Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website, www.rhysy.net



Monday, 25 March 2013

VY Canis Majoris - Or, Be Afraid. Be Very Afraid.

Recently I came across the following image :


Supposedly it depicts VY Canis Majoris, the largest known star. Not the most massive or the brightest, but the one with the largest diameter - a truly terrifying 1420 times bigger than our own Sun. It's a beautiful and inspirational image by the talented artist Facundo Diaz. But is it accurate ?

The image grabbed my attention, but the properly-cited physical characteristics on Wikipedia are what's really stunning. Other images have tried to put into perspective just how frickin' gargantuan this behemoth is, but I want to tackle this in a slightly different way. The most natural approach to me is to consider what would happen if, somehow, our Sun was magically and instantaneous swapped with VY Canis Majoris.

The result, in a word, is death. Complete and total death. Mercury, Venus, Earth, Mars, the million tiny worlds of the main asteroid belt, and even Jupiter - all gone. Jupiter, in fact, would find itself about 70 million miles below the surface of the star. It's a thought experiment to make Schrodinger's Cat look like the kindest act of animal welfare.



"Surface" in this case is a tricky concept. Although the star is staggeringly vast, it doesn't have all that much mass, considering. It may have a volume 10 billion times greater than the Sun, but its mass is only 17 times greater. That gives it an average density of next to nothing - a thousand times thinner than ordinary air (of course, the interior regions where nuclear fusion is occurring will be very much denser).

Not only is the outer part of the star a pretty good approximation to a vacuum, but the gravity would also be only about a quarter as much as on the surface of the Earth (on the surface of our own Sun, gravity is about 28 times higher than on Earth). And that means that the nice, well-defined spheres in the above images are just wrong. It might be better to think of it as a searingly hot, blinding nebula than a star. It might not even be spherical.

What it definitely would be is utterly spectacular. From the orbit of Saturn, the star would fill half the sky. It would appear less like a star and more like a stupendous flaming wall of fire.

I cannot hope to match the beauty of Diaz's render, but I shall give it a damn good try.

Actually, what it would look like to the naked eye is not much of anything, because it would cause instant blindness. The surface brightness of the star (i.e. how much energy it's putting out per square meter) is about one-sixth that of the Sun, which for all intents and purposes makes no difference. Imagine the Sun filling half the sky, and you get the idea.

In fact it's somewhat worse that that, because the amount of energy received by a luckless "observer" would be about five million Watts per square meter at Saturn - roughly 5,000 times what you get from the Sun on Earth. So there'd be no need to worry about the blindness because there wouldn't be time. The rings of Saturn - if not actually the planet itself - wouldn't last long either.

This means that whether Diaz's wonderful image is accurate or not is a moot point. It would be a sight, quite literally, of such awesome power that no human eye could ever see it.

Moving further out, things get better - slowly - but only because they can't very well get any worse. At the orbit of Uranus, the Star (it deserves a capital letter, and a better name) still absolutely dominates the view.



If we move along a planet, we come to Neptune. Now we can start to see that the Star is embedded in a nebula. This isn't surprising, given the insane amount of energy output, the low surface gravity and density - effectively, it's evaporating, wrapping itself in its own death shroud, if you will.

I cheated a little bit here. The "nebula" is actually an image of the Large Magellanic Cloud - a nearby galaxy - as seen in the infra-red.

At Pluto, the Star still appears much larger in view than the Sun does from Earth. Just in case I haven't hammered the point home yet, it's absolutely ginagorous.



The most distant man-made object is currently the Voyager 1 spacecraft. One hundred and twenty times further away from the Sun than Earth, it's somewhere that can only be described as cold, dark and incredibly lonely. Not so if our puny Sun makes way for YV Canis Majoris. Astonishingly, at this huge distance it would still span 6 degrees. The Sun as seen from Earth spans only 0.5 degrees.

Voyager model from NASA.

Oh, and I almost forgot. One day, this star is going to explode. Isn't science fun ?

17 comments:

  1. Very nicely done. Are there any calculations as to the temperatures you would experience at the various surviving planets, Saturn, Neptune, Uranus, Pluto? What would that mean for their moon environments?

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  2. When you say Jupiter would be located 70 million miles below the surface, are speaking with respect to its semi-major axis, perihelion, or aphelion?

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  3. I'm just going to leave this here: http://en.wikipedia.org/wiki/NML_Cygni

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    1. _____________
      _____________)- - - - - (eye)
      ( )( )

      I'm just going to leave this here

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    2. http://en.m.wikipedia.org/wiki/UY_Scuti

      And I'll leave this here. :)

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  4. Thanks !

    Saddletramp : Usings this online tool (http://www.astro.indiana.edu/~gsimonel/temperature1.html), the temperatures would be as follows :

    Saturn : 816K 542C 1009F
    Uranus : 555K 281C 539F
    Neptune: 455K 181C 359F
    Pluto : 345K 72C 161F

    Only Pluto becomes remotely habitable, and then only for extremophiles. Saturn becomes more than hot enough to melt lead, while Uranus and Neptune are both well above the boiling point of water.

    Anonymous (1) : That's the semi-major axis, but there's not a vast variation - it would never get anywhere near emerging from the "surface" of the star.

    Anonymous (2) : Very interesting. I guess I should probably emphasise that there's considerable uncertainty in estimating the size, by a factor of a few... it could be as "small" as only 600 solar diameters across, or as large as 2000.

    And to whoever linked this on Reddit - thanks !

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    1. So, would the solar winds wind up stripping away the gas layers of the 3 giants? Or at least Saturn?

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  5. Excellent post. As a kid, I used to imagine 'falling into' Jupiter. This is exponentially scarier, although arguably you'd live 'longer' in Jupiter.

    Quick question: I remember reading somewhere there could be a supernova soon from a relatively close star in this galaxy. If that happens, can you explain how and why it possible for a star lightyears away to 'cook' our planet?

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    1. Thanks Austin !

      I'm not sure which particular star that could be - Betelgueuse ? Eta Carinae ? - but, reassuringly, it turns out that there are no dangerous stars nearby :
      http://earthsky.org/space/will-a-nearby-supernova-harm-life-on-earth-in-2012-nah

      Though, as the article says, if a supernova _did_ go off very nearby, it would certainly be a bad thing. The article does a better job of explaining how and why, but in a nutshell, a supernova shines (very briefly) as brightly as all the stars in our galaxy, combined. An explosion as bright as a hundred billion Suns... well, you can imagine why that would be a problem !

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    2. I guess an extra pair of sunglasses wouldn't work...

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    3. I'm afraid that swapping our Sun with VY Canis Majoris is, on balance, a really bad idea. Please don't try this at home.

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    4. Well except is VY Canis Majoris went off, it wouldn't be a supernova. It'd be a hypernova because the star is so big. But you are right, because it is nowhere near us.

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  6. May I have your permission to re-host the images in this post, with appropriate credit and a link back to here, on my own personal blog?

    http://www.everydaysciencestuff.com/ha-puny-earthling/

    At the time of posting, I wasn't aware of where these images came from, found them on some random persons imgur account - so the bottom bit will gladly change, if approved :)

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    1. Yes, please do !

      Linking is appreciated (either here or my main website, www.rhysy.net) but I don't really mind, lord knows I've linked other people's images enough times without credit...

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    2. Credit given, linking to this specific post :)

      Thanks a bunch :)

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  7. A very entertaining work, thank you! :)

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  8. Very scary but very interesting at the same time
    Thanks for writing this also to what other people have been saying if a super nova were to happen near our solar system maybe in our local cluster, then it might in a couple million years create life of some sort as supernovas are the circle of life in space

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