How Many Galaxies In The Universe?

Cover: Astronomer Gerard Bodifee with wife TV Presenter Lucette Verboven

Day Six. By this stage in the life of the Universe (11,500,000,000 years ago) there exists something of the order of 170,000,000,000 (170 billion) galaxies each containing between tens of millions and billions of stars.

The name ‘galaxy’ is from the Greek word for milk which is how they described the whiteish appearance of the band of light we know as our home galaxy. Each of the 170 billion galaxies is indeed a group of stars, but much more than that. They almost certainly hold billions of planets, star systems, star clusters and in most cases, a massive black hole near the centre. They also contain a lot of material in the form of vast clouds of gas and dust particles that are mostly the remains of stars that have exploded at the end of their lives.
It has become clear that every galaxy has more mass (the ‘weight’ of something on earth) than we can measure by adding up what we can see. (A cannon ball in space may be weightless but it would still make a mess if it hit you because it still has the same mass.) This non-reflective material is predictably called ‘dark matter’ which incorporates ‘dark energy’. As Einstein showed, matter and energy are different forms of the same thing.

Galaxies come in a range of sizes and types from the dwarf galaxies with only 10 million stars to the big boys that can have one hundred trillion (that is, a hundred million million). 100,000,000,000,000. That’s a lot of zeros and it’s hard to imagine a number so large, especially when we are talking about very hot objects the size of our Sun, which itself is a million times bigger than the earth.
In between there’s not a lot going on, but technically, intergalactic space is not empty, but just as near to it as all get out, maybe 1 atom per cubic metre, which is sparse by anyone’s standards. In the popular press, quoting distances is done in light years, however the scientists in this field tend to use the parsec, which is 3.26156 light years. One might be forgiven for thinking that it’s not exactly a round number, an easily remembered number or one that trips lightly off the tongue, so why use it?
When we see a star, at the same time we also see stars further away. Six months later, when we are on the opposite side of our orbit around the sun, that particular star will appear to have moved slightly compared to the background stars. That’s because we are looking from a different angle.
You can close one eye and look at something close, then look through the other eye and it seems to move against the background. The parsec number is defined by measuring this angle. If it appears to move by 1/3600th of a degree, that is one parsec (like 3,600 seconds in an hour) the light would take 3.26156 years to reach us. The cosmologists need really big numbers and even light years aren’t big enough for the job, so parsecs it is.
Over the years, a fair number of galaxies have been catalogued, tens of thousands in fact but here is the interesting bit, there are 5 main catalogues and they don’t have all the same galaxies listed. Even when they are, they have different identification numbers.

Take Messier 109 for example. In the Messier catalogue, as you would expect, it’s number 109 but in the other catalogues it’s also code number NCG3992, UGC6937, CGCG 269-023, MCG +09-20-044 and PGC 37617. Clever boys.
Now, in the rest of the scientific world, the custom is to assign a name to whatever is being studied, even the least significant, invisible to us, microbe. Here we have entire galaxies, billions of times larger than our whole planet and all they get is a number. Well, up to five numbers actually, depending on how many catalogues we are looking at.

It should therefore come as no surprise that someone thought the galaxies were not getting a fair deal and decided to make a new catalogue, called with great imagination, ‘The Catalogue of Named Galaxies’ to give them a bit of dignity and some spiffy names.
Belgian astrophysicist Gerard Bodifee and the classicist Michel Berger began their new catalogue with one thousand well-known galaxies. Each were given a meaningful and descriptive name using the methods of other sciences like biology, palaeontology and anatomy. For example the aforementioned Messier 109 became the new ‘Callimorphus Ursae Majoris’.
Hell yeah, that’s a much better name.

One cannot pursue the subject of galaxies without mentioning ring galaxies, unusual in that they don’t seem to have much in the middle. They are thought to form in a type of bull’s eye collision when a smaller galaxy passes through the middle, cleaning up so to speak, leaving a neat ring of stars behind.
It looks like that might be what happened to the Andromeda Galaxy, perhaps more than once as it displays a multi-ring structure. There won’t be many rings left after the next collision when our own massive Milky Way smacks into it head on, which it is scheduled to do soon. (Soon in galactic terms, about 200,000,000 years from now.)

Despite the prominence of our galaxy and the millions of other large and very large galaxies, in sheer numbers, dwarfs rule. In fact you could say most galaxies are the smaller variety, perhaps only one hundredth the size of our Milky Way and often host to fewer than a hundred million stars.
Some of these are so small, light photons only take 326 years to travel from one side to the other, almost like crossing the street in galactic terms. They have all the same shapes as their larger cousins, elliptical, spiral and irregular, although the dwarf ellipticals require a little imagination to classify them that way.

Checking out our neighbourhood reveals that twenty seven of the local galaxies are dwarfs and that in itself is somehow surprising, but better yet, the mass of each dwarf (equal to about ten million of our Sun) seems to be very similar, regardless of the number of stars. This, it is argued, lends weight if you pardon the pun, to the conjecture that dark energy/matter is indeed the dark horse in gravitational theory.


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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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