The Supermassive Podcast - 51: Into The Interstellar

Episode Date: March 26, 2024

Izzie and Dr Becky are venturing into interstellar space. What is it? Where is it? And what do we know about it?  With special thanks to Professor Martin Barstow from University of Leicester and Professor Chris Lintott from the University of Oxford. Chris' new book "Our Accidental Universe" is out now.  Check out the RAS's video on Pons Brooks here: https://ras.ac.uk/news-and-press/news/12ppons-brooks-how-and-when-see-devil-comet The Supermassive Podcast is a Boffin Media production for the Royal Astronomical Society. The producers are Izzie Clarke and Richard Hollingham. 

Hello and welcome to the supermassive podcast from the Royal Astronomical Society with me, science journalist Izzy Clark and astrophysicist Dr Becky Smethurst. This time we're venturing into interstellar space we're gonna be learning oh I liked that Izzy thank you for that yeah what is it where is it and what do we know about it? Andbert massey the deputy director of the royal astronomical society is with us too so robert where exactly does the solar system stop and then interstellar space begin like i presume there's not like a boundary as such like there's not a sign you are now entering interstellar space no it's more like a kind of fading away as you'd expect. So

much is in astronomy, really. So imagine the sun's gravitational force. Okay, it's infinite, but it fades off with the square of the distance. So, you know, you get to a point where it's infinitesimal or it's not significant. One of the definitions or one of the ways we can think about it is the influence of the solar wind. So the stream of particles that comes off the sun, and that has a certain pressure, pretty low, but a certain pressure certain pressure and that drops off too as you get further and further from the sun and one of the definitions is that when you get to towards the well not not the edge of the solar system which goes a long way but get get towards the influence of interstellar gas where those pressures balance out the so-called heliopause that's a point at which you can

argue that you're entering interstellar space. However, there are lots of things that orbit the sun beyond that point, beyond the edge of the heliosphere. And we have sent probes, Voyager 1 and 2, Pioneer 10 and 11, space probes that weren't designed to do this, but worked so well that they crossed out into the heliopause. At least the Voyager probes were able to measure that. It was so cool when that data came back. I was like, look at it.

It was like that drop off. We're outside. You could really see it though in the data, this drop off of solar particles and like increase of cosmic rays. I was just like, that's cool. It definitely is. And this is the point where you can argue that the sun's gravity carries on a long way beyond that in a significant way.

And there are lots of things orbiting the sun through what you might define as interstellar space in that sense too. So you can take different definitions, but if you take things like comets, they're quite a long way towards the nearest star, what you might define as interstellar space in that sense too. So you can take different definitions, but if you take things like comets, they're quite a long way towards the nearest star, towards the Alpha Centauri system. So it is definitely a fading out,

more than a kind of signpost saying, as Izzy said, you are now entering interstellar space. It doesn't quite work that neatly. And the Sun's influence goes out a very long way indeed. As it happens, I really like the interstellar medium and interstellar space because it was the subject of my PhD, specifically things like the Orion Nebula and so on, all these clouds of gas and dust that are just sitting there between the stars. So I've always had some enthusiasm for it. And if you look at the kind of things that astrophotographers like to take pictures of, it tends to be objects in interstellar space, you know, whether that's

giant clusters of stars or a beautiful nebula. it's something that appeals to most of us. Cheers, Robert. We'll catch up with you later in the show for some more questions and stargazing. So what is it actually like when we leave our little neighbourhood of the solar system and the heliosphere? Well, I spoke with Professor Martin Barstow from the University of Leicester, who studies this mysterious medium. And obviously, I started with the biggest question, which is, what exactly is interstellar space? It's mainly nothing, which is a good description of it. In fact, when I published a paper on it,

the Telegraph produced a cartoon about it saying we've discovered a lot more nothing than we thought we knew about. And you look at interstellar clouds and you see these beautiful pictures from Hubble and from James Webb. But you're looking at these things over a great distance. And actually, if you were there, the amount of material would be tiny, less than one atom per cubic centimeter. So it's very low density material. very low density material, but you do find regions that are a bit higher and even regions like the one we live in, which are very low density indeed, which are far lower than that. So it's very thin. It's a vacuum basically, wherever you are. But when you see the effects integrated over tens

and hundreds of light years, it actually becomes very significant. Okay. Okay. So you said there that we're a sort of low-density region. What does that mean exactly? How do we compare, say, to the rest of interstellar space? So we live in a region called the local cavity, sometimes called the local bubble. And we believe that this region was created by the exploration of a supernova several million years ago.

And the blast wave from the supernova, maybe several supernova, we're not entirely sure, swept out this lower density region. The typical densities around us are about a factor of 10 lower than you would see on average okay and so what is that stuff that that we're talking about here you know in terms of the chemistry of interstellar space what is out there i know there's not a lot of it but you know if we came across some stuff what are we talking about here it's mostly hydrogen and helium okay like everything else in the universe. But there are also traces of other materials. And interstellar space is part of this overall process of cosmic recycling. The material that is in the interstellar medium right now is produced by stars throughout the history of the universe. So we see all the usual things in trace amounts, such as carbon, nitrogen, oxygen, and silicon, and iron. And I could go on, but basically it's the debris of stars that has been thrown off either when a star becomes a white dwarf

at the end of its life, or in a few cases in a supernova explosion and obviously we talk about like interstellar space is so vast you know there's not a lot of stuff that does that mean it is just cold is that what we're talking about no it is basically a vacuum yeah but different parts of it exist at different temperatures although what we mean by temperature isn't really how we experience it here on Earth. We feel the warmth of the atmosphere and we can touch things. We get transmission of heat to us. So when we talk about temperature in interstellar space we're talking about how much the atoms have been sort of what we call excited by either collisions or by radiation.

So if we have a supernova explosion nearby, that shockwave collides with the surrounding interstellar gas and it excites it up to high temperatures. So it's heating it up. And those temperatures can be high as 10 million degrees. A lot of interstellar space is quite cold, a few degrees above absolute zero. But also there are warm phases where we don't have quite such a large amount of heating as we get from a supernova explosion. And these regions exist in balance with each other. So it's quite a complicated sort of physical picture of what's going on. Yeah, yeah, definitely. So what do we know about interstellar space? And what are still some of those questions that we need to understand?

I think the structure is something that is quite hard to follow. One of the problems is that we're actually sitting in the middle of it. If we were looking at where we live from the outside, it would be easier. If we look at distant galaxies, for example, we can look at a galaxy like the Andromeda galaxy, and we can see all the bubbles of gas that were created by historical supernova explosions. But probing that structure when you're actually in the middle of it is really tough. And you've got to infer what that structure is from a whole set of observations, from different telescopes in different wavelength ranges. In 1977, we had the launch of the Voyager missions, the crafts that made it to interstellar

space. So how important have they been in our understanding of interstellar space so how important have they been in our understanding of interstellar space they're really hugely important um although again it's a complicated picture it's not quite as simple as we've just reached interstellar space yeah yeah we live in the solar system and the the local environment of the solar system is strongly determined by the sun. And the boundary, what we call the heliopause, which we would judge as the boundary between the solar system and interstellar space, moves according to solar activity. So Voyager has given us a great picture of how that boundary behaves. So I think they really are in interstellar space right now, but for a few years it wasn't obvious because the boundary kept moving past the spacecraft and then back again

in response to solar activity. So that heliopause is the sort of nominal boundary between ourselves and interstellar space, but it's a bit porous. And actually, we do see interstellar material flowing into the solar system. It's not a solid boundary that wraps around the solar system and protects it from what you might loosely call the outside environment of the rest of the galaxy. And what would you say have been some of the big findings or the big moments from those Voyager missions? I think the really big one was actually locating the heliopause. We all knew it

existed but there's a difference between a hypothesis and an experimental examination of what's going on there and how it behaves. And Voyager isn't just about taking nice pictures, which it did of the planets on its way out there. It's measuring the energy of the surrounding gas, the fraction of electrons that are out there. So it's doing a real in situ diagnostic examination of that region in a great deal of detail. Yeah. And outside of those Voyager missions, how do we actually study interstellar space? Well, Voyager are the only two spacecraft that have ever got there. There are plans, not necessarily very mature, to actually try to send another probe out there which is specifically designed to study the

heliopause and travel out into interstellar space and measure in situ what's going on out there. But it will take a long time to get there. So we are limited to using ground-based telescopes and space-based telescopes to examine interstellar space, basically by observing stars. So the material that is in the interstellar medium, we see shadowed against the background of the starlight, and we see absorption lines blocking that light as it travels on its way to us. And so if we can isolate

which of those lines are actually interstellar rather than those that belong to the star, then we can use them to understand the temperature and the density of the gas and learn a lot about the overall 3D structure if we can look at enough stars. Thank you to Martin Barstow. So, Becky, what are some of the challenges that researchers face when it comes to studying interstellar space? Because it's not like, you know, it takes so long to get there. You can't just keep sending, like, craft and things like that. Yeah, well, I mean, like Martin said as well,

there's not much to look at um i was thinking you know that saying like you can't see the wood for the trees right yeah this is like the the opposite of that like you can only see the wood like the entire forest like you can't see individual trees here at all so you know the trees being like the individual molecules in that metaphor which i don't know i quite liked um so we can really only study the interstellar medium like on large scales which makes things tough right and if you add in the fact that a lot of the information we want to get out about interstellar space is actually blocked by the earth's magnetic field or our atmosphere

right which you know for life great um but if you're an astronomer interested in the interstellar medium not so much you know we've only been able to study it from the ground with radio waves for example so radio lights that make it through the atmosphere so particularly emission from like hydrogen it gives off this specific radio wavelength of 21 centimeters it's quite like a famous emission from hydrogen and from that you can sort of track where gas is in interstellar space but everything else you you know, infrared studies, like for example, with JWST that we've seen recently, there's a credible like the Carina Nebula, right?

The cosmic cliffs as they were dubbed. Like any infrared studies just have to be done from space because that light gets absorbed by our atmosphere. So until like satellite telescopes got launched, like this field of study was quite literally didn't get off the ground, if you will pardon the pun, Iz. I'll allow it and do you think it's likely that we will see another craft venture into interstellar space in our lifetime? I mean I hope so I think if you'd asked me last year I would have said in our lifetime was a bit unlikely but then nasa announced this thing

earlier this year which i almost was like i'm sorry is this actually come from yeah yeah what this doesn't make any sense um they announced the idea that use lasers to power a huge swarm of tiny spacecraft and when i say tiny i mean like ready the laser yes but like we're talking like a few grams these spacecraft right and they would launch them towards proxima centauri b which is the nearest known exoplanet to us it's about four light years away in one of our closest star systems now because these spacecraft are so light if you smack them with a powerful laser right the energy from the particles of light in that laser hitting them acts a bit like a wind on a sail on a boat right and so they can be pushed off in one direction and all that energy from the laser accumulates meaning that they can actually be accelerated to

like 20 percent of the speed of light because they're they're so like they're so like just little gram spacecraft little teeny tiny things yeah so that you know they travel there through interstellar space investigate both interstellar space and then the planet itself and then return to earth all within the span of about 40 years blimey which sounds so sci-fi right and the fact they've also dubbed these coherent pico spacecraft swarms i mean swarm cps's you know but it would be a swarm of these almost like i mean you're thinking like nanobot sci-fi like marvel stuff almost right now but like they would be these just tiny little spacecraft and these huge big swarms that would just get sent out towards proxima surta to be on

a big laser but like the thing that came out from nasa was essentially like a technology feasibility study and they reckon like we'll be able to do this by the middle of the century so like 25 years time like i can't wait now i'm like yeah bring on the swamps oh my gosh okay well that is i mean it does it sounds like something from a sci-fi film but we will see if and when that happens maybe 25 years just stay tuned guys we all know things yeah exactly and i'm sure people will want to know like we've talked quite a lot about you know the physical properties and the chemistry of interstellar space but what about science of life of interstellar life i mean what this is all you want i'm just saying people want to know and by people i mean me what are

some of the sort of things that researchers would be looking for in interstellar space itself right you're just a bit limited to looking at what molecules are present so you know biosignatures as we call them whether they're sort of signs of life existing um obviously on a planet that's great because you can actually have life there but in terms of in interstellar space you're looking to see if you just have the molecules present that you will need for life so that, you know, if there are stars forming in the nearby regions, you can say, okay, well, you know, the molecules are there at least, you know, things like water and phosphine and dimethyl sulfide, but also just like all of the organic sort of compounds that you might need,

lots of hydrocarbons, things that and while and while i doubt like full-on like phosphine might be prevalent in the interstellar medium in the interstellar space there's you know somebody chemistry needed to create it obviously you could at least have all those molecules present like i said um and if these little swarms got to proxima centauri like it might be very obvious straight away if there's life if there's at least an advanced civilization there in terms of any signals being given off that they could detect but it'd also be a case of you know like spacecraft flying through the atmosphere you know like we do you know with saturn's moons and jupiter's moons and things like that where they sort of dive bomb them and swoop

off again just to get a taste of what molecules are present there So we've talked about what we know about interstellar space and our attempts to reach it. But sometimes an interstellar visitor swings by our neighbourhood. We've glimpsed two interstellar objects in recent years. But what are they? Luckily, this is something my colleague Professor Chris Lintott studies at the University of Oxford. And you might actually know Chris from the BBC Sky at Night. Well, his latest book, Our Accidental Universe, tells the story of surprise discoveries. And one of those is about the most famous recent interstellar object called Oumuamua. So I kick things off by asking him to define what an interstellar object is.

Well, for once, as astronomers, we've given things a sensible name. So interstellar objects are objects that come from beyond our solar system, from interstellar space. But we usually mean sort of asteroid-sized things, so tens, hundreds, thousands of metres across, that are travelling through space and which we encounter in the solar system. We have seen precisely two of these so far, but we think there are many, many, many, many, many, many, many, many, many, many, many, many trillions of them out

there. Amazing. And we'll get onto those specifics in a moment. But how do you know if something is from another system? So the first clue is speed. So these things arrive fast. And in fact, the two that we've seen, Umumura and Borisov, were coming towards the solar system so quickly that they can't have been bound to the sun by its gravity. So we do see comets coming in from the outer solar system, but they have a typical speed. These things are on orbits whose direction and speed tell you that they must have come from elsewhere. Okay, okay. So let's get on to the two objects that we've seen in our system. So let's start with the Oumuamua. What did that look like? And when did that come and visit us for a

short while? So this was the Oumuamua was discovered by the Pan-STARRS telescope in Hawaii, hence the name. Oumuamua is Hawaiian. It translates roughly as the scout from very, very far away. Okay, love that. Which is quite nice for the first interstellar object. It appeared as a dot, a point of light, moving in the Pan-STARRS images, and even with very large telescopes, and even with the very large telescope, it never appeared as anything more than a point of light. We saw it as it was leaving the solar system.

It had already come in past close to the sun and was shooting out by the time we discovered it. And it was already something of a mystery. We expected these things to be like comets. We think if they've escaped another solar system, they probably come from the edge of the solar system. Comets come from the outside of the solar system, so they should be icy. If you take an icy body, you put it near the sun, it should develop a coma, an atmosphere, and a tail. Umumu did none of those things. It just traveled on its merry way without apparently being bothered by its visit to

the solar system. And there were a few mysteries about it. So one was that its brightness was changing rapidly. And so the most likely explanation of that is that it's an unusual shape. So originally, people thought it was maybe cigar-shaped, a few hundred metres across, maybe 50 metres in width, tumbling through space. These days most people think it was a pancake shape. But the same principle. If you look at the face of the pancake, you see a bright dot.

If you look on the edge, you see a fainter dot. And then the other mystery is that it appeared to speed up as it left the solar system, or at least it didn't slow down as much as it should have done because of the sun's gravity. And so trying to work out how that could have happened, especially in the absence of cometary activity, has left us with something of a puzzle. Okay. Were you able to find out anything more about Oumuamua? Like how quickly do you have to like react to these things when you see them? Well, we only saw Oumuamua? Like how quickly do you have to like react to these things when you see them? We only saw Oumuamua for a bit less than a couple of months. And it's quite unusual in astronomy, because we will never get any more information about this object. So what we know now is pretty

much all we will ever know. There's not a lot more to say we've got colour, it's sort of reddish. So it would fit in colour with the bodies that lurk in the coip about in the outer solar system and so there's some idea that it had been exposed to interstellar space for a long time and that the processes that happen they can turn ice into this sort of reddish color the thing that that's annoying is it's tempting to think okay i know the orbit it ideally we'd be able to trace that back and it would point to a particular star. And people did try to do that. But actually, in the galaxy, things get jumbled up pretty quickly. And so we can't really say where it came from. It seems to have come from

a set of stars called the Pleiades Moving Group, which includes the Pleiades cluster, but those stars are scattered right around the galaxy. And so we know what family it came from, but I can't tell you which star system it came from. There was a suggestion, especially once people thought it was cigar-shaped, there's this famous sci-fi story, Rendezvous with Rama, in which a cylindrical spacecraft travels silently through the solar system and leaves mysteries in its wake. So people were speculating. We did listen in just in case Oumuamua was sending

radio signals home, but no sign of that. Okay, there is one more thing that we need to clarify here. There's a lot of talk about it potentially being a spaceship. So can you clarify why that that isn't the case? Sure. Well, why it probably isn't the case. So the first thing to say is it's not a stupid idea, right? If we are visited by spaceships, you know, I tell people who ask me about UFOs all the time that I don't believe in them because astronomers would have seen them. So spotting something coming from interstellar space, visiting the solar system for a brief while and heading off, that fits the profile. The idea has got a lot of publicity from a guy called Avi Loeb at Harvard, who's written books and is adamant that it's likely, he says, that Oumuamua is an alien spaceship. What he's doing is he's pointing to all the ways that Oumuamua is unusual.

He says it's an unusual shape. He says this acceleration can't be explained. And in particular, he puts a lot of stock on the fact that Oumuamua appeared to move with what's called the local standard of rest, which is the sort of average speed with which stuff at the sun's radius moves. So his idea is that you'd expect a probe to hang out and then swoop into a solar system. But the truth is, if you talk to people who study asteroids, then they tell you that we haven't looked at many 100-meter class asteroids. Oumuamua was much more interesting to people than a normal asteroid. So we don't really know what the shapes of those asteroids are. So maybe this isn't unusual. You can explain the acceleration by cometary activity, even a little bit of cometary activity that we wouldn't have seen. And it turns out when you

model the population of interstellar objects, Umuamua is not that unusual in how it moves through the galaxy. So the difference between me and Avi is that he puts the odds of Oumuamua being natural at a trillion to one. And then says, therefore, we should take it seriously at Zanes. I think if you talk to people who study these things, if you do the maths, then Oumuamua is unusual at a 50 to one chance. So if we get another hundred from Rubin, we should get two Oumuamuas. And as long as that's true, then I think we can say we've ruled out the weird alien spacecraft hypothesis. And it's interesting anyway

I don't, you know, it's fine to speculate about aliens, but it annoys me sometimes that people think that the object's only interesting if it's an alien spacecraft This is a bit of another solar system that's flown through us, that we spotted with telescopes, that we got to observe and puzzle out its mysteries It's been great fun. And I think we should respect that without having

to put aliens inside it. And then we had Borisov that came and visited in 2019. That's right. So how is that different? What was Borisov like? So Borisov, I think, saved a lot of us a lot of angst. So at that point, we had this one object, we'd only seen one of this population of objects, it was weird and so you at this point where no one knew whether all interstellar objects were weird like umuamua and our theories were wrong or whether we just picked up a strange object borisov which was discovered in 2019 by an astronomer in kazakhstan i think for whom it's named just looked like like a normal comet. All comets are interesting, and they're interesting in their own way. And there were measurements of things like

the carbon monoxide ratio that are interesting. But it basically behaved as it should. It arrived in the solar system, it developed a coma and a little tail. It may have broken up. Some of the images, particularly from Hubble, show at one point two nuclei appearing. So it's possible that its encounter with the sun has broken up, but it too has fled back into interstellar space and has disappeared from our telescope. So we've got two, one normal comet, one slightly unusual, both fascinating. And we're now sitting here waiting for the third interstellar object to turn up. Yeah, so how do you classify between an interstellar object and a comet? So it's the orbit that matters, because it's a

classification that depends on its origin, right? So it's an interesting thought, actually, because our solar system, we're used to thinking about asteroids and comets and so on. But there's good reason to think that many of the things that formed in the solar system have been scattered to interstellar space. So some evidence for that comes from looking at the Kuiper Belt from the region with Pluto in, where many of the objects out there appear to be binaries. So Pluto has its large moon Charon, but Arrokoth, which is the little Kuiper Belt object that New Horizons flew onto after it visited Pluto, that was a contact binary. It looked a bit like a snowman with two halves.

And there are many other binary ones as well. And one explanation for that is that we think the Kuiper Belt was about 1,000 times denser than it is today. So that means that 99.9% of the mass in the Kuiper Belt has been scattered into the galaxy. So there are little bits of the solar system flying throughout the galaxy, and which may be interstellar objects for other people. And we receive bits of other star systems in return. Oh, amazing. I mean, is it likely that you could look back at previous comets and then,

you know, reassess whether they might be interstellar objects? Yes, we've been doing that. So I've been doing some work with my marvellous PhD student, Matthew Hopkins, who's been working out the details of what we'd expect from a population of interstellar objects based on what we know about stars and how they move in the galaxy. And so we've played this game. So we've gone back in particular to a comet that's really close to my heart, which is Comet Arendt-Roland, which visited in 1957. I love that name. It's good, isn't it? Aaron Rowland. It feels like Aaron Rowland should have opened the batting for England in 1837 or something. Yeah, exactly.

Or he's the guy at the local pub who's always there with a half pint. We all know an Aaron Rowland. Exactly. Anyway, he's also got a comet. So comet Aaron Rowland was a bright comet in 1957. It was bright, it came on an unusual orbit. And so people said, maybe this is an interstellar comet. I'm not sure many people would have believed that, but we've been able to check.

And there's no evidence that that or any other comet that we've seen is interstellar. Then you've got to ask why we've only found two of these things if they're really common in the galaxy. And the answer is they're really hard to see. They're small. We don't always look for small, fast-moving things in the outer solar system. And so we think that at any one time,

on average, there should be at least something the size of Oumuamua, this side of Neptune, passing through the solar system. But we're not yet very good at seeing them. Hopefully, we'll be able to build up a larger stock of these things in the next few years. Thank you to Chris Lintorintor okay quick question becky though what is the future of better understanding interstellar objects like what would researchers like chris be looking for yeah i mean chris is literally like ready and waiting for another one just for context like yes chris is my office next door neighbor in the physics department at oxford and i just i'm just waiting for the day

that he like leaps up with joy because there's been another one spotted or something because i mean he's got dedicated time on telescopes to follow up on anything that we might find and he keeps joking to me that now he's actually written a book about how we've only ever spotted two another one will surely be found now that it's been published because you know yeah the universe is just a stickler for sod's law right so yeah yeah um how we spot these objects though is is like the big thing right all the hopes are really sort of on the vera rubin observatory which is this huge all sky survey with a eight meter mirror to scan the sky every three nights that should be coming online next year 2025 things crossed yeah again things get bushback so we'll see um but that survey has been designed to detect basically anything that

moves or changes across the sky in that time so every three days basically so asteroids yes any flickering stars like variable stars variable growing black holes all that kind of thing but hopefully some of these interstellar objects as well and like if people are being optimistic about how many we'll find like chris and colleagues reckon a hundred or so interstellar objects over about a 10-year survey if you could get that then you've got a population of these objects to study so you can start doing statistics of things like, you know, what are they all made of? And can we test our models of how we think stars and planet formation occurs and how many objects then get thrown out in that chaotic process of planet formation and stuff as well.

So it's a really exciting field. And it's also a really fun way of answering those kind of questions, right? Like you'd think, okay, planet and star formation, it's literally just, you know, make some models in a computer and run it and see what happens. And the idea that you could actually then test that with something that's swooping through our solar system because it's been kicked out of another one is just amazing.

I was just thinking, as Becky was saying this, there's actually a European Space Agency mission called Comet Interceptor. And this would tie up really nicely with it. It due to launch i think around 2030 and it will sit there at the i think it's the l2 point waiting for a comet to come into the solar system either a long period one or an interstellar one and if if if the timing is right it'll see this thing and then it will fly towards it which i think is super super cool. Oh my God, that's so great. So it may be the chance to take a close look at something like Oumuamua or another object of that sort.

So cool. How did they get that funded with that many ifs? I don't know. You'd have to ask Geraint Jones. He's the PI on it. Who wrote that proposal? Because I need it. Yeah, he's MSSL.

I need tips from that guy on writing proposals. He did. It always seems like a really cool mission to me. It sounds incredible this is the super massive podcast from the royal astronomical society with me astrophysicist dr becky smethurst and science journalist izzy clark shall we get on to some questions yes because the listeners always send in the best ones so yeah they're so good so um uh let's go with this one becky elijah phelps asks

if you were placed halfway between the milky way and andromeda what would you see i like this question elijah um i mean if you're thinking about interstellar space not much as you said it's very sparse it's like saying can you see air no but in terms of like the milky way and andromeda, like I'm sure that'd be a site, you know, that's worth writing home about. Because you've got to remember even now, right, that Andromeda, the galaxy spans six full moons across on the sky, even though to our eyes, it only looks like a sort of fuzzy, spongy patch, right? That's maybe a quarter of the size of the full moon something like that that's what we see

right it's just because it's so faint right now if you went halfway between milky way and andromeda you half the distance to andromeda which means andromeda doubles in size so that means it'll be about 12 full moons across in the sky now the milky way would look similar in the other direction maybe a little bit smaller, but again, a lot fainter than we're used to seeing. You definitely know that there were two galaxies there, but you wouldn't see probably all of that grand spiral structure like we're used to seeing like in Apple desktop backgrounds, for example, the famous M31 image, because your eyes are just not that sensitive enough to pick up all the light at that distance. But I bet if you took

a camera, that would definitely be your new desktop just not that sensitive enough to pick up all the light at that distance. But I bet if you took a camera, that would definitely be your new desktop background after that. Okay. And Robert, the kid Flash wants to know, how big is the observable universe? Well, when Izzy and Becky say good questions, I always sit there thinking, yes, hard questions. No, no, this one's not too terrible.

I mean, the answer is enormous. You know, essentially we think based on the rate of expansion in the universe and what we understand about its evolution, that it's about 93 billion light years across. So what that means is that the most distant objects are about, at this point in time, 47 billion light years away. And it's easy to confuse that with the fact that the most distant things we see we see them as they were 13 billion years ago or so because the lights traveled to us for that time but over that 13 billion years the universe has got a lot bigger

and i think the other mind-blowing thing about this is look this is only the observable universe there's all the bit beyond it that we just can't see because the light from those objects hasn't reached us in many cases may never reach us so we'd have really no idea no credible idea how big the the whole universe actually is okay and becky rafa kazima sent this question about gravity which is how does gravity work in interstellar space like like normal uh you know it just depends what you're closest to do you remember what robert said before when he said that like the sun's gravity is infinite it's just as you it drops off with the square of the distance so at some point it just becomes indefinitely small the pull towards the sun right so if you think about if you're halfway for example between the sun and

its nearest star system like the Alpha Centauri system. I was obviously really influenced by the previous question here, wasn't I? If you're halfway between the sun and Alpha Centauri in interstellar space, right? Alpha Centauri, the system is ever so slightly heavier than the sun. So you would feel an ever so slightly stronger gravitational pull to that system rather than the sun. So if you were completely stationary at that halfway point and you waited a long enough amount of time, you would eventually start to drift that way very, very slowly. You know, because remember, you know, Einstein's theory of general relativity, which is our best theory of

gravity, it says we can think of gravity as the curvature of space so when you're very close to a heavy object the curvature is is a lot greater so it's a very steep curve whereas far away it is still curved it's just not curved a lot so as any diy knows if anything isn't perfectly level right you'll know about it if you're if your nice patio that you've built isn't perfectly level, water will eventually make it to your house and start pooling there after a certain amount of time. Right. And it's a similar thing, right? As long as you're on this curved space, you will eventually start drifting towards whatever the nearest object is that has the biggest pull towards you amazing what a great analogy

i thought so too thank you and robert brian has emailed with this question as he says i've been wondering if multiple extrasolar objects were observed in a small amount of time would that imply that they came from the same origin and if so would that give enough data to give a good idea of where those objects roughly originated uh yeah brian great question i mean the answer is sort of yes i think if we saw objects ejected from the same systems you'd expect them to be moving you know somehow in a commonality with the parent stars they were they were thrown out from and as the sun runs into them the most likely place that we'll see these things is on average in the direction of the solar apex,

which is the point in the sky the sun appears to be moving towards as it moves around the galaxy. So if we saw them in a short period of time, then that would imply some kind of concentration. It's not trivial to decouple because you have to allow for the changing appearance of the skies, the Earth moves around the sun

and all those things. But if you do that work and you see them coming in some kind of stream, it would be really interesting. I think it would be really hard to necessarily pin down exactly which star they came from because they're likely to take millions of years to reach us. And, you know, even those stars move, you know, they move quickly, but we don't see the star changing that quickly. But over millions of years, we really would.

So it might be quite difficult to identify exactly where they came from but it would be intriguing I mean this is also I suppose finding these things is that again as Becky mentioned earlier the kind of thing the Vera Rubin Observatory can do so if we start to see you know more of these objects then perhaps we can start to see those patterns so I think the answer Brian is that this would be really intriguing but it might just be very difficult to track down exactly where these objects had come from even if they we think they have a lot in common okay thanks robert and if you're listening to this and you're like i have a question for the team then do email it in you can send it to podcast at ras.ac.uk or find us on instagram it's at supermassive pod we do all of them. We don't always have time for all of them.

We've had to start doing bonus episodes for you because you all sent in too many questions. But thank you. I appreciate it. Keep doing it. Thanks. So shall we finish with some of our usual stargazing? What can we see in the night sky this month, Robert?

And there's something that I'm very excited for. So let's start with that, please. Shall we start with the solar eclipse of the daytime sky in the night sky this month, Robert? And there's something that I'm very excited for. So let's start with that, please. Shall we start with the solar eclipse of the daytime sky in the US? If you're in the US on a track from, and I think Mexico as well, on a track through from Texas right up to the northeast and Canada, then you can see a total solar eclipse if you're in exactly the right place. Now, we're all excited about that. But what you might not realise is that you can also see

a tiny partial eclipse in parts of the UK as well. So as it happens, this is unusual because it's only the western half of the UK where it'll be visible, and Ireland and the best places in theory if the weather is good, is say somewhere like Donegal in North West Ireland. It'll be at sunset, so it'll be quite a challenge. And that's for a number of reasons, like if you have a cloud, any clouds along the line of sight, then you're not going to see it so those kind of things will make it harder but it but i think it's well worth trying for if you've got a clear sky

but but but as ever we always have to say get a certified solar filter of some kind from a reputable supplier because the setting sun can look safe but if you sit there and stare at it it's not going to do your eyes any good it can do serious damage so do do take those usual precautions but you know we'd be delighted to see images of this uh this eclipse which everybody rightly in the us is getting excited about for it from uk soil as well now beyond that the clocks are going forward so we're getting shorter nights but finally a little bit warmer weather i know i know shorter nights but warmth you know it's that fine balance isn't't it? What do you want? Exactly. Perfect condition.

She wants to move to Hawaii, basically. Pretty much. So it's very much spring stars, you know, and we've got these wonderful groupings, like we've got Arcturus in Boeotis, the herdsman underneath the plough, Spiker in Virgo, another bright star there, and then groups like Coma Berenices, which is the wonderful name of Berenices' hair.

And one of the things about this time of year is that they're all this is away from the milky way so the milky way tends to is running through places like to the minasuris to the east of orion and it's running in the summer sky but in the spring in the autumn you've got the time of year when it's not so prominent but what that means is that you're looking out of our galaxy so you can see other galaxies more easily and there are thousands of them that are visible if you know where to look so if you get a good pair of binoculars and you look at the constellation of Virgo you can see things like Messier 87 which will only admittedly look like a fuzz but it's a fuzz that happens to have a dramatic jet coming out of a black hole if you look at images online so you can have the satisfaction of seeing those things for yourself and this month April is also the time to see Comet Ponds Brooks which is low in the northwest at sunset we've talked about this

before I finally got to see it it was a challenge it did need a pair of binoculars it does look pretty much like a small fuzz maybe it'll get a bit bigger some of the images you see online are obviously a lot better because cameras collect light for longer and they see the tail and the colour and all of those things but under perfect conditions if you don't have a moon or light pollution and you've got a very clear sky you might get to see it with your eye and a good night to look out for it is on the 12th of April when it's underneath Jupiter which is still hanging in there low in the west so it's a bit of a signpost and it happens, and I guess we should put this in the links, we've created an RAS explainer video as well,

which links to some star charts that a friend of mine put together. So do take a look and send us pictures of those if you can. You know what else is with it on the 12th of April? With Jupiter and the comet at the same time in the same part of the sky. Do you want to guess? Go on. It's the total moon. Yeah, there we go. Ding, ding, ding ding ding i was like i'm not saying it

i'm not saying it but it would be really nice though because actually like uranus is also just above as well so there's this sort of like you know square of uranus jupiter the moon and ponds brooks that if you i think if you're an astrophotographer you've got to you've got to try and get those four together oh my gosh if someone gets that image please send it to us yeah i'm really hoping to please it'll be that that yeah that's a crescent moon we'll have a nice earth shine as well it should be really good think about the phenomena and all those different in that like one picture like earth shine comet j Comet, Jupiter. It would be great. It would be so good.

Well, I think that's it for this month, except for one final question from Steve Newmontial in Canada, who says he loves the podcast. We're having an eclipse here on the 8th of April. Maybe you've heard. My question is, how important are pre- and post-eclipse parties? Like the best bit. i have done one i think the most glamorous one i did was in i haven't seen eclipse for a long time but i remember uh 2005

something like that anyway 2001 in manapools in a in a game park in zimbabwe where after the eclipse we were all served champagne and so on and watching it in the you know the briny that was that was yeah that was definitely that sounds pretty great it was pretty awesome watching the wildlife moving around us and things as well i was gonna say to steve let me ask you a question when you go to a wedding what's the best bit is it the actual ceremony itself or is it the party afterwards i think we all know the answer to that as a guest if you've never seen one before I think the best advice is often

don't try to take many pictures of it just enjoy the view during totality and only during totality because we don't want anybody's eyes damaged pick up a pair of binoculars and look at the sun it's just incredible

see all the prominences the moon literally moving in front of it it's absolutely amazing but do not do it when the eclipse is partial because that's be very careful because it brightens very quickly as well right at the end of totality so yeah you do get warning i mean it when it when it brightens up it's very obvious that it's about to end so you then just look away so just be sensible cool and i want to give a shout out as well to there's a map by the eclipse company that's really good because it also has along the map of like

showing you you know what time things start and all that kind of stuff it has events that are organized by the communities in those locations and like links to all the web pages so i think we'll pop a link to that in the um the podcast description for people in the us who want to plan what they're doing that sounds amazing um and i it makes me laugh every time we say don't look at the sun um because we always get emails saying like we know stop saying it but safety is important guys so if this is the first time you've come to the podcast we'll keep saying it and we'll always say yeah oh and uh do say hello to producer richard if you see him eclipse spotting in northwest

pennsylvania he's the only one of us that's going and we're not draws at all. But you can follow him and his wife, Sue Nelson, at Space Muffins. Yes, we love Space Muffins. And we'll be back next time with another bonus episode. And after that, we've mentioned a few times in this episode already, the Voyager missions. So those are the crafts that we've sent

into interstellar space. And we've had a request from listener Suraj to do an entire episode on Voyager. So we will. Yeah, you're welcome, Suraj. I'm excited for that one. Contact us if you try some astronomy at home. It's at supermassivepod on Instagram,

or you can email any questions you have to podcastras.ac.uk and we'll try and cover them in a future episode. But until then, everybody, happy stargazing.