The Supermassive Podcast - BONUS - Can A Star Orbit Another Star?

Episode Date: March 12, 2025

If black holes are so dense, how can gamma rays jet shoot out from it? At what point in space does our sun become invisible to the naked eye? And a question about CubeSats, black holes and detecting alien civilisations... It time for Izzie Clarke, Dr Becky Smethurst and Dr Robert Massey to take on your questions!Got a question to add to The Supermassive Mailbox? Email it to [e-mail] or message us on Instagram, @SupermassivePod. There's no such thing as a silly question. In fact, the sillier, the better! The Supermassive Podcast is a Boffin Media production. The producers are Izzie Clarke and Richard Hollingham. Hosted on Acast. See acast.com/privacy for more information.

Hello and welcome to another bonus episode of the Supermassive podcast from the Royal Astronomical Society with me science journalist Izzy Clark, astrophysicist Dr. Becky Smethurst and the society's deputy director Dr. Robert Massey. We are getting ready to dive into your questions. Remember you can email them to podcast.ras.ac.uk or you can send us a message on instagram at supermassivepod. Okay, Robert, let's start with this email from Jimmy Muir. They say, Hi, I love listening to your podcast. And I'm fascinated with everything space related. A quick question for you is how far would you have to go before our own sun would be invisible to the naked eye? Could you see it

from Pluto maybe? And if so, how far away would you be before it disappeared? Thanks for everything and keep up the amazing work. Well, Jimmy, the sun is definitely visible from Pluto and even there it's about 300 times as bright as the full moon is on Earth. So magnitude minus 18, that's really quite bright. And it varies a bit depending on how far away Pluto is. It's got quite an eccentric orbit, so you could probably knock a magnitude or two off that sometimes, but it would still be bright. It would still be really, really bright. Intriguingly, NASA actually have a bit of a website where you can look up Pluto time, which sounds bizarre, which is when the sky on Earth is as bright as it is at noon on Pluto. I checked this and for me today, here in Sussex, it's five minutes after sunset.

Still quite a bright twilight sky, it is not a faint star. You have to go a really long way for the Sun to be invisible to the naked eye. It's a brighter than average star, but nothing like as bright as the brightest ones in the galaxy. So if you assume a limiting magnitude of 6.5, which is the definition that astronomers use, you know, the weird magnitude system where higher numbers means fainter, then the Sun is that bright from a distance of just over 70 light years. So to put it in perspective, the nearest stars are a bit over four and a bit light years, the Alpha Centauri system, and from there the Sun would be about magnitude 0.5, so quite bright, but fainter than some in the sky. But what it means, I think, in the sense of perspective,

it gives me is that if we traveled, if we were able to travel across the galaxy, the Sun would very, very quickly become quite unremarkable. Its light would just be merged into that mass of hundreds of billions of stars. It would just be another bit of the Milky Way. So, you know, for us, obviously, it's absolutely vital. It's entirely responsible for us being able to exist on Earth, but go, you know, even 20, 30, 40 light years away and it drops down into the background. Okay, thanks, Robert. And Becky, George Ellenberg says, Hi, guys, love the podcast. I am so addicted to it and eagerly await each new episode. I have listened to every

single episode." Thank you very much. I have a question if you guys can answer it. It's about black holes. If black holes are so dense and have so much gravity that not even light or photons can escape its gravity, then how can there be jets of gamma rays or particles or x-rays shooting out from it? Is it because everything from radio waves, low energy, to light, x-rays and gamma rays, high energy, are all photons, and that the x-rays and gamma rays simply have enough energy to escape the black hole's gravity well? If that's the case, falling into a black hole doesn't seem like it would be absolute. Would love to hear your thoughts on this. That is a great question, George, because I know it's probably incredibly confusing

that we talk about these growing supermassive black holes with jets and outflows of x-ray light. They are the brightest objects in the entire universe. And yet in the same sentence, we'll say things like, it's a black hole where nothing can escape. You know? Um, so the key thing to get across here is that all of these jets and outflows and this incredibly high energy light that you've heard about are not coming from the black hole itself, they're not coming from beyond the event horizon, that point of no return where you'd have to be traveling faster than the speed of

light. So all of these jets in this high G light are coming from what's known as the accretion disk around a black hole. So this is where if you have material drawn to a black hole, black holes are spinning because the stars that made the black holes were also spinning. And so you pull that material down into this flat disk in the same way that if you take a bowl of pizza dough and throw it up above your head,

it flattens out into a disc. And if you're on a merry-go-round, right, and you're spinning, you can feel that force pushing you outwards, right? That's what makes it into this flat disc. And so you've got all of this material that's being pulled down into this incredibly thin, incredibly hot, flat disc

that's basically all swirling and vying to be the stuff that falls into the black thin, incredibly hot flat disc that's basically all swirling and vying to be the stuff that falls into the black hole, right? But there's all of these collisions that's stopping stuff from getting too close and sending it back out again. That's what's helping to heat up the material as well. It's also traveling at huge speeds. And so all of this friction is, first of all, what allows the accretion disc to heat up and glow so that we can see it.

And that's usually in x-rays or visible light and UV light and it's how we find a lot of the, say, the smaller black holes in our own galaxy that are formed when stars die and go supernova, but then also supermassive black holes at the centers of galaxies as well, that are like millions to billions of times the mass of the Sun. And so what can happen is if you have too much material trying to get into that accretion disk around the black hole, because the accretion disk is glowing, it's actually putting a force back outwards because the pressure from that all that high energy light hitting into the material coming in.

So almost if you've got too much material in there, it then starts to put too much of a force outwards that you end up with these outflows back out again that send material back out. But again, it's only from the accretion disk. Nothing's actually crossed the event horizon yet. The jets are a little bit more complicated because it's something to do with magnetic fields. And the age old thing is that you never ask us about magnetic fields because they complicate things. Give us a lesson on magnetic fields please. Yeah, there's lots of complex magnetic field lines, all sort of like, you know, everything spins, everything just gets incredibly complex essentially. And so we know that jets are somehow launch magnetic fields, but people are still working

on like the exact mechanics of what's going on there. And again, the jet is being funneled from the accretion disk up and over the poles of the black hole nowhere near the event horizon. And that's how it can sort of escape from those regions. And this is what I always try and get out, right? Black holes are not hoovers, right? It's only when you get too close that stuff can't escape. But if you're still, you know, far enough away, which in terms of astronomical terms, even like two event horizons away, right? You're still fine. You still, if you travel at the speed of light, can get stuff to escape. And as soon as you got stuff traveling at the speed of light, it's when it starts to glow. And that's how we end up seeing it was such incredibly energetic light.

Oh, great question, George. Okay, Robert, Sylvia Rodriguez in California has a question about stars. She says, Hello all. Thank you for such a wonderful podcast. As a child, I grew up loving all things space and it's been a joy rekindling that love as an adult. That's lovely. I'm wondering if astronomers have seen a star orbiting another star. I know binary stars exist where they all orbit each other. But how about one star orbiting a central star? Will the star that is orbiting eventually collide with the central one? And would that star that is orbiting eventually collide with the central one and would that star that is orbiting technically be a planet? So Sylvia, this is a really intriguing question and to answer it, in every circumstance both stars

are going to move. They're technically orbiting around each other around a centre of mass rather than one absolutely sitting still. And that's true in the solar system with the planets as well, the sun does move. But it's just That's because they have a mutual gravitational interaction rather than just one object putting on the other. It is possible to have a system where you've got one which is much more massive and that wouldn't move if it had a very light companion going around. It wouldn't move very much, I should say, but still move a bit. I'm pushing it here and I was trying to think about the maximum possible ratio for normal stars, normalish stars at least. If the most massive stars may be pushing it 200 times the mass of the Sun, we're in orbit with the lowest mass, red dwarfs, and

if you ignore brown dwarfs, maybe a 13th the mass of the Sun, you get a ratio of 2,600 times. For a comparison, the Sun is about 1,000 times as massive as Jupiter, so it's not actually that far off that kind of system. There, the Sun is about 1,000 times as massive as Jupiter. It's not actually that far off that kind of system. There, the Sun moves about its own diameter. In the massive star red dwarf system, then the massive star would move a bit less than that. But that said, we could detect it quite easily with the exoplanet radio velocity measurements we've got, where we're looking at very small shifts in stars as planets go around them as they tug on them. And it turns out there's a paper a couple of years ago by

Madeleina Reggiani and her collaborators and they found O stars which are among the most massive of the sort of normal stars until you get into evolve ones and so on. And they have that, those stars some of those have masses about 15 times that of the Sun and companions a quarter of the Sun, so a ratio of 60, but you know not getting towards the kind of thing I was describing. But what we wouldn't do is describe the companions as planets, they're still very different to planets because they're shining through nuclear fusion, they are stars. And as for your final thing about whether they would collide or not, well it really depends on how close they are together but that's unlikely, in a more likely outcome with an O type star, a very massive star, is that it's going to

explode as a supernova and the smaller one is then ejected from the system. And we see examples of where that happens, where we can see high speed stars moving through systems. So good question there. I just love the idea of a high speed star just like like moving through lighting. They're moving quite quickly. Exactly. And Becky, we have a recording from a listener. So here's Keith. Dr. Becky and Izzy, thank you so much for making an awesome podcast. Dr. Becky, you mentioned the idea of launching a CubeSat at a black hole,

and it made me think about the effects on the CubeSat itself and a potential connection to SETI. As a satellite approaches the black hole, it made me think about the effects on the cube set itself and a potential connection to setting. As the satellite approaches the black hole, time would slow down for it. This means if any ancient civilization had the same thought for probing the edge of a black hole, the probe would experience a time shift that could put it millions or billions of years into the future, maybe even close to our time. This can be a possible way of detecting ancient tech near black holes from millions of years ago. What are your thoughts on this?

Oh, Keith, it's brilliant of an idea that is like the fact that we could find this like ancient tech just orbiting around a black hole. I think the chance of us picking the one black hole of the around a hundred million or so in our galaxy, the Milky Way that just happens to have already had like a CubeSat probe sent to it by an ancient civilization. They're low. I think that's overstating how low they are.

And even if we did find something, just to really turn your question on its head, working out how recently that probe had been sent to the black hole, I think would be near on impossible. But also if you think about it, our probe would also be affected by time dilation. So by the time our probe had sent back where that it had found something, we could be long gone. So I think as fun as the idea is, I don't think we should perhaps put too much scientific energy into figuring that one out just yet. But never mind, I do love it when we have a cheery end to the podcast. And that's it for this week. Do keep the questions coming or the recordings, you know, you can email them to podcast. Yeah, that was fun.

Yeah, that's the first time we've done that. You can email podcast at rs.ac.uk or find us on Instagram at SupermassivePod. We'll be back in a couple of weeks with an episode on astronaut training and going back to the moon, which I am so excited for. Izzy, I can't wait to hear who you've got as our special guest for that one. But until next time, everybody, happy stargazing.