Don’t Panic

Milky Way
Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space. Your planet Earth orbits its parent star, the Sun, at a distance of about 150,000,000 kilometres. For comparison, that’s 3,750 times the distance around the Earth’s equator.

The Sun accounts for 99.86% of the matter in the Solar System, so you might consider it as the Sun plus a few little specks of dust, one of which you live on, like a tiny microbe. The Sun’s neighbours are the three stars of the Alpha Centauri system. A and B are quite like the Sun (10% heavier and 10% lighter) and orbit each other every 41 days, like a spinning dumbbell. The runt of the family, C or Proxima, is only a tenth of the size and orbits much further out, taking about half a million years to make a complete circle. For the next 30,000 years or so, it holds the status of nearest star to the Sun, until its orbit moves it further away.

Proxima is four and a quarter light years from the Solar System, or 40,000,000,000,000 kilometres. Or 2.6 million times the distance between Earth and Sun; 10,000,000,000 times the distance round the Earth’s equator. Nothing but empty space in between. (Well, maybe a few wisps of hydrogen gas.) But that’s just the backyard. The Sun and its neighbours sit in the suburbs of the Milky Way Galaxy, in the unfashionable Orion-Cygnus spiral arm, at about 25,000 light years from the galactic centre. The Sun orbits the centre of the Galaxy every 250 million years, meaning that it’s been right round about sixteen times since condensing out of a cloud of dust and gas and lighting up.

The Galaxy, like most large ones, is shaped like a symmetrical fried egg, but because the Sun is close to the central plane, out in the egg white, the view of the galactic centre is obscured, and Earth-based astronomers have found it difficult to construct a realistic picture of what’s going on and how it’s structured. In recent years though, a fairly detailed description has been put together.

The main discovery is the Monster. Stars near the centre of the galaxy are whizzing in orbit around something which is very, very heavy, but surprisingly small. Four million times the mass of the Sun, yet only 40 million kilometres across, in fact. (If it was placed in the Sun’s position, the orbits of all the planets would still lie outside its perimiter.) The Thing can’t be seen directly, but the only explanation for it consistent with current scientific knowledge is that it’s a huge black hole.

I like to think of a black hole as a flaw or fault in space. When you pile in more and more matter, gravity pulls it tighter until, suddenly, space rips. What happens inside is mysterious, but from the outside, what you have a a bit of space that is cut off by the event horizon. Nothing inside can ever get out. The usual picture of a black hole is that it pulls in and eats stars, planets, dust and gas in huge quantities, but the one at the centre of the Galaxy seems to be fairly frugal, although the energy of what is probably a disc of hot matter spiralling in to its death can be detected on Earth by radio astronomy. (In fact, it’s one of the strongest sources of astronomical radio energy arriving on Earth.)

It is now thought that the centre of every galaxy will contain a supermassive black hole. The Milky Way Galaxy’s neighbour, the Andromeda Galaxy, is about two and a half million light years away, and is thought to have an even more massive one at its core. Milky Way and Andromeda are the two largest galaxies in what is called the Local Group, thirty or so galaxies which are gravitationally bound and orbiting each other. The Local Group is just one of about a hundred groups of galaxies making up the Virgo Supercluster, a structure over a hundred million light years across.

I don’t know about you though, but I have absolutely no idea what a hundred million light years actually means. Or four million solar masses, or forty trillion kilometres. To be honest, I don’t even have a real “gut feel” for the size of the Earth. I can walk to my nearest village in under forty minutes (there’s even a chemist’s) and that seems like quite a long way, even though you couldn’t see it on a globe.

But all that doesn’t matter. We may not be able to understand big numbers intuitively, but we can still use logic and mathematics to get answers — call that “science”. But I’m struck by the fact that “Intelligent Design” does the opposite, given that the main argument is always that some feature of biology “can’t possibly” have evolved, even over millions of years. Which is a bit like saying “150 million kilometres to the Sun? That’s impossible – there’s no such distance.” Space is big. Time is big too.


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