Distances in our solar system are hard to imagine. At its heart is our sun, some 864,938 miles (1,391,982 km) in diameter. Our sun is just one of the stars in our milky way galaxy that is thought to have between 2oo and 400 billion stars. But then our milky way galaxy is just one of the galaxies thought to be in total near 2 trillion in our universe. To help with our comprehension of these vast distances we first consider the sun as scale down and then consider our milky way galaxy as scaled down.
Imagine the sun as a 15 cm (6 inch) ball: on that scale —
Mercury would be a tiny grain of sand 6.4 metres away from the sun.
Venus would be a grains of sand 12 metres away from it.
Earth, a grain of sand 16.5 metres from the sun.
Mars a slightly smaller grain of sand 24 metres away.
The gas giant Jupiter would have 1.75 cm diameter and be about 84 metres away.
Saturn, also a gas giant would be 1.43 cm diameter 155 metres away.
Uranus, also of hydrogen and helium 0.63 cm diameter and 311 metres away.
Neptune, with gases like the others, also 0.63 cm diameter and 487 metres away
In August 1977 Voyager 2 was launched before the September launch of Voyager 1. The Voyager 2 Spacecraft was to go by each of the four outer gas giant planets, leaving Voyager 1 to be the first of the two to enter interstellar space. It did so in August of 2012 and travelling at 17 kilometres a second.
Voyager 2 travelling at 15 kilometres per second did likewise in November 2018. They are both now out of our solar system and in the region between stars where scattered material remnants of stars that died millions of years ago reside. Both spacecrafts continue to emit data that takes hours to reach us on earth.
Our sun is just one, and not a particularly remarkable one, of the vast number of stars in our galaxy, that as said earlier, we call the Milky Way. Our nearest neighbour star is Proxima Centauri. Often referred to as Alpha Centauri C, it has links with a pair of binary stars (Alpha Centauri A and B) that are in orbit about one another. Light from that near neighbour takes 4.24 earth years to reach us. Light from its binary neighbours take 4.44 years to reach us. Such distances mean that even if we travelled at 20 km per second (faster than Voyager 1 and 2) it would take 15,000 earth years to get to that closest neighbour star.
The Milky Way galaxy is a pancake shaped galaxy with spiral arms and a bulge in the centre that includes a black hole whose mass is two million times that of our sun. The solar system is sited on an edge of the orion spiral arm that is between the perseus arm and an arm called scutum – centaurum. It is located more than half way between the galaxy centre and its edge and just slightly above its equatorial plane.
Two dwarf galaxies are merging at the edge of our galaxy at a location diametrically opposite the solar systems location. The merging is unlikely to involve collisions as the space in galaxies is enormous and because our galaxy has grown from such mergers in the past. The galaxy is rotating and it is estimated that the solar system will take between 200 and 250 million years to rotate around it once.
Our ancestors were more familiar with the night sky than we are. Very few of us now see the milky road of dense stars crossing the heavens as they did. Only those living in or visiting remote areas get to see it. It is our view along the plane of our pancake like galaxy. The less dense stars we see are the one’s above and below us in that pancake like structure
Still on the scale where our sun is a 15 cm, 6 inch diameter ball
Our milky way galaxy would be about 100 million km in diameter.
Our galaxy would be about 1 million km thick. That’s some pancake.
The solar system would be some 26 million km from the galaxy centre.
The solar system would be about 65,000 km above the galactic mid plane.
The andromeda galaxy would be 200 million km in diameter.
The distance to the andromeda galaxy would be 2,500 million km.
Now imagine our galaxy as a 15 cm (6 inch) disc: on that scale —
The andromeda galaxy, is a barred spiral galaxy and, on this new scale, it is now a disc about 2o cm in diameter and about 4.9 metres away. In good conditions you can see it as a fuzzy area in the heavens and away from the scale we are using it is 24,000,000,000,000,000,000 km away, that’s 24 quintillion km. Yes you can see that far.
It, along with the Milky Way is one of about 30 galaxies in what is termed the “Local Group”. This “local group” includes two galaxies that are clearly visible in the southern hemisphere namely the Large Magellanic Cloud and, back on the 15 cm milky way scale, of tennis ball size and 30 cm away) and the Small Magellanic Cloud (a ping pong ball in size and 35 cm away). These latter two galaxies are regarded as satellites of the Milky Way Galaxy.
A group of over 1000 galaxies called the Virgo Cluster is on this same scale about 55 metres away. That cluster is no bigger in size than the local group but having many more galaxies in it means it has much more mass. This additional mass is pulling the “Local Group” toward it. The Virgo Cluster and Local Group together form part of what is called “The Local Super Cluster”. It is about 168 metres long on our scaled galactic scale.
The universe has many super clusters in all directions. The entire visible universe, on our galactic scale would be a large sphere about 28 miles in diameter and containing over 200 billion galaxies and as earlier said possibly as many as 2 trillion. Surely our earth and its life forms cannot be unique in such an extensive distribution of energy and matter.
Most of the smaller distances in space are measured in light years. Light in a vacuum travels at 299,792,458 metres per second. A Light Year is the distance light travels in one earth year. It is just less than 6 trillion miles or just less than 10 trillion kilometres in metric. A parsec, the preferred unit in Astronomy, is about 3.26 Light Years. Hans Solo made reference to it in star wars.
You have to realise that when looking at the stars you are viewing objects as they were many, many years ago. Some will be as they were many millions of years ago. Many will no longer be where we see them, some will be bigger, some smaller, many will have changed colour, some may no longer exist. In viewing the heavens nothing we see is “as is”. Everything is a multiplicity of views of the past, brought to us by light’s journey through space and as distorted by our atmosphere. Such light is focused by the lenses in our eyes and terminates on the retina’s at the back of our eyes, where the information received is transferred to our brains so providing the image we see.