by far the most common kind of star in the universe, making up roughly 70-80% of all stars. We know they make planets, and they even preferentially form small rocky planets similar to Earth. That in turn implies that most Earth-like planets in the cosmos may have red suns in their skies. Our own home world may be an exception, so studying these disks around red dwarfs puts our own origins into contrast.

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