Where do the stars go in the daytime?

EPISODE TRANSCRIPT

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The topic of today’s episode was suggested on Twitter a while back when I asked people to tell me what kind of questions their kids asked them. Stuff about stars popped up A LOT so this is our first attempt to answer some of those questions. I mean, we all know the sun is a star but do we know how it was formed? Or what stars are actually made of? Are there different types of stars and how can you tell them apart? Do we know how many stars there are in the galaxy and how do we count them? What do stars do to the planets around them and what happens when they die?

I asked Dr Donna Rodgers-Lee to help me answer these questions. Donna is a theoretical astrophysicist based in the School of Physics at Trinity College Dublin. Donna's research focuses mainly on stars like our Sun which are very numerous in the Universe. She is investigating the effect of very energetic particles (like the particles accelerated at CERN) produced by the young Sun which cause the aurora observed on Earth and how these particles may have influenced the beginning of life on Earth.

Donna did her PhD at the Dublin Institute for Advanced Studies working on the early stages of star formation and considered the influence of magnetic fields on the evolution of young stellar systems. She then took up a postdoctoral research fellowship at the University of Hertfordshire in the UK where she worked on the influence of massive stars in spiral galaxies like our own galaxy, the Milky Way. She joined Trinity last year where she is currently a postdoctoral researcher and is part of Prof Aline Vidotto's "Cool stars and exoplanets" research group.

I began by asking her where do the stars go in the daytime?

Donna: Well, the very short answer is that the stars don't go anywhere! It's actually just because the sun is very close to us so it's really bright, and during the daytime the Earth spins, because it's a globe - well, not because it's a globe but it is a globe and so it spins - and during the day when we are on Earth, we can see, we're facing directly towards the Sun and so the Sun is the brightest star in the sky. At night time, the Earth, when we are looking in to the sky, we're facing away from the Sun and we're looking out towards the universe, and then because the Sun is no longer as bright we're able to see all the other stars.

Jenny: Ok, so because the Sun is closest to us, that's why it's the brightest star?

Donna: Yes, all the other stars are actually very far away. They're much, much further. So we orbit around the Sun in our solar system and then the other stars are very, very far away. It's sort of like during the daytime, if you go outside, and you look up and notice there's a streetlight that's still shining. You never notice that it's - it doesn't add to the brightness of the day, and it's only if you really look and stare at it you notice that it's turned on. So it's like that with the other stars. They're still there in the sky but they're just so dim that the Sun completely outshines them.

Jenny: Right, ok! So what is a star?

Donna: Well! A star is really just a very, very large ball of fire in the sky. It's held together by its own gravity, so it's quite dense and the gravity holds it together in a ball. It's bright and it shines because it's burning like fire on the inside.

Jenny: The question that I always think of though is could you actually stand on a star if you could withstand the heat, if you weren't going to like burn up, or would you just fall through it?

Donna: I think you just fall through it because it's quite... it's got a very sparse - like, the air on Earth, it's got this atmosphere. So it has an atmosphere as well, so you would probably travel quite a long way down through the star before you got to anything that was really solid enough to stand on.

Jenny: Fun times! Are there different types of stars?

Donna: There are actually. The classification of stars was started in the 1900s and it was pioneered by a number of women who worked in Harvard. So the classification they came up with, we still use today. It was originally proposed by Annie Jump Cannon, and so basically the Sun is quite a common, somewhat "normal" star. But there are stars that are much hotter than the Sun, and they're much brighter, and they have a different colour so you know like fire, if it's a bluer flame it's much hotter. Similarly, there are hotter stars that are bluer, and they're more massive so they're very heavy and they weigh more and then there are also much lower mass stars than the Sun, and they're even redder than the Sun is. Basically the colour of the star kind of helps you tell if it's hot or not, and that's how you classify them.

Jenny: Aha! How are they formed then, and what are they made of?

Donna: Stars, if you think on the biggest scale of the galaxy, and within the galaxy... So our galaxy is, the Milky Way is a spiral galaxy so it's like a big disc of material that has these spiral arms in the disc. Sort of like a fried egg! If you look down at a fried egg, there's a dense central core which is the yolk and then there's the white around it, and in the white patch there's these spirals. And those spirals, we can see them in other galaxies as well. You can spot the very, very hot stars. They're very blue, and they're very bright. So we can observe those in other galaxies. And just beside, or close to, those blue stars there's these very dark patches in the observations. Also, actually, if you look at the Milky Way bar up in the sky, you can see these dark patches across the stream of brightness, and that's basically dense material, so it's dust. And that's where stars form. So if you want something to form a star, it has to first be very, very cold because as something is very cold it starts to shrink and get smaller. This dust material and gas in the Milky Way, in the spiral arms, that's the densest part of the galaxy. There's the most material packed in to those places, and so it slowly, as it cools, it contracts and then at some point it becomes so dense that it becomes gravitationally bound so it's stuck together with gravity and it can't escape, and then it forms a star. Stars generally form together in groups, so you don't usually have one star forming on its own. You have a star forming region, and so there could be a few hundred or a few thousand stars forming together.

Jenny: Are some types of stars more common than others?

Donna: Yeah, that's why the Sun is quite a common type of star. So stars that are lower in mass are much more common, so there's about a hundred types of sun or lower mass stars, really small little things, for every big star. So big stars are very rare, and you don't see very - well, we do see many of them, but when you're trying to search for them when they're very young, it's hard to spot them, to watch the process because we don't see very many of them. 

Jenny: Do we actually know how many stars there are in the universe?

Donna: We're able to come up with an estimate. We can try and count up all the stars in our own galaxy, and then basically say, well if we know how many galaxies there are in the universe we can say for every galaxy there are a billion stars, like as an estimate. And then if there are a billion galaxies in the universe, there are a billion billion stars in the universe. So there is some sort of an estimate which is very, very, very large number which is probably like a billion billion billion! There's a huge number of stars in the universe, but there are obviously, you know, very dim types of galaxy that we don't see in the universe so we don't really know are there a lot of low-brightness galaxies out there that we just can't see. So we can come up with an estimate for how many stars there are.

Jenny: So even just around us, say in our galaxy, how would we physically go about counting them?

Donna: I think for our galaxy... so the galaxy rotates like the solar system, so the Earth orbits around the Sun, but then for the galaxy the stars all orbit the centre of the galaxy and so they all have a similar speed that they're moving at and so you can look, and also you can tell by looking if something is a star or a galaxy. So if you image the sky at night, and do this over and over and over again, things that are moving at the correct velocity you can say they're in our galaxy, and also you're not going to see stars from other galaxies because they're too dim to see, like, individual stars. You might see all of the stars together, but not individual stars. So basically you just watch them move in the galaxy, and say they're part of the galaxy, and the instruments basically just stare at the sky for hours and hours and hours and then they count up all the points in the images.

Jenny: So it's basically 'look and count'?

Donna: Yep, think so!

Jenny: Do we know how old stars are, or how would we even be able to know that?

Donna: When the universe began, there were a population, the first population of stars that formed, formed under very different conditions to the Sun. The Sun is a second generation star, so it didn't form in the beginning of the universe. So there were stars that were born that were very massive, they were very different to the Sun, and when they disappeared - so they created, in the beginning there was only hydrogen in the universe and those stars initially created heavier elements. And so then when the Sun formed you can tell that it has some of those elements in its core or in its shell, or in the interstellar medium around the Sun, so you can tell that the Sun formed after those stars. And then also by the age of it. I said that the Sun is like a fire, so it has a finite amount of fuel so you can age stars based on if you know how much fuel they have, so you can work out how old they are, and also you can look at the composition of the star to figure out how old it is. But it is quite hard to age stars because you can age the sun because it's very close to us, but it's harder to get the same types of observations for stars that are further away.

Jenny: Do stars have any effect on the planets that are near them?

Donna: Yeah, that's a very important aspect of research at the moment that we actually do a lot of work in Trinity on. So if the planets are too close to the star they get very hot, and so they're not very pleasant for life. That's why for Earth, Earth is located in the habitable zone - it's called the habitable zone of a star - and that's where liquid water can exist. So if you're too close to the star, like the Sun, the water evaporates off the surface of the planet, and you think of liquid water as being something that is good for the development of life on planets. And if you're very far away from your star, the water all freezes in to ice, and that's not very helpful either. There's this region around stars where we think it would be good for planets, good for life if planets live there. For planets that are far from their stars, if the star is not too hot, if it's like the Sun if you're very far from your star you're going to be very cold. So, like Pluto - which is not a planet! - it's very cold and icy, and so it's not very hospitable for life. But if you're very close to the star it's so hot, and it also strips... so it can evaporate the water, but it can also strip the atmosphere unless you have a very strong magnetic field which binds, traps the atmosphere. The Sun spews off material in a wind, and this wind effectively strips material off a planet so it's not good if you're too close. It can also distort the shape of the planet as well, it'll like flatten it into a kind of, if it's a globe, it'll flatten it more so that it bulges at the equator, and can actually rip material off the planet as well if the planet is too close to the star. So it's not good if you're too close.

Jenny: So stars are pretty violent?

Donna: Yeah, they can definitely eat their planets!

Jenny: You mentioned that stars have a finite amount of fuel, so does that mean that stars can die?

Donna: Yes, it does. And what happens to them when they die depends on how big they are to start with. So the Sun is not very big, and so when it dies in a few billion years - it's still got plenty of fuel left - but when the Sun dies it basically means that it runs out of hydrogen to burn so it doesn't have that fuel, so it starts to do other things. And that means that the star eventually kind of puffs up, and gets very red, and very, very large and then it cools over time and becomes what is known as a white dwarf. But if you're a more massive star, so if you're eight times as heavy as the Sun or bigger than that, when the star goes through and burns all its fuel, it's so heavy it can collapse down on to itself which causes a supernova explosion. A supernova is one of the most energetic events in the universe, and when a supernova goes off in another galaxy it outshines the entire galaxy, which would be a hundred billion stars and if you look at it, you will just be able to see one star. That's how bright they are when they go off. And so they happen in our galaxy every like fifty to a hundred years, so they're not very common in our own galaxy and we often have to look for them. Well, it's easy enough to look for them because they're so bright, so we can look for them in other galaxies as well. So yeah, there's a very different fate depending on if you're a small star or a big star, and actually when a supernova goes off that's how you can form a black hole as well - the thing left behind. So when it collapses on to itself, the core shrinks really rapidly and it can form either a black hole or a neutron star which is also another type of very, very dense star.

Jenny: I'd always wondered what black holes were!

Donna: Yeah, that's one way to form them!

Jenny: So we finish every episode by asking our expert what their favourite thing is about being a researcher, so what is your favourite thing about being a researcher?

Donna: I think for me, my most favourite thing about research is that you get to answer that question of 'but why?' You're able to spend a lot of time thinking about these questions when the only, you know, there is no... the outcome you're looking for may have benefit to society, but you're able to satisfy your curiosity and you're learning more about the physics of the universe and how things work. The sort of research that I work on is linked to how life begins on other planets, and so for me that's very interesting because it's just a curiosity, you know, for us to know our place in the universe and whether the solar system is a unique type of place to live, or is it something that happens very frequently in the universe and there loads of other worlds with life just like Earth, and so for me that's great that my research allows me to look at that, and I really like my job!

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Thanks to Donna for talking to me today! As always you can find a full transcript of this episode at tcd.ie/research/researchmatters. You’ll also find links to more information about some of the things Donna talked about during this episode, like the European Space Agency and the European Space Observatory. For anyone interested in reading more about Annie Jump Cannon there’s a book called The Glass Universe written by Dava Sobel that’s all about the women astronomers who worked at the Harvard Observatory and the stellar classification system they devised that’s still in use today.

Remember to send us your suggestions for future episodes any time! Tell us what you want to know!

Thanks as always to Tim Nerney who composed our music and Conor Reid at Headstuff who helps me with the production side of things. And thank you for listening! Make sure to subscribe and like wherever you listen to your podcasts so you don’t miss out on what you want to know!