We run regular stargazing sessions and interact with many people with a huge variance in understanding of the night sky. Often we find that people are not aware of how the Solar System relates to the galaxy and how our Galaxy relates to the rest of the Universe. Also a lot of people struggle with understanding how our star, the Sun, relates to the other stars in our galaxy.
This is not a bad thing as many of these things are no longer taught in school and if it’s something that you never run into then you’ll probably not have much cause to learn. Those of us who are obsessed with the night sky are able to rattle off all sorts of facts and figures about the different celestial objects without too much of a problem. The purpose of this post is to demystify some of the basics of stargazing so you can have a a good place to start from when you have a look in a telescope.
First the Solar System, this is like our little village of planets, which all come in different shapes and sizes. At the middle of the Solar System is our Sun, our very own star. It’s the same type of object as all of those stars that we see in the night sky except with our one it is very close so we get to feel it’s heat rather than just see a point of light.
As far as stars go, our Sun is pretty common and lucky for us it is pretty inactive. It’s presided over our solar system for nearly 5 billion years and based on its mass it should give us another 5 billion years, though the last one billion of that will be rather unpleasant for planets like Earth, Venus and Mercury. Solar systems are all about individual stars, they are the group of objects orbiting a particular star. So our solar system includes our Sun and all of the the planets as well as the asteroids and comets and anything that is orbiting the Sun or any of the planets.
The Sun is huge, if you lined up 110 Earths side by side that would be the diameter of the Sun. The Sun lies at the centre of the Solar System about 150 million kilometres from us. There are other solar systems out there beyond ours and the nearest is around Proxima Centauri, the nearest star to the Sun. In fact we now know that many of the stars we see at night have planets around them so have their own solar systems.
The planets orbiting our Sun are those that you’ll be familiar with; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. We see these planets in the night sky roughly along the path that the Sun takes across the sky. The planets, other than Uranus and Neptune, are visible to the naked eye and at least one of them can be spotted most nights. Compared to the stars we see, the planets are very close and even the most distant, Neptune, is only four billion kilometres away. Whereas the stars are tens of trillions of kilometres at the closest and 150,000 times that for the most distant in our galaxy.
Beyond our solar system is the rest of the galaxy. I always describe it as like a city made up of billions of solar systems (villages) like our own. Our Galaxy is called the Milky Way and we live about two thirds of the way out towards the edge. It’s an enormous distance to the centre of our galaxy, if you could travel at the speed of light it would take you 27,000 years to get there, so we say 27,000 light years distant.
Our Galaxy holds somewhere between 200 billion and 400 billion stars and at night we can only see about 3000 of them, if there are no clouds. Our Galaxy is arranged as a big flat spiral with a bit of a bulge in the middle where a super massive black hole is.
Beyond our Galaxy
There are other Galaxies beyond our own and for those of us in the Southern Hemisphere we can see two other Galaxy in the night sky, the Magellanic Clouds. These are very small irregular galaxies and also very close. There are hundreds of trillions of other galaxies out there in the universe.
The Earth orbits the Sun each year, it took us humans quite a while to figure that out! and our Sun orbits the centre of the galaxy every 250 million years. Our Galaxy orbits other galaxies in the local group of galaxies, kind of like a large collection of cities, and that group of galaxies orbits another group of galaxies. That’s our universe!