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The Milkyway

A galaxy not so far, far away. Photo - DGW

We've been having an un-seasonally fine week here in Perth with sunny days and clear skies in the middle of winter so, I thought I'd take advantage of the clear skies and try out some astro-photography.

I hadn't looked at the galaxy in a long time and taking time to sit back and see it, gave me a unique perspective on how lucky we really are.

One thing that is unique about astro-photography is you need to expose your shot for at least 25 seconds and a photo would take a minute to render. This was something that I was unfamiliar with. I was so used to having instant feedback with everything. I hit record on my camera and it would record in real-time, swiping on my phone, scrolling through Facebook and Instagram; it was all instant, but this had a cost. Everything is so fast-passed that often I'd not fully engage in these moments.

Waiting for that 'click' gave me time to sit back and soak it in. It was quiet. The air was still and all I could hear was some distant sirens and the occasional muffled voices from neighbours as they carried across the night sky. I hadn't really looked at the Milkyway in years. I didn't really look up at night like I used to as a kid. Most nights I was snug up in my roofed house and if I did look up, there was too much light coming from the suburbs to really notice the stars. But, here I was waiting for my camera to click, forced to slow down.

My thoughts took me back to those times as a kid and I remember camping with my grandad. My siblings and I all sat around the campfire and my grandad would offer $2 to the first grandchild who spotted a satellite. You could see so many stars back then that the satellites felt sparse, but looking up now, almost every few minutes a satellite zipped by. It made me want to venture out somewhere far from light pollution, but for now I'd try my luck with the backyard.

I looked around for star clusters and realised much of the stars were focussed into just one band across the sky, the Milkyway. I'd seen the Milkyway many times, but never had never really thought much of it, but tonight, a question popped into my head. Why was the Milkway a band of stars across the sky, when we are in it? Why doesn't the cluster of stars surround us?

I remembered seeing a picture of what our galaxy looked like and it dawned on me that the Milkyway was actually a spiral disc and that band of stars in the sky was actually the arms of the spiral. This meant that above and below the Milkway were other parts of the universe, galaxies and suns shining back from beyond our own. But this only lead me to another question, why is it a disc and not a sphere like Earth and the rest of masses in our solar system?

I took my last photo and called it a night as the temperature dropped to 7 degrees Celsius, but I couldn't stop thinking about that question. I knew it involved gravity, but I couldn't workout why space created a 2D shape and other times a 3D shape. As I waited for my photos to copy, I did what any modern day questioning mind would do who wants answers; Googled it. I stumbled upon this video >> See end of post. It is hard to get your head around, but it explains how it works.

How spheres and discs form in the universe

So, basically there are three factors that create these shapes in the universe; the force of gravity, the density of matter and rotational spin.


Gravity shapes, moves and attracts matter. In space it acts like a plughole, pulling matter in and is the force that forms masses.


Matter is the physical stuff that makes up everything; such as gases, atoms and particles.


Masses are a collective of matter.


The density of matter is determined by how much those bits are compacted together. If it's rock for example, it has a high density because all the atoms are jam packed together. If its gas, it has a low density because the particles are further apart.

The other thing is, density of matter effects gravity. This is where things might break your brain, but the denser the matter, the more gravity it has and the more it can withstand the force of gravity. So, a rock for instance has a high density and stronger gravity and can also withstand its own force of gravity, so it won't collapse on itself. While, a cloud of dust and particles has very little density and gravity, so it collapses easily.

So, the denser the matter, the more it pushes back against the force of gravity, the less dense the matter, the less resistance it has to gravity.


In the case of creating a sphere, matter has swirled around and formed into a mass that is dense enough to have its own gravity, but also enough density to create pressure that resists gravity. Once it reaches an equilibrium its pressure is what stops it collapsing in on itself and forms a sphere.


As matter is drawn in by gravity, it spins around the centre, colliding with other matter until it reaches an equilibrium; where both forces from matter and gravity reach a point where they cancel each other out.


In the case of spirals and discs, smaller pieces of matter are gravitating towards a giant mass; a nearby a planet, much like Saturn and its rings or a blackhole. The surrounding matter clusters as it orbits, reaching equilibrium and this cluster forms a disc.


In the case of the Milkyway, the gravitational mass is a giant blackhole and the surrounding matter are the suns and solar systems that spiral around its centre. So, this is why our galaxy is a spiral disc and not a sphere.

If you got through all of that and understood it, give yourself a big bloody big pat on the back! Now you know why things form spheres and discs in the universe.

Out of all the solar systems in our galaxy and all the galaxies in the known universe, it just goes to show how crazily-lucky we are to be on this blue planet; just a grain of sand amongst trillions and trillions.


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