Friday, 27 September 2013

Astrophysics Corner, Part 3 – The View from the Edge of the Galaxy

Astrophysics Corner, Part 3 – The View from the Edge of the Galaxy
In The Witches’ Stones – Book One - Igniting the Blaze, Dav Castillo explains a few facts about the Planet of the Amartos to Sarah Mckenzie:

"We're not about to let you out exploring at nighttime.  And even if  we did, it's pretty dark out there when the sun's down.  We're at the  very edge of the galaxy and there's no moon.  You wouldn't be likely to see much."

So, what would the view from the edge of the galaxy be like.   We asked our astrophysics consultant, Scott Olausen (a PhD candidate who is lead author on several Astrophysical Journal papers) for his view on “the view from the edge of the galaxy”, which he kindly submitted below.  Note that the planet is offset from Earth about 140 degrees eastward ,  at about galactic longitude 320 and 40,000 plus light years from the center of the galaxy, which puts it in the outskirts of the galaxy:

The first difference is that there'd be a lot fewer stars in the sky. Here on Earth, we can see about 5,000 to 6,000 stars in a dark sky with the naked eye. The vast majority of these stars are located within a few thousand light years of us, which isn't far compared to the size of the galaxy; we aren't seeing stars from halfway across the galaxy. Now, it turns out that the density of stars decreases as you get farther away from the center of the galaxy, so a planet near the edge of the galaxy (e.g. 40,000 light years out, compared to about 25,000 for the Earth) will have fewer stars in its neighbourhood than we do, to the tune of around 25% of how it is in our neck of the woods. Suddenly the number of stars visible even in the darkest sky drops to no more than 1,500. Other objects directly related to stars like nebulae and clusters like the Pleiades would similarly be more rare.


Another object that would look considerably different is the Milky Way, which as observed in the night sky is a faint band of light formed by starlight from the disk of the galaxy. On Earth, that band stretches all the way across the sky, though it is patchy and naked eye visibility varies. Conversely, on a planet at the edge of the galaxy the Milky Way couldn't possibly cover more than about a third of the sky or so. There's simply no galaxy to see in the other directions! It's harder to say, however, whether the Milky Way would be brighter or dimmer than it is on Earth. We see it as quite patchy because dust blocks a lot of the visible light, and the amount and distribution of dust is going to vary from place to place in the galaxy.

Apart from the Milky Way itself, there are a few galaxies that are visible from Earth with the naked eye. The brightest are the Magellanic Clouds, two relatively nearby dwarf galaxies (about 160,000 and 200,000 light years away) that are probably in orbit about our own. As it happens, the Planet of the Amartos is closer to these galaxies than the Earth is, enough that they might look about 20% larger and 50% brighter. Of course, the other visible galaxies, Andromeda and M33, are millions of light years away, so distant that they'll look basically the same from anywhere in the Milky Way Galaxy.

Lastly we'll consider globular clusters, which are groupings of hundreds of thousands or millions of stars, bound by gravity into spherical shapes, contained within the galaxy but not confined to its disk. About four or so globular clusters are visible with the naked eye from Earth, and I expect the numbers would be similar on the Planet of the Amartos.  Although they're less common further out in the galaxy, they're also easier to see since they're out of the disk of the galaxy so you don't have to worry about dust obscuring their light. There are even a couple globular clusters, NGC 362 and NGC 1261, that lie halfway between here and there so should look the same from both planets, but unfortunately they're too dim to be seen without binoculars.

Scott’s most recent paper,  “The McGill Magnetar Catalog” (an extensive catalog and population analysis of all known magnetars), has been submitted to the Astrophysical Journal.  A pre-print version is available at:


He has published several other papers on magnetars and high magnetic field pulsars in the Astrophysical Journal, one of the most prestigious journals in the field.


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