Monday, January 06, 2014

Relative Dimensions In Space

                                      Composite by Tom Buckley-Houston

This image of the Moon and the Andromeda galaxy in Earth's night sky (made from an original photo of the Moon taken by Stephen Rahm, and an ultraviolet image from NASA's GALEX mission) was all over the internet last week. It shows how big the Andromeda galaxy would seem if the entire span of its spiral arms was bright enough to see with the naked eye (usually only the core is visible without augmentation). The Andromeda galaxy is 140,000 light years across, but it's 2.5 million light year away. The Moon is just 3400 kilometres in diameter, but it is of course much closer - a little over 360,000 kilometres away at perigee, or 1.282 light seconds. A nice illustration of relative distance and size.

The eponymous artificial world of my Confluence trilogy orbits a black hole where most of the Large Magellanic Cloud used to be. The LMC is a satellite galaxy of the Milky Way, some 163,000 light years from Earth. It's about a tenth of the size of the Andromeda galaxy, but because it is much closer it seems much larger. In Earth's sky the Andromeda galaxy is about 3 angular degrees across (there are, of course, 360 degrees of sky), about six times the size of the Moon. The LMC is about ten degrees across, and the Andromeda Galaxy is about the same size of our Milky Way galaxy, that is, roughly ten times the size of the LMC. So if you stood on Confluence and looked back at the Milky Way on a dark night, it would be about 100 angular degrees across, filling almost a third of the sky. Imagine the sight:
Framed on one side by the bluff on which the Aedile’s house stood, and by the chimneys of the paeonin mill on the other, the triple-armed pinwheel of the galaxy stood beyond the edge of the world.  It was so big that when Yama looked at one edge he could not see the other.  The Arm of the Warrior rose high above the arch of the Arm of the Hunter; the Arm of the Archer curved in the opposite direction, below the edge of the world, and would not be seen again until next winter.  The structure known as the Blue Diadem, that Yama knew from his readings of the Puranas was a cloud of fifty thousand blue-white stars each forty times the mass of the sun of Confluence, was a brilliant pinprick of light beyond the frayed point of the outflung Arm of the Hunter, like a drop of water flicked from a finger.  Smaller star clusters made long chains of concentrated light through the milky haze of the galactic arms.  There were lines and threads and globes and clouds of stars, all fading into a misty radiance notched by dark lanes that barred the arms at regular intervals.  The core, bisected by the horizon, was knitted from thin shells of stars in tidy orbits concentrically packed around the great globular clusters of the heart stars, like layers of glittering tissue wrapped around a heap of jewels.  Confronted with this ancient grandeur, Yama felt that his fate was as insignificant as that of any of the mosquitoes which danced before his face.


Anonymous Anonymous said...

The human eye is actually remarkably sensitive: a 6th magnitude star (the faintest that can be seen) corresponds to something like ~ 10 photons/second striking the retina. Unfortunately, it's no good at integrating, which is why vistas like that remain the province of photography.

January 06, 2014 10:57 pm  
Blogger Hakan said...

Wouldn't be also dimmer than the Galaxy as we see it - we're closer to it than the LMC and it's hardly visible unless you're in a nice piece of country with clear sky with no artificial lighting around? Just like a reverse-shadow in the sky?

January 07, 2014 1:40 pm  
Blogger Paul McAuley said...

Hi Hakan - good point. A fair bit of our galaxy is visible in the night sky - all the stars in the sky. But the core as seen from Earth is, as you point out, fairly dim, because all the bright stuff in the centre is obscured by dust clouds. I'm not sure how tilted our galaxy would be in the sky of the LCM, but I've used novelist's license to assume that there is some tilt, so the bright core can be seen.

January 07, 2014 4:33 pm  
Anonymous Anonymous said...

The LMC is about 33 degrees above the plane of the Galaxy (the SMC is about 44 degrees from the plane), so the center of the Milky Way would be much more visible from the LMC than it is from our vantage point within the Galactic disc.

January 07, 2014 8:53 pm  
Blogger Paul McAuley said...

Thanks, Phil. Good to know.

January 07, 2014 9:49 pm  

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