The History of the Universe

Day 4. How To Make A Galaxy

Imagine if our universe was a book and on important days, someone opened the book and wrote about it. ‘Day’ 4 would have been such a day.

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. Given that the distance between galaxies is many times more than the width of a galaxy, 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 (decommissioned in 2013) showed us 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.

References and links

The Milky Way galaxy
The first nuclear fire
A galaxy described properly
Our neighbour, the Andromeda Galaxy

(You can see ALL the interesting ‘days’ in the magnificent science poster which you can download and print.)


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