The Universe on a Platter
(or “Have I Bitten Off More Than I Can Chew”)
When I was young, I was as guilty as anyone when it came to playing with my food at the dinner table and daydreaming. The reasons varied. The conversation or the food may have been somewhat less than stimulating. As a child, whilst pushing my peas around, I would dream about distant stars and spaceships. Today, I am much bigger, still play with my food and still think about astronomy. However, these days I tend to dream about the problems that people, starting out on their road to the Universe, have in comprehending our marvellous skies. The Andromeda Galaxy is so far away that the light we see takes two million years to get to Earth. How does one explain that to people who think it is a long way to the next state? After a while, one can almost hear the click, as a persons mind shuts down. If mental attention was analogous to light, I am soon left standing in the dark! I suppose none of us are capable of truly comprehending the distance of a light year. With practice we eventually look upon this concept like an old friend (we don't ask how he or she works). Lets face it ‘space’ is well named because there is certainly a lot of it!
So here I am struggling with the enormity of the Universe. If only it was as simple, and as tangible, as this dinner plate. So lets see… push this small young pea into the middle of the circular plate and call it… Earth (how many people do you know that name their vegetables!?). That would make the Moon a pinhead sized ball on the rim of the plate, going all the way around about once a month. Well that was easy, Ive started my journey in shrinking the Universe (I bet there are some people who wish it was this easy to reduce their waistline). To push further into space I find it is necessary to leave the table and even the room (thats one way to escape the boring conversation). Lets set off in the direction of the inner Solar System. If Venus happens to be at its closest to Earth, and you are observant, you may glimpse this pea sized object near the front door (a slightly smaller pea compared to the earth). We leave my abode, cross the lawn, and perched on the front fence is a pinhead sized Mercury. Crossing the road it isnt difficult to find the small beach ball sized centre of our Solar System, the Sun. We have now travelled only about 50 metres from the dining table.
Thinking it is time to walk off my dinner, I decide to have a stroll up the street. In a matter of a few minutes, the inner Solar System is left behind. Tiny Mars has been passed, and about 300 metres from our Sun we encounter the gas giant, Jupiter. This golf ball sized king of the planets, sits proudly in the centre of a circular coffee table. The orbits of the four largest and brightest moons, discovered by Galileo, can be found on the top of the table, running on concentric paths. The orbit of the outer of these moons, Callisto, has a diameter of about 1.3 metres. Like our Moon, on this scale, these satellites also appear pinhead in size. In reality three of these bodies, Io, Ganymede and Callisto are larger than our Moon; Ganymede being the largest satellite in the Solar System.
A further 200 metres on, magnificent Saturn appears as a ‘ping-pong ball sized object, imbedded in the centre of its saucer sized rings. Our walk continues as the marble sized planets Uranus and Neptune slip by. Finally, at a distant 2 kilometres from home, the orbit of minute Pluto is crossed (another pinhead sized ball). From this isolated spot the Sun is no longer visible as a ball; just a distant point. From Pluto, the Sun would be just a bright star in the sky and day-light would be like twilight on Earth.
Looking outward into deep space, our next stop, the nearest stars are far beyond any further walking. For at this scale, Alpha Centauri would be a pair of beach balls at a distance a little larger than the diameter of the Earth! It is obvious we need to return home to my dinner plate and make some dramatic modifications to my scale of our universe (or move to a larger world?).
Its time to cram more onto my quite ordinary plate. The reduction is painless and the entire Solar System now sits neatly in this circular area normally reserved for my meat and vegetables (I dont know how the table can take the weight). At this scale, it is now pointless talking about the sizes of the planets. The Sun is only 3/100ths of 1 mm in diameter! A quick glance at my revised scale and I reach for the car keys. Driving down the road, I finally catch and pass Alpha Centauri. It is less than 1kilometre from home and consists of two pin-points of light separated by about the width of a fist. Knowing I need to cover some ground, fast, I head for the freeway. I quickly reach the speed limit and realise I am travelling at close to 4 million times the speed of light! Upon reflection, it would have been easier to borrow Sagans ship of the imagination - less wear and tear and a lower fuel bill. The local neighbourhood of stars is quickly passed (the closest 30 stars all lie within a 15km circle from home).
After about an hours driving (a distance of about 100 kilometres), my first goal is reached, one of the nearest star clusters - ‘the Beehive. This is the grand daddy of all bees nests. It is hard to imagine that this 2km ‘swarm, consisting of hundreds of stars, is the same object which Hipparchus (130 B.C.) referred to as the ‘little cloud. While I am stopped, it is worth considering that one of the closest planetary nebulae is also about this far from home - the Helix. From Earth, this faint ring looks about the size of the Moon. Out here, the Helix shows itself as an even fainter, diffuse spherical shell of gas which is about 1 km in diameter. Being so close, and looking so big, it is easy to miss it completely. When you are in a city you dont notice the smog, from a distance you wonder how anyone can breathe (you know, the old forest for the trees routine).
Another 2 hours, down the road, and the nearest stellar nursery is reached - the ‘Great Nebula of Orion. This mighty 6 km cloud is illuminated from within by young stars. It looks a little like a storm cloud, that glows from internal electrical discharges.
To push on and reach the centre of the Milky Way is a nice dream, but unfortunately a little impractical. I would run out of roads and dry land. The centre, from the Earth, is about twice the distance between Sydney and Perth. In fact, the diameter of our entire galaxy would be twice that of the Earth! Unfortunately, even at this scale, to push beyond the Milky Way, I would need to invoke astronomical distances. Time for another revision of the size of the Universe (its a lot of fun playing God).
I am back in my dining room and my innocent dining plate takes on its greatest challenge - to support the entire Milky Way galaxy (if I kept on reducing the scale so dramatically, I wonder how many further jumps before I set off my own ‘Big Bang?). Light now takes over 100,000 years to cross the diameter of my ‘china friend! The nearest galaxies, the ‘Magellanic Clouds, are the size of two baked potatoes, at about an arms length distance from my ‘Milky Plate. The Andromeda Galaxy would be another similar plate to my own, across the room, a little over 5 metres away. Scattered around the room would be another 2 dozen or so pieces of vegetables which completes the local group of galaxies (perhaps the result of some extragalactic food fight?). You see even galaxies prefer to hang around in families.
The nearest major group of galaxies is the Virgo cluster which has about 3,000 members. It is well known, in amateur circles, because about 100 of these are bright enough to see in an average size telescope. This group lies at a distance of about 200 metres and the core of the cluster is about 30 metres in diameter.
I can now push out towards the edge of the known Universe. Some distant quasars are believed to be about 4,000 million light years away. When the light (that we now see) left these objects, the Universe was probably about a quarter to half of its current age (depending on who you read). On my current scale, this distance is reduced to a more comprehensible 13 kilometres.
Well, thats it, Ive run out of Universe. I dont intend to further struggle with cosmological distances (Ill let greater minds tackle that one). I think its time to wash up the plate, pack up my calculator, and get out my telescope. Its a clear sky tonight and it is time for my eyes to soak up some of its beauty.
(For those who, like the author, like playing with numbers, my standard ‘Astronomical Plate (ap) is as follows:- 1 ap = 270mm or about 1.8 X 10-12 astronomical units. I doubt you will find any standards association that will recognise this unit of measurement)