The Milky Way

Dear Diary. Day Five. After an unimaginable time span of 1,800 million years after the ‘Big Bang’ stars in this area ignite forming the Milky Way, our home galaxy.

It’s somehow comforting to think we have neighbours, perhaps lots of them, in our locality. Our “town” in the Universe is called the Milky Way and as far as we know, there are between 200 and 400 million suns much like ours in town. Now 200 million stars is a big difference, but you can’t just count them.
The main problem is that our solar system is a fair way out of town, on one of the big avenues (the Orion–Cygnus arm) and there are three others just as big. (Actually we are not right on the avenue, more like a side street off one of the main avenues.)
On top of that there are all the other stars around the centre as well so scientists have to work out the approximate mass of the galaxy and divide the answer by the average size star and you get a rough idea of how many stars there are in town.
When you think that our solar system has 8 planets made up from the leftovers from the sun’s birth, it’s hard to imagine all those other suns out there don’t also have at least a handful of planets too. To add to the fun, recent data from the Kepler space mission points to planets that are not attached to stars, just wandering about, probably a couple of hundred million of them.

Getting back to planets doing the right thing, the data strongly suggest that there are up to 40,000,000,000 planets orbiting stars in the habitable zones and 11,000,000,000 of those look just like our Sun. This is just in our galaxy so all that adds up to a lot of neighbours, but don’t expect a visit tomorrow. The nearest star to us (other than the Sun obviously) would take more than four years to get to and that’s only if we can work out some way to travel at the speed of light and we don’t bump into a speck of dust or something a little larger. The closest one that might have an earth-type planet is 12 light years away.
The reason the neighbourhood has a milky look about it is that our vision has only evolved to help us find things to eat and avoid others that might want us for lunch. Our eyes did not evolve to see stars, which is why we can only see about 10,000 of them (all in the Milky Way, although some argue Omega Centauri is just outside our galaxy) meaning we can see one star in 40,000. (An exception is the temporary super-bright flash of the death of a star, a supernova). The light from the rest blends into the band of light we see on dark nights. The dark patches are caused by interstellar dust that masks the light from the stars. In the Southern hemisphere, where the dark patches are most prominent, one of the most famous is the emu, close by that Australian icon, the Southern Cross.

It must have come as quite a surprise to Galileo, to see so many stars when he put his telescope up to his Mark-1 eyeball in the year 1610. He was the guy who worked out the earth was not the centre of anything and got belted up by the Catholic Church for saying so. All the way up to the 1920’s scientists thought the Milky Way was the only show in town. Man, were they wrong, and by a margin that’s impossible to grasp. There are literally billions of other galaxies out there (170 billion to put an approximate figure on it) most of them holding between millions and billions of suns and you can’t even see one star with the naked eye, only a few distant galaxies of stars.

Our galaxy is a spiral, that is, a centre disc with 4 major arms and fairly big as galaxies go, nothing like the real big ones but not a tiddler either at 120,000 light years across. If you thought of it as a very big city 120 kilometres across, our suburb is 27 kilometres out of town on the Orion–Cygnus arm. To get our size into perspective, if our Solar System was one inch or 25mm across, the Milky Way would be about the size of China, the USA, Australia, Canada or Brazil. As we see it from Earth, orbiting the Sun and rotating every 24 hours, the Milky Way passes overhead twice a day.
In downtown Milky Way, you will find “Sagittarius A star”, a supermassive black hole, perhaps a reminder of a few cities you’ve been to. You wouldn’t want to visit this one. It’s about 4.5 million times heavier than the Sun which is about a million times bigger than the Earth. The rest of the stars rotate around this point like a big pinwheel with the arms bending back as though they were in the wind. It takes us (meaning the Sun and our solar system) about 240,000,000 years to go around once even though we do it at a cracking pace, about 220 kilometres every second so it’s a rather long way around.
What is really weird though, is one would imagine that the further out a star is from the centre, the faster it must be travelling, however that is not what scientists have found. Most stars are moving somewhere in the 210 – 240 kilometres per second range regardless of their proximity to the middle. This seems to provide evidence for unseen matter, or dark matter as it has been dubbed, that is responsible for the variation in gravity needed to make this work.
Even that seems a stately pace when you consider the Milky Way itself is belting along at 600 kilometres a second on a collision course with the neighbour galaxy Andromeda which has 3 times as many stars although scientists think the total mass is not too different.
One fleetingly pleasant thought about our Milky Way is that is has a couple of bars. Apparently about two thirds of spiral galaxies have a bar or two but as you guessed you’ll never have a refreshing nip in one of these. For obscure reasons concerning the flow of gaseous material, the central collection of stars, including the black hole have formed up into a bar shaped structure that works mightily hard as producing new stars. We make on average, one new star every year.

I guess that means in another couple of billion years, we may have new neighbours popping up on newly formed planets, but on the other hand, some stars are running out of hydrogen so there are probably lots of planets being snuffed out every year too. It’s a rough neighbourhood.


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