#GalacticAnatomy: Dissect the Milky Way

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As readers of a blog like this (and many others), it’s pretty safe to say that you’ve probably bumped into a book or two in your time.  No surprises there! Who read fantasy novels?

Yeah, me too.

Hands up those who used to love examining those little maps that seemed to be a standard part of many fantasy and science fiction novels? I’ve always loved maps; the hours lost in imaginary places. To be able to see the roads travelled by a band of adventurers, or the grim fortress overlooking some craggy windswept peak where the reluctant hero made their last stand was an important part of the story. It gave the events within the narrative a sense of place.

fantasy-maps
A randomly generated fantasy map. I’ll bet your imagination is already populating it with peoples and events… Image: Mark Frauenfelder, Boingboing.net

Well, the ultimate map is all around us. It’s the Universe. A couple of posts ago I explained the joy of the unexplored vista; of finding out what lies out in the unknown. This has always been my fascination with science and studying the Universe in all it’s infinite facets. Many moons ago I purchased a copy of an Australian science news magazine; Australasian Science. I couldn’t resist the picture on the cover, showing a generic artists impression of a galaxy, and a headline promising the secrets of the Milky Way.

This was a map, a road map to the greatest fantasy kingdom in the history of creation.

The article was a good one (so believed much younger Ben), and I actually still have that very magazine lying around somewhere. To be completely honest much older Ben still thinks it’s a cool article.

I will find that magazine and show it to you. It’s one of a few science publications that changed my life. That is actually a good topic for a future blog post right there.

Moving on. This magazine article introduced me to galactic anatomy. It dove into something called the GHZ, which stands for Galactic Habitable Zone. In addition the article taught me a little about the structure of our very own Milky Way. Obviously this was not a scholarly journal piece, although it was careful to include solid references where it could.  I learnt of things like metallicity, and a mysterious ring of this mysterious  stuff called the Galactic Halo, encircling our little ol’ Milky Way. Most likely a great many other galaxies possess such a band as well. Apparently the Halo is is a band of dark matter: that mysterious cosmic MacGuffin, which plays an as yet unknown role in galactic metabolism: akin to the way junk dna plays a role in the comings and goings of genetic information.

The Milky Way is a pretty standard galaxy, one of about 2 trillion in the observable universe!

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“My God, it’s full of stars….” A tiny slice of the sky, courtesy of Hubble. Image: Wikimedia Commons

Of course, standard can mean so many things (especially in this day and age), so I’ll put up another post when the fancy takes me regarding the galactic bestiary. For now, we’re going to focus closer to home. That’s not to say that the Milky Way is at all insignificant. It’s a vast armed spiral of stars and countless other stellar objects, self contained and packaged by collective self gravitation. It’s a cosmic gated community, encircled by a moat of dark matter, warding off the extragalactic night. Containing over 100 billion stars (and several billion planets) this behemoth is approximately 100,000 light years across- not counting our dark matter moat.

That’s a long way.  As Douglas Adams so succinctly put it:

Space is big.

A light year is the distance in which a photon of light travels in one year. Light travels at 186,000 miles in a single second. As Steve Irwin would have said: crikey!!

crikey
She’s a fair dinkum ripper!

The numbers are unimaginable. The Milky Way is vast beyond comprehension. The crazy part is however, that as far as galaxies and cosmic objects go the Milky Way is fairly modest in size. To quote a favourite line from the (unfairly) maligned movie “The Phantom Menace”:

 

N.B. I will often throw in a cheesy sci-fi reference. That’s just how I roll. Stories are an excellent resource, because they can sometimes help you see how the world works.

Welcome to the Neighbourhood

The Milky Way is one of three larger galaxies, forming part of a greater cosmic community called The Local Group. Andromeda and the Triangulum galaxy make up the other two heavyweights.

Local_Group.svg
The Local Group. With a diameter of approximately 10 million light years, the Local Group contains 36 galaxies or dwarf galaxies. Image: Wikimedia Commons

Ten million light years is a lot of distance. But it doesn’t stop there. The Local Group is part of a greater group: the Virgo Supercluster. This is in turn part of the Laniakea Supercluster. For, after the mere 100,000 light years of the Milky Way itself, things just got silly. This post doesn’t venture beyond the immediate neighbourhood of the Milky Way. As for the great beyond, to quote “The Neverending Story”; that’s another story, and shall be told another time.

We have nearly 50 smaller galaxies orbiting us! Three are of note. The Large and Small Magellenic Clouds are visible with the naked eye, and have been since before Man came down from the trees. It’s getting harder and harder to see them. Urban sprawl and light pollution from the world’s cities is inexorably removing the sky from the collective memory of the human race. If you ever can get a chance to get away to remote places, where the night sky is still awe inspiring, then do it! It is absolutely worth it.

 

The Milky Way, like all other galaxies has a structure. Over time our view of it has changed. It has progressed in the minds and eyes of observers throughout history. Native Australian mythology believed the band of stars stretching across the night sky to be some kind of river of the night, along which supernatural beings and the spirits of fallen warriors would travel between worlds. I have to admit, mythology really puts a fascinating spin on these things.

Whilst it may not be a river, the reality of it is just as interesting. As mentioned previously we see the galaxy as a band across the night sky. This is because we are looking along the plane of the galaxy. We are, after all, embedded deep within it, some 26,000 light years away from a supermassive black hole, some 4 million times the mass of our own sun. This monster is busily devouring neighbouring stars and clusters in the bulge which comprises the galactic core.

Universe Sandbox ² - 20170521-230725 Video
A hungry black hole prowls our galactic core…

Oh, to live in some science fiction movie, watching the spectacle of stellar destruction. Future tour operators really will have some impressive vistas to explore!

Let’s move on to this “Galactic Habitable Zone”. It sounds like some interstellar demarcation in a cheesy space opera. To my own biased ears it sounds positively mythical; like a really cut down version of the Galactic Empire.

Or, it just sounds cool.

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The blue band represents the GHZ. Image: NASA/Caltech

Readers of previous space based posts will have come across references to something called the “Goldilocks Zone.” Another word for this is the Habitable Zone. We on Earth only exist because we’re smack bang in the middle of it. Venus happens to orbit just within the inner cusp of our Goldilocks Zone, whereas frigid Mars lies at it’s very outer limits. It’s the zone around a star where the temperature is amenable to liquid water existing on the surface of a planet, hence greatly increasing its habitability. Of course life needs a little more than this, but it sure helps. As far as we know, no life can exist without liquid water.

The recently discovered TRAPPIST-1 system comprises a red dwarf sun, with FOUR planets within its habitable zone! In the above video, TRAPPIST-D, E, F, and G all lie within the TRAPPIST-1 Goldilocks zone, and so are all hypothetically capable of at least bearing life.

As these planets (and those within our own solar system) orbit their parent star, so does our solar system move around the galactic centre, with a galactic year lasting some 250 million years! What then, makes our position in the Milky Way so special?

As they say in the real estate business, location location. Consider you’ve just moved into a large city, looking to set yourself up. You start by looking for a home. They all look nice. You’re not overly fussy about the house itself, but it has to be in a good spot. There’s a fantastic place on the outskirts of town. Big, roomy, plenty of space to accidentally lose the kids. There’s also this nice little flat smack bang in the CBD. How do you weigh these two places up? The edge of town is nice, and there are plenty of open spaces, but that’s a drawback. There’s nothing out there. Like, really. No one to talk to but trees and ageing hippies. In the centre of town, there’s an insane buzz. Nothing ever closes. Everywhere you look, there’s plenty of action. But that also worries you. Also, the crush of humanity in the inner city is nuts. There’s no room to swing a cat anywhere. You’ll lose your mind within two weeks of moving in. No thanks.

So, you look to somewhere in the middle. Out in the suburbs, life is a little boring, but that’s probably for the best. Not much crime or craziness, with just enough to keep your brain from going stale. Oh, and you don’t want to be on a main road either. Little kids and traffic don’t mix. That decides it then.

Our position in the galaxy is something like that. Too close to the galactic centre and life would never have made it. It’s vastly overcrowded with stars and stellar clusters (not to mention Sagittarius A*) and supernovae rates are off the charts. You know what that means for life? Curtains. The Galactic Core is a hellhole of cosmic radiation. Life wouldn’t even start there.

Out on the rim things aren’t much better. When categorising stars, astronomers group and observe stars under two main types: Population 1 and Population 2.  Population 1 stars, like our own sun are deemed metal rich, or have high metallicity.  In the context of stellar populations metals are any elements heavier than hydrogen and helium; the most abundant elements in the universe. Such stars are common in the disc, gradually thinning further out toward the galactic rim. Heavy elements are crucial for the construction of all other material in the universe, ie planets and the organic compounds needed for life.

Out in the rim, Population 2 stars dominate. These stars are older, and are metal poor. Not much new stellar construction goes on out here. Stellar nurseries; the cosmic factory floor are more common in the disk.

So what’s all this about main roads? Revisiting the real estate analogy, roads are bad news, right? Hard core traffic is plain dangerous. Well, if we take another look at the galaxy (any spiral galaxy for that matter) we see that tremendous arms radiate outward from the bulge at the centre:

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A typical barred spiral galaxy, just like the Milky Way. Image: Wikimedia Commons

These arms are the main roads. Densely populated with throngs of stars and solar systems whizzing around the centre. Lucky for us the Sun is situated between two of these highways, free from too much interference from nasty neighbours. It whiles away it’s time, drifting peacefully through the quiet galactic suburbs. Life, as we’re finding out, is extremely tough but there’s no way on God’s green earth it would have started at all if conditions on earth were too apocalyptic.

I’ve left my favourite part of the galaxy til last. The galactic halo. The very name is evocative, carrying obvious religious connotations. We don’t really know it’s there, due to the fact that it is likely composed of dark matter. That stuff gets around, doesn’t it? It exerts an invisible pull on the large scale universe, drawing space and time into a new dance. It encircles the galaxy, like the diffuse corona of light surrounding the heads of saints in old paintings. To date, no one really knows what dark matter is; only what it does. It’s something like the wind: no one can see it but we can all see what it does. Something this mysterious is like science candy, and it’s the subject of much debate.

After all. Remember those maps I was talking about at the start of this post? My favourite parts were the hazy lands that lay outside the story….

This has been a slightly longer post, but it’s pretty hard to squeeze something 100,000 light years across into a few paragraphs. Please feel free to leave a comment. There’s nothing better than meeting other science buffs.

Have a good one.

Ben.

P.S., The European Space Agency’s Gaia Probe has been hard at work imaging and tracing the position and movements of over 1 billion stars, enabling us to gain a highly detailed picture of the galaxy and it’s development. I’ve provided a link to a YouTube video which visualises the movements of two million stars over the course of five million years.

Wanna keep up with Gaia? Follow them and catch all the galactic fun on Twitter!

 

References and other stuff you may like:

 

https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field

https://www.scientificamerican.com/article/sagittarius-satellite-spiral/

https://en.wikipedia.org/wiki/Satellite_galaxies_of_the_Milky_Way

https://www.universetoday.com/22285/facts-about-the-milky-way/

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