Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy

Main Sequence Stars – Size Matters

With this article, I’m going to talk about the size of stars and what that means for astronomers. I’m going to talk about what fuels them in their early lives, why there are so many different types, and also start hinting towards what happens after they die. The biggest challenge with this topic really is distances. I mean, how do we even start to comprehend the sizes of these massive nuclear power plants? But, to start, let’s look at the best example we have – the Sun.

Below is a picture of the Earth (click to make it bigger), which I’m choosing as our starting point because it is the biggest object that we can properly think of in size.  The radius of the Earth is approximately 6,300 km, and has a circumference of 40,000 km at its equator. If you were to drive a Bugatti Veron at its top speed (429 km/h), it would take you 93.415 hours to drive around it. Not bad. Now, below that picture of the Earth is a picture of the Sun, with a radius of 695,500 km, and the Earth. The Earth is that tiny little dot beside the arrow to the left of the Sun.  That same journey in the Bugatti would take you 424.4 days to complete. And here’s the scary part – the sun is only a medium sized star!

The Earth

The Earth

The Earth and Sun, side-by-side

The Earth and Sun, side-by-side

Ok, great. Now we have some sort of sense of scale. Where do we go from here? Well, how about comparing our sun to other stars? To do this, I’m going to need to talk very briefly on how a star is made – there will be another, indepth post about this later, right now I’m just going to glance over it. A star is formed when large gas clouds full of Hydrogen collapse due to gravity. Sadly, when I say collapse, I don’t mean a dramatic this-all-happens-in-5-seconds amazing movie. The collapse of a cloud can take thousands to millions of years, but more on that another day. Essentially, when enough of this Hydrogen is packed into a small enough space – BAM! FUSION! The hydrogen atoms start hitting off of each other so hard that they stick together to form Helium, and a star is born.

So, stars are formed from clouds of Hydrogen. So how do we get different types of stars? It’s quite simple really – the more Hydrogen is in the cloud, and the more of it that clumps together, the bigger the resulting star. Below is a picture of the Earth (not visible on this scale), the Sun and Sirius, the brightest star in the night sky, all beside each other. As you can see, Sirius is larger than the Sun (it’s radius is approximately 1.7 times the Sun’s radius), and, more interestingly, it’s also more blue. “Blue?” I hear you say “What does blue have to do with this?”. Well, when we’re talking about stars, colour means everything!

Sirius, the Sun and Earth all beside each other.

Sirius, the Sun and Earth all beside each other.

Have you noticed when you look at a candle, or even better, a bunsen burner, that the flame is red on the outside, orange in the middle and then blue towards the middle/bottom of the flame? That’s because the flame at the center is hotter than the outside. Hence, orange means hot, blue means very hot. And see how Sirius is more blue than the sun, and bigger? See where I’m going with this? Essentially, the bigger a Hydrogen burning star is, the hotter it is, and the hotter an object is, the quicker it burns up it’s fuel! So, looking at that picture of the Sun and Sirius, we’d think that

  1. Sirius is bigger than the Sun
  2. Sirius is hotter than the Sun
  3. Sirius will burn through it’s fuel quicker than the Sun

Hence, Sirius is going to have a shorter life than our sun, and should be younger than it! And observations have confirmed this! The Sun is roughly 4.6 billion years old and is predicted to have another 4.6 billion left before it’s done with its Hydrogen, while Sirius is only 0.3 billion years old and only has about 0.7 billion years left. Hence, we can say, at least for Sirius, that bigger, hotter stars burn Hydrogen quicker, and life shorter lives. Well, let’s take another example.

Sun, Sirius, Beta Centauri

The Sun, Sirius and Beta Centauri

Above is yet another picture, this one featuring the Earth (now completely invisible as it is less than a pixel big), the Sun, Sirius and Beta Centauri. Beta Centauri is one of the biggest Hydrogen burning stars that we know of. It has a radius about 10 times that of the Sun. That same Bugatti Veron ride as before would take 4,244 days. Now we’re starting to talk big. You might ask “If it’s so big, why can’t we see it properly in the night sky?”. Well, mainly because it’s 3,302,000,000,000 km away. As Father Ted would say: “These ones (in our case the sun) are near. Those ones (Beta Centauri) are far away”.

First thing to notice about that picture – Beta Centauri is, again, more blue than the Earth and Sirius! Which means, it’s younger than both of them! Infact, Beta Centauri is only about 14 million years old, and is due to use up the last of it’s Hydrogen very, very soon. So now, using the 3 stars above, I’m going to make a bold statement – by observing parts of the Universe, if we can see big blue stars (like Beta Centauri) then we know stars formed there very recently, or that these stars are about to pop. If we only see stars like our Sun, or smaller and cooler stars, then star formation stopped there a long time ago, and these parts of the Universe are pretty old.

This is a very useful trick for astronomers, as they can look at our Galaxy, the Milkway, and see where stars are forming. This gives us a better insight into the structure of our galaxy.

Now, one final thing. The title of this post is “Main Seqeunce Stars”. I haven’t told you what that means yet, but I’ve told you what they are! That’s right. When astronomers talk about Main Sequence Stars, they’re talking about stars that are burning Hydrogen! But this means that there must be other types of stars right? Well, there are. And quite a lot of them. You see, when a star is finished burning Hydrogen, it will get bigger again, until the temperature and pressure inside get so high that the Helium they made begins fusing. This star is then referred to as a Red Giant. Now, this means that all the stars I’ve talked about above are going to get bigger in the future, when they’re done with Hydrogen. And when I say bigger, I mean a LOT bigger. And then, when they’re finished with Helium as their fuel, they’ll get bigger again, until they can fuse whatever else they have in their cores! Infact, there’s a very long sequence of the elements that a star will fuse together (more information here, but I warn you, it’s quite heavy). So what I’m gonig to leave you with is this.

Below is a picture of NML Cygni. the largest star known to us. And when I say large, I mean it is an absolute monster! This star, known as a Red Hypergaint, has a radius of 1,650 times that of our sun. The journey in the Buggatti would take 1917 years to complete. Infact, if we replaced the Sun with NML Cygni, the star would extend out so that it nearly swallowed up Saturn. So, basically, it would eat us whole. Scary stuff

The Sun, Sirius, Beta Centauri & NML Cygni

The Sun, Sirius, Beta Centauri & NML Cygni

I’ll talk about why stars get this big another day. But, for today, all I want you take away is this – when talking about Hydrogen (Main Sequence) stars, the bigger they are, the brighter they are and the faster they burn. One of the largest known Hydrogen stars is Beta Centauri, and it should run out of fuel very soon. Finally, by looking at regions of the universe and seeing if blue stars are there, we can tell if star formation has taken place there and how old that system should be. Pretty neat, eh?

And finally, remember – the bigger they are, the harder they fall.


If you have any comments on this post, or have any ideas for what I should talk about on Monday, please let me know in the comments section below! Otherwise, I reckon the next post will be on the Pillars of Creation, where stars are born! Also, all of the above images were made using Universe Sandbox. It’s a fantastic program for playing around with if you have an interest in space!


About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

6 comments on “Main Sequence Stars – Size Matters

  1. peegee94
    January 31, 2013

    Hey bud !
    That was actually, dare I say…. Interesting ! But I have a couple of questions for ya.

    1. You say that stars have helium. Is that where we get our balloon helium from….?

    2. The harder they fall…? As in when a big star burns out it will fall hard on top of us?

    3. Why do stars shoot? That should be a topic in one of the next blogs!

    4. If stars r made up of hydrogen does that mean its possible to create a star in a chem lab?

    5. Is the beta star the bright one that we can see at night here?

    6. Is the moon a star?

    Think that’s bout it for now mark but I’m sure next time I see u ill have more 😉 good job on the blog bud. See u soon xxx

    • irishphysicist
      January 31, 2013

      Hello Peegee!

      1. Surprisingly, yes! All of the Helium we have either was created in the big bang, or was made in a star billions of years ago and just got shoved into space when the star blew up, and then ended up being part of Earth when the Earth formed a few billion years later.

      2. No, we’re pretty lucky that they won’t fall on us. What I mean about the star falling (and I’ll go into this in a few weeks) is when a star becomes too heavy, then everything in it starts falling in towards the center really fast. This is a kind of tricky one to explain, so I’ll go into it in later weeks.

      3. Stars don’t actually shoot. What we see are pieces of rock burning up in the Earths atmosphere. As you said, I might end up talking about this soon.

      4. Theoretically, yes, we could make a star in a lab. Infact, that is the aim of Nuclear Fusion – to smash Hydrogen nuclei together to create energy (and more Helium, hence more balloons for everyone!). Sadly, fusion is turning out to be very tough – I’ll definitely do a post on this.

      5. Beta isn’t the brightest star in the sky – that title belongs to Sirius! Infact, if you live in the Northern hemisphere, you can never see Beta Centauri as it’s located very close to the South Pole.

      6. The moon isn’t a star. There are several theories into how it was made. The most popular is that, during the formation of the solar system, the Earth hit a smaller planet. A lot of this other planets mass would have ended up here, but whatever was left over made the moon! This explains why the composition of the moons surface is very similar to places on Earth – again, this is probably something for the future!

      Thanks for the great questions Peegee, I hope I answered them ok, and I’ll try answer more of them better in the coming weeks.


  2. keith
    February 1, 2013

    Hey Mark,

    Decent attempt to make your subject more accessible to the great unwashed masses. I look forward to reading more. I also hope, probably as much as you, albeit for different reasons, that Beta Centauri does die in my lifetime.

    ” If you were to drive a Bugatti Veron at its top speed (429km/h), it would take you 93.415 hours…”

    Conversely, if you were to drive a Veron at top whack around the equator (assuming that it were possible), it would cost you €934,150,000.

    • irishphysicist
      February 2, 2013

      Hi Keith,

      Thanks for the feedback, it’s much appreciated. I didn’t look at the Bugatti problem from that way before, but I like it – forgot how much it is to drive one of those things.

      Out of curiosity, why do you want to see Beta Centauri pop?


      • keith
        February 2, 2013

        why do I want to see it pop?

        Cosmic pyrotechnics as the late, great Sir Patrick Moore once phrased it 🙂

  3. Pingback: How it’s All Going to End: The Wandering Black hole (Part 4) | Let's Talk Physics

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This entry was posted on January 30, 2013 by in Astrophysics and tagged , , , , , , , .
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