Morgan, the protagonist in the Written in the Stars quartet, and I, have spent our working lives studying Galaxies. This is the first one he saw when he was looking through the 36-inch telescope at Steward Observatory atop Kitt Peak in Arizona, back in 1969.
This particular image was taken much later with a huge telescope 3.5 meters in diameter fitted with an ultra-sensitive electronic camera integrating the light for two whole hours. So imagine how little if anything Morgan would see with the naked eye which integrates for all of a tenth of a second, through his much smaller telescope: virtually nothing at all. That’s why he struggled for half an hour to even convince himself that it must be there, and why he felt that studying galaxies could become the lifetime challenge he was looking for. And so it came to be.
NGC 7331 just happened to be the nearest Spiral overhead his telescope at the time. All the spiky stars are stars in our own galaxy the Milky Way, just hundreds of light-years away, while the spiral lies 45 Million light years beyond them, yet is one of the closest such giant spirals to us.It is about a hundred thousand light-years across! The smaller galaxies lower down are even farther off. Morgan was to spend most of his astronomical life studying galaxies whose light had set out towards him when dinosaurs still ruled the Earth, and some so far away that the Sun and Earth didn’t even exist when their light was emitted.
Even now we know little about these beasts; they are full of mysteries. For instance they spin so fast that they ought to fly to bits. So what holds them together; certainly not the stars we can see in them — their gravitational force would be far too weak. Some astronomers mutter about “Dark Matter”, but what is that, and why have we detected no sign of it after 50 years of searching with every ingenious means we can think of? Then we have no idea how galaxies formed, not if the Universe is really expanding, for their infant stages would have been far too fragile to survive the Big Bang. Galaxies remain a challenge for any young person who would like to spend their lives trying to understand what is out there.
Hard as it was to see, it turns out that NGC 7331 is one of the most visible galaxies there are. Over the course of 50 years Morgan and his colleagues were to find much much dimmer specimens using radio waves instead of light. The next montage shows some typical specimens first located with the Parkes Radio Telescope, then imaged in visible colours with the Sloan optical Telescope in New Mexico, both absolutely state of the art instruments. Its worth studying this montage:
Believe it or not every postage-stamp contains a galaxy, some so dim one can barely spot them. This illustrates the “Visibility of Galaxies” problem which Morgan discovered in that caravan on the Teifi estuary back in 1975 [See my book Against The Fall of Night]. The problem is that, dark as it appears, the night sky is not absolutely dark so that any galaxy dimmer than our sky would be invisible. And why not? Could most of the structure of the Universe be hidden from sight? Morgan, Cotteridge, Cockle and other astronomers spent their careers wrangling over that question. There’s a great deal to think about here.
Broadly speaking there are two types of giant galaxy, Spirals as above, and Ellipticals. As we can see next, Ellipticals look like giant swarms of bees ( actually very old stars).
Ellipticals are just as mysterious as Spirals but in somewhat different ways. For instance NGC 4889 seen here fades imperceptibly into the sky; so where does it end and therefore how big and how massive is it? And why are Giant Ellipticals only found in Clusters? And where have the cold gas and the young blue stars gone that light up Spirals? And why do some of them, like this one, have colossal Black Holes in their cores? And how do these giant Clusters hold together when the gravitational forces required are hundreds of times too weak , if only visible material is responsible. Once again we run into speculations about “Dark Matter” which sound a bit like The Emperor’s New Clothes to me. In other words we are mired in mysteries which challenge anyone with Curiosity. By the way, this colossus is about 300 Million light years away, its light having set out towards us even before Dinosaurs evolved and 50 million years before the great Permian-Triassic extinction, which wiped out 90 percent of the the species on Earth. Yet in astronomical terms it’s almost next door, only seven times as far away as our Spiral friend NGC 7331.
We said most galaxies dwell in Clusters containing anything between a few and tens of thousands. Here is a very large Cluster Abell 1689, 2 Billion, not Million, light years away:
The further away we search , the further back in time, the greater cosmic mysteries become. For instance what you can actually see here is about one per cent of the Mass actually present. We know that because the galaxies in the cluster are whizzing about so fast that the cluster would have dispersed unless there are overwhelming amounts of “Dark Matter” holding it together [Or else there’s something even more mysterious going on.] This dramatic tendency of galaxies to cluster made it very difficult indeed for Tom Morgan and his friends to find “Hidden Galaxies” because the hidden ones would tend to get lost amid a crowd of ‘Visibles’, requiring great precision to get at the truth.
When we professional astronomers talk to the public we tend, for sociological reasons, to emphasise what we do know, as against what we do not. Now that I’m retired I can admit that most of the time, at least in extragalactic astronomy, we have little idea of what is really going on. Yes there are fools only too ready to rush in with half-baked explanations such as “Dark Matter” or “Dark Energy”, but I doubt that many will stand the test of time because they ignore Ockham’s Razor, a very profound and vital principle of Common Sense [See my book Thinking For Ourselves or a post on this site entitled Fuzzy Thinking and Ockham’s Razor]. In the mean time they not only rob us of mystery and wonder but they can hold up the search for deeper truths. For instance the invention of fictional “Land Bridges” held back the discovery of Continental Drift by a century.
I hope this post encourages readers, especially the young at heart, to retain their sense of mystery because, as Einstein put it : ” The most beautiful thing we can experience is the mysterious. It is the source of all true art and science”.
You can look up hundreds of more wondrous images of galaxies by going either to stsci.edu, NASA’s Space Telescope institute, or eso.org the European Southern Observatory which runs the world’s largest optical telescope (The ‘VLT’) in the Chilean Andes. But beware of beautiful coloured images which look like pizza advertisements; real galaxies don’t look anything like that, they’re far more subtle and infinitely more difficult to decode.
Tags: cosmology, galaxies, hidden galaxies, my books, ockham's razor
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