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Chapter 1: What makes Polaris such a mysterious star?
Scientists have a problem with Polaris. From afar, the North Star's permanent position in the night sky has guided us for centuries. But from up close, it seems to behave in confusing and bizarre ways. And the closer we look, the less it makes sense. Polaris doesn't act as our stellar models predict, and it is surprisingly hard to get basic measurements for it.
How can a star be so clearly visible to our eyes, but evasive to our scientific instruments? Sounds pretty ironic. So let's dive into everything we know and don't know about this enigmatic star. I'm Alex McColgan and you're watching Astrum. Join me today as we unravel Polaris' hidden companions, explore its peculiar behaviour, and reveal why this star could reshape our map of the universe.
No, really, wait and see. It's a moonless summer night, and you've wandered to your favourite star-gazing spot. Looking up, you recognise Urza Major, which contains the Big Dipper, a constellation of seven stars in the shape of a ladle. You follow the two pointer stars up and up until you hit a noticeably bright star in the sky. This is Polaris.
It sits almost exactly above Earth's rotational axis, so while all other stars appear to wheel around the sky, this one stays put. At the North Pole, Polaris appears nearly directly overhead. As you move south, it appears lower and lower in the sky until you reach the southern hemisphere, and it disappears behind the horizon.
But while we've relied on Polaris to navigate the world, it turns out that Polaris itself is far more mysterious and far more complicated than anyone once believed. To us down on Earth, Polaris appears as a solitary, unchanging point of light. That is, unless you use a telescope. In 1779, William Herschel did, and he discovered Polaris had a second star, Polaris B, in a wide orbit.
120 years later, in 1899, astronomer William Wallace Campbell noticed Polaris A had variable radial velocity, suggesting it might have another companion star, one not visible through a telescope. In 1929, a spectroscopic study confirmed this, and we only got our first images of the third Polaris star, Polaris AB, in 2006, thanks to the Hubble telescope.
These three stars are bound together by gravity in a triple star system. The brightest component, Polaris AA, is what we think of as the North Star. It is an evolved yellow supergiant, 5.13 times the mass of our Sun and 46 times wider. If you placed it in our solar system, it would reach over halfway to Mercury. 2.8 billion kilometres away is its close companion, Polaris AB.
Together, these two stars form a binary system that completes an orbit every 29.6 years. The third star, Polaris B, circles this inner duo every 40,000 years, at a distance of 386 billion kilometres. That's about 24 times further than Voyager 1 is from us after 48 years of travel. Both Polaris B and A are yellow-white dwarf stars, about 500 times fainter than their primary star.
Polaris B is 1.39 times the mass of our Sun, while Polaris AB is slightly smaller at 1.26 solar masses. But Polaris' triple star system is the least interesting thing about it. Its primary star, Polaris AA, belongs to a rare class of stars known as Cepheid variables, which, as it turns out, are one of the keys to mapping the cosmos.
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Chapter 2: How did astronomers discover Polaris' hidden companions?
And in the depths of the southern constellation Scutum, We think that we've found the largest one yet. But just how big can a star get? And are there even bigger beasts waiting to be found? I'm Alex McColgan and you're watching Astrum. Join me as we climb the cosmic beanstalk into the kingdom of the universe's giants. You might think our Sun is pretty big, and it is.
With a radius of 700,000km across, if the Sun were a football, or a soccer ball for you Americans out there, the Earth would be 109 times smaller, the size of a 2mm wide peppercorn. Even if you added up all the mass of all the solar system's planets, the Sun would still be 743 times more massive than all of them combined. But when it comes to other stars, our Sun is nothing special.
Some of our closest neighbours, the Alpha Centauri binary stars, are a similar size, and Sirius A is twice as big. But how big can a star actually grow? To understand that, we need to look at stellar evolution. So our journey begins here, in the heart of something known as the Hertzsprung-Russell diagram.
Independently invented in the early 1910s by both Danish astronomer Einar Hertzsprung and American Henry Norris Russell, it plots stars' temperatures against how bright or luminous they are. And running down the centre of this diagram lie what's called main sequence stars. This spine is where most stars spend the majority of their lives.
If they're here, it means they're in a stable phase of existence. Having gone through the chaotic molecular cloud collapse of birth, a main sequence star is now continuously fusing hydrogen in its core, generating energy that pushes outward against the brutal inward force of its own gravity. It's a balancing act, and once it's reached, a star is said to be in hydrostatic equilibrium.
Almost all of the stars on the main sequence are in this state of so-called rest, but that doesn't mean they are all the same. Our Sun sits comfortably in the middle of the Hertzsprung-Russell diagram. It's a fairly average G-type main sequence star.
The G is what's called the spectral class, which essentially classifies stars by their temperature and therefore colour into a seemingly arbitrary naming system of OBAFGKM, where O is the hottest and M is the coldest. I remember it using the mnemonic, oh be a fine girl, kiss me, do with that what you will.
The Sun and other G-type stars have surface temperatures around 5778K, giving them the yellow-white hue that we're all familiar with. Their cores steadily fuse hydrogen into helium, converting about 600 million tonnes of hydrogen per second and emitting energy that, in the case of our Sun, has powered life on Earth for billions of years.
But if you move up or down the main sequence, the stars change and a pattern begins to emerge. It's probably not surprising that dim stars are usually cool, and that the hotter a star gets, the brighter it gets too, at least on the main sequence. But what might be less intuitive is how a star's mass relates to this. And there is a clear correlation.
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Chapter 3: What is the significance of Cepheid variables in astronomy?
We don't know. And unfortunately, we don't have the technology to find out. Yet. What I do know is that humans can't really wrap our heads around anything bigger than a few thousand kilometers, so the scale of the biggest stars is far beyond our comprehension. They are cosmic monsters. But in the grand scheme of things, even these stars are tiny.
Galaxies are tens of thousands of light years across, millions of light years apart, and individual filaments of cosmic web stretch billions of light years through space. The universe is so vast, we can't even begin to pretend to understand it all. That doesn't mean we can't try though. And bigger isn't always better.
I think living on a tiny planet around an average star is working out pretty well for us so far. Betelgeuse is a super interesting star. Not only does it have an incredible name, but it's one of the closest red supergiants to us, meaning that while it is cooler than the majority of star types, it has an enormous diameter.
If it was the star in our solar system, everything up until the asteroid belt would be contained within it. It's about 700 light years away from us, a lot further away than most other visible stars, but because it is so large, it's the 10th brightest star to the naked eye in the sky, and brightest in the infrared. It's easily visible as the left shoulder of Orion.
If you do look for it in the night sky, it is also visibly redder than any of the surrounding stars, and it does a lot of twinkling. There's something else very special about Betelgeuse. It is likely to explode in a supernova at any moment, although I say that in astronomical timescales. That means it could still take 100,000 years. How do we know that?
Well, you see, large mass main sequence stars, or stars in the adulthood phase of their existence, are powered by the nuclear fusion that goes on within their core, converting hydrogen to helium. This fusion creates an internal pressure, which combats the effect of gravity wanting to compress the star into a smaller volume.
However, eventually, the hydrogen fuel in the core will run out, having been converted to helium, meaning the fusion process stops, and the star's core can't overcome the effects of gravity anymore.
The core compresses, but if the star is massive enough, the compression will trigger fusion again, this time with the helium in the core into heavier elements like carbon, with this process repeating for oxygen and neon.
With every new fusion cycle, the star's internal pressure expands the diameter of the star until it begins the red supergiant phase of its life, when the core is being converted predominantly into iron. Red supergiants can't fuse anything beyond iron, so once the fusion stops, the star collapses completely into a supernova. And that's where we are at with Betelgeuse now.
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