‘Neighbours, everybody needs good neighbours…..’
It used to be a programme that I religiously watched every day as a youngster at 5.35pm – before the Simpsons at 6pm of course – and I learned a lot from Harold, Lou, Madge, Susan, Karl and whoever Holly Valance’s character was! But let’s increase the scale of what we can determine as neighbours. Let’s go beyond houses in a street, even nearby cities, even neighbouring countries, even adjoining continents, even local planets, and even past our nearest stars.
Let’s talk about our neighbouring galaxies! But first let’s talk a bit about our own galaxy, the Milky Way!
Named after a well-known chocolate bar, the Milky Way contains around 300 billion stars, in which our own star sits about 30,000 light years from the centre point, around about the suburbs. The galaxy itself is about 120,000 light years side to side, and bulges in the middle at about 16,000 light years thick (but out by us it just 3000 light years wide). Someone should write a song about this by the way, maybe Eric Idle perhaps?
If you are wondering, a light year is the distance light – the fasted thing we know – can travel in one year. So if we travel at the speed of light it will still take us 120,000 years to cross the Milky Way, which I actually remember now is called so because of an ancient Greek legend in which one of the Gods threw a hissy fit and catapulted a giant pot of milk across the sky, and the chocolate bar came afterwards. Incidentally the ancient Greek word for milk is galakticos, where we get the word galaxy from.
The Milky Way is a spiral galaxy, containing 2-4 spiral arms depending on what side of the debate you are on. They spin around a supermassive black hole at the centre of the galaxy, taking 240 million years to make one full rotation. To give you an idea of how long that is the last time our planet was in this position in the Milky Way, the dinosaurs still roamed the Earth!
You can actually see the Milky Way with your own eyes, all you have to do is head to the countryside, and if it is dark enough you should be able to see a glowing pathway through the night sky. It contains all the light from billions of stars in another spiral arm, which due to the sheer number of stars and distance away blends together into one band across the sky. All the stars you can see are also contained within the Milky Way too, but there are other objects you can see which are not contained within our galaxy.
In the southern sky you can see the Large and Small Magellanic Clouds, which are dwarf galaxies that orbit the Milky Way. On top of these there are another 12-24 other dwarf galaxies that cannot be seen with the naked eye, but which orbit our own galaxy.
In the Northern Sky just now however you can see the nearest galaxy similar to our own, the Andromeda Galaxy. Along with Triangulum Galaxy, the Milky Way and Andromeda make up the Local Group of Galaxies, incorporating 54 galaxies overall, but with the above three spiral galaxies the dominant ones. The gravitational centre of this group is located somewhere between the Milky Way and Andromeda, with each galaxy moving with respect to each other. What’s more is that the Andromeda Galaxy is actually moving towards us and is on a collision course. So much for being good neighbours!
Andromeda is currently 2.5 million light years away, and over the next 4.5 billion years will slowly start to dominate our night time sky as is approaches, which would be pretty damn impressive to look at, but once it gets here then it is not good news for the Milky Way.
The thing is Andromeda is much bigger than the Milky Way, and contains around 1 trillion stars, and when two galaxies collide, the bigger, stronger one starts to strip away all the stars, gas and matter from the smaller one, and incorporates it into its own disk. This process will take about another billion years, but once finished the Milky Way will be no more. There is a theory that the reason Andromeda is so big already is that it has already collided with another galaxy. Below is a video simulation of what the collision might look like.
Now, there are only three major galaxies in our local group, but billions more in the rest of the universe, and the Hubble Space Telescope has been taking many photographs of these beautiful star clusters. But the other day the telescope produced the following photograph of a group of galaxies.
The universe is smiling at us!
Now, the two yellow eyes and white nose are galaxies, but the curves of light that make up the smile and the shape of the head aren’t actually what they appear to be. They are actually produced by a phenomenon called gravitational lensing, where by the sheer mass of the two galaxies bends space around them, and as a result when light from objects behind the galaxies passes by this gravitational effect it curves into a ring shape. They are called Einstein rings, and only occur when the source of original light, gravitational lens and observer are exactly in a straight line. No doubt a once in a lifetime photograph!