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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How To Make A Galaxy

Day Four. Essentially a galaxy is a whole lot of stars clumped together, but in terms of distance, “clumped together” hardly gives an accurate impression of the size of a galaxy.

Take our own galaxy The Milky Way as an example. Our Sun is just one of somewhere between 200,000 million and 400,000 million similar stars but to get from one side of the galaxy to the other, well you’d need to pack a big lunch.
Technically, it’s possible to build a craft that could travel close to the speed of light. It would have to be very large to accommodate enough fuel to burn constantly for several years, but eventually it could reach speeds approaching 186,000 miles per second. At this speed you could get to the middle of our galaxy (once you decide where exactly that is) in about 20,000 or maybe 30,000 years. There’s probably not much chance of visiting another galaxy anytime soon.

In Universal terms however, galaxies are not that far apart and they tend to be in clusters too, anywhere from a couple of dozen to a several thousand. Virgo for example is a super-cluster and has something approaching 2,500 galaxies. Three of these galaxies are really giant ellipticals and each one is around a million light years across. Compare that against our own humble spiral’s 100,000 light years across. We’re actually in a relatively isolated group of only 50 galaxies including Andromeda, which we will get to shortly.
We shouldn’t assume that seen one galaxy you’ve seen them all. Our home galaxy is the spiral type full of extra gas and dust with long arms in which new stars are being formed continuously. Other types have practically no gas clouds and have different shapes too, including lenticular, elliptical galaxies and irregular galaxies like the dwarf Sagittarius galaxy currently being ”eaten” by the Milky Way. (It rotates through us at a right angle to the disc and every time it passes through, more stars are ripped off to become part of the Milky Way.)
Generally the galaxies with the dust forming new stars have had the least interaction or collision with other galaxies, whereas those who have been though multiple mergers tend to have “smoothed out the bumps” and mopped up most of the free gas, no longer producing new stars.
While most galaxies formed early, not so long after the Big Bang, recent data from NASA’s Galaxy Explorer telescope shows that at least some galaxies have formed in the last couple of billion years, which is not long if you are a universe.
Back when the universe was young, there were a lot of atoms of hydrogen and helium not doing much, but over time (a time scale beyond our imagination) gravity pulled them together to form clouds that by their sheer size had accumulated so much mass and gravity, became so strong, the clouds so dense, the temperature so high, we had ignition as hydrogen atoms fused under enormous pressure to start creating more helium.
These first stars tended to burn out rather quickly but gravity was still collapsing clouds and pulling the whole mass into slowly rotating disks. These attracted even more gas and dust and eventually became the size of a galaxy. Inside the rotating disc new stars formed.
Those discs of gas that were spinning slowly tended to use up all the gas making new stars and are the lens shaped galaxies we see today that no longer make stars. The faster spinning discs formed spiral arms and continue to produce stars today, in our galaxy about 1 every year.

Some of the small galaxies can have a mere 10,000,000 stars. Ten million like our Sun, which is itself a million times bigger than the earth, may seem like a lot of activity.
Imagine having a dollar for every million stars in a galaxy. If you owned a small one, you’d only have $10. If you owned the Milky Way, you’d have at least $200,000 in the bank because our home galaxy has 200 – 400 thousand, million stars.
Now if you owned a really big galaxy, you’d be rich by anyone’s standards. The larger ones have up to 10 trillion stars, which would give you 10 million dollars in the bank at just 1 dollar for every million Suns. Of course you would have to be careful how you treated the super black hole in the middle.

Ellipticals make up about 60% of the galaxies and mostly they are a lot smaller than our spiral Milky Way but a few are bigger, a lot bigger. To some extent this is a result of the number of collisions that have occurred and that has shaped and to some extent, torn apart galaxies. The few (a relative term) that are a lot bigger are the most massive galaxies in the sky. They’re a bit messy in that their stars are not lined up in an orderly fashion around a flat plane disc. No, the ellipticals have stars orbiting all over the place and mostly they are old stars and you can just about guarantee they will also have a super massive black hole at the centre. These are the ones that have the 10 trillion stars and look a bit like an egg shape or maybe a football if you don’t follow soccer (the round-ball game).

Spirals make up about 20 percent of the galaxies and generally they are the brightest so they are the most of the most visible to us. All galaxies are held together by gravity but one of the curiosities of spirals is that instead of most of the mass (and therefore gravity) being in the centre of the spiral, most mass resides in the outer edges. Spirals come in 3 main varieties and they all produce new stars on a regular basis.

The irregulars make up the rest and mostly consist of large clouds of gas and dust but have no spirals arms and have a fair mix of new and old stars. For the main, they tend to be a lot smaller than the Milky Way.
While the distances between the galaxies is quite large, when compared to the size of the galaxies, they are relatively close, certainly closer to each other in proportion than stars are to one another.
Being relatively close means a certain amount of jostling, galactic pushing and shoving. Mostly this is shadow boxing at best as the gaps between the stars is so big, they hardly ever physically collide, fist on jaw, but the gravity of the component stars twist and warp the shape of the galaxies.

Our own Milky Way is about to take on the slightly larger Andromeda galaxy which is on a collision course with us as we speak. They are so large, the gap between them is only 25 times more than their diameters. When they meet, at a leisurely 500 kilometres a second, there will be little chance of stars directly colliding, but gravity will severely distort the shape of the combined mass. After they pass through each other and throw out a few unfortunate stars into intergalactic space, they will slow down, essentially stop and start moving back towards each other again for another collision. Eventually they will become one. Of course, we will not be around to witness the best bits as the process takes somewhere between a couple of hundred million years and a really long time.

The final result of combining these two spirals will probably be an elliptical-shaped galaxy, but it could still end up an even larger spiral. We can only speculate.


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