Our own galaxy, the Milky Way, has between 200-400 billion stars, of which our Sun is only one. The scale is unfathomable, and yet we are no where near understanding the immensity of the universe surrounding us. Our galaxy is just one of approximately 30 galaxies that make up our Local Group, and just as our solar system is made up of great and complex orbits, so is the Local Group. The entire structure slowly revolves around its centre of gravity which is located somewhere in the 2.5 million light years that separates its two largest galaxies: the Milky Way, and M31, also known as the Andromeda galaxy.
Every star we see, up to 10,000 in Earth's greatest vantage points, is within our own galaxy. Why can't we see them all at night? Due to the artificial light generated that you are most likely using to read this. Light pollution blocks out most of our spectacular nocturnal displays and stars, and there is one particular light that is of huge intellectual significance. In the Northern hemisphere, the constellation Cassiopeia can be seen in Autumn and Winter. Shaped in a “W” between the Little Dipper and the North star, (and located more precisely at 0h43m, +41 degrees 16' for the amateur astronomer,) the entire galaxy of Andromeda is just visible to the naked eye as a smoky ball of light below the right most “V” of the “W”.Credit: historyoftheuniverse.com
The light from this seemingly tiny smudge travelled ~2.7 million years to reach our retinas, emanating from our closest major galaxy. Andromeda is by far the largest member of the Local Group, with around 1 trillion stars, (over twice that of the Milky Way, the second largest.) The diameter of the disc is 200,000 light years across, also twice that of the Milky Way. Its spiral shape has a skeleton of interstellar dust clouds, two beautiful arms composed of starlight, and a double nucleus at its centre; evidence of its merge with a smaller neighbour only tens of millions of years ago. The double nucleus is made of two dense knots of stars, with 5 light years between them. The true centre of Andromeda is a super-massive black hole of 30 million solar masses, probably caused by an immense super nova, (the death of a star.)
The first force human beings enquired into and understood was gravity, and if Newton taught us anything, we know that the apple will always fall to the Earth from the apple tree. Larger and more massive objects exert a higher gravitational pull that smaller bodies are unable to resist. Our own planet Earth was accreted and formed from smaller planetesimals that attracted each other and then collided. These planetesimals were in turn, formed from the pull of smaller clumps of matter towards each other, and so it continues, down to a sub-anatomical level. Gravity is the weakest of the known forces, but the most pervasive, and similarly, this same force is what pulls the Milky Way and Andromeda together until they must inevitably collide and fuse.Credit: Seth Jarvis, 2009
No-one panic just yet. Although the two neighbours are being drawn together at 120km/second, they won't collide for another 5 billion years. Once they are close enough for their spiral arms to touch, it will take approximately 900 million years for the nuclei to fuse and form the centre of a new galaxy, consisting of almost 1.5 trillion stars. It would be a truly spectacular sight, to view our Local Group from its gravitational centre, and our orbiting doesn't end there. The Local Group id part of the Virgo Super-cluster, an even larger gravitationally bound family of more than 100 galaxy clusters. We don't know how long it takes for the Local Group to rotate once around the Virgo super-cluster, but we know that Super-clusters are also bound together, forming galaxy filaments, or great walls. We are part of the Pisces-Cetus Super-cluster complex, and beyond that, who knows?