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Galaxies

During the first billion years after the big bang, the first galaxies appeared and began organizing themselves into clusters and superclusters.
Freshly formed galaxies experienced violent phases in connection with their violent central black holes.
As the black holes rip stars apart and surround themselves with whirlpool-like disks of hot gas,vast quantities of energy are released. Over time, these Quasars and Active Galactic Nuclei slowly die down.
Gravity pulls matter together into galaxy-sized structures.
When a slightly overdense region of space with the mass of a galaxy begins to collapse, dissipation and cooling stop the collapse as the material approaches a kind of galaxy-sized structure.
Most of the first galactic seed regions had some slight degree of rotation, with a small amount of angular momentum.
Angular momentum is conserved during the resulting collapse. Rotating disklike structures naturally tend to form. These galactic disks support the beautiful spiral patterns that we often associate with galaxies, as shown in the image of The Great Spiral Andromeda Galaxy, at the top of this page.
When we look up into the night sky and gaze at the Andromeda galaxy, we're looking at it as it was about 200 million years ago. Which means it is 200 million light years away. This light has been travelling towards us before even human beings existed.
Starlight from the farthest known galaxies has been traveling toward us for more than twice the age of the Earth!
Everywhere we look in the sky, we see countless billions of galaxies, ranging in size from fewer than a million stars to a trillion or more. And from tens to hundreds of thousands of light years across.
Some are oval-shaped, filled with elderly stars, whilst other galaxies, like own own Milky Way Galaxy, are rotating spirals full of young stars and glowing gas.
More than half of all galaxies are ball-shaped, Elliptical galaxies, filled with old stars with no spiral arms of disk.
Ellipticals have very little dust and gas and no stars are being born inside of them.
Spiral galaxies such as the Milky Way and the Andromeda galaxy, have an elliptical central hub of old stars surrounded by a flat disc of stars containing two or more spiral arms.
The spiral arms are full of young stars, bright nebulae, gas and dust.
About a third of all galaxies are spirals or barred spirals.
Barred spiral galaxies have a straight bar of material running through the hub and extending on either side.
The bar is made up of stars in motion. It's thought that this pile-up of stars is only temporary.
At the centre of our Galaxy there is a supermassive black hole with the mass of 2.5 million Suns. We're about 25,000 light years distant from the galactic centre. (See "black holes" topic page.)
Our Milky Way Galaxy and neighbouring Andromeda galaxy are a part of what is referred to as the "Local Group" of galaxies, along with more than thirty others.
Gravity is pulling our Galaxy and the Andromeda galaxy towards each other at about 100km per second. These two galaxies will collide in about five billion years time (coincidentally around the same time as the Sun dies. So Earth will either be devoured by the Sun's red giant phase or be thrown out into the coldness of deep space when these two galaxies collide.)
The heat of two colliding galaxies instantly spawns thousands of hot new stars.
When galaxies collide, the collision causes each galaxy's gravity to pull at the material of the other. In the centre, gas clouds crash together, while at the edge, stars are thrown out into deep space.


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