The Beginners Series – The Universe to Earth

The first time I really thought about space I didn’t consider the distances of what I was looking at. I remember as a kid thinking about the Moon, just being there and not really worrying about how far away it was. When people first start out in astronomy they often are in awe of the distances involved. This article is aimed at an introduction to how big space is so those new to astronomy can have a basic appreciation of the stuff they are looking at when they check out the night sky.

The Universe is incredibly large, calling it huge would be a massive understatement, it is unfathomly massive. It’s so big that we cannot see all of it, we can only see the bits that are/were within 13.8 billion light years. This is called the observable universe, the actual universe is considerably larger at about 46 billion light years. The difference is because over the 13.8 billion years of the life of the universe, it has been expanding in every direction. The Hubble Ultra Deep Field has given us a glimpse back in time to the near beginning of the Universe and the Hubble eXtreme Deep Field, even further (a little different than the Thomas Digges map above from 1576).

Within this universe there are billions and billions of galaxies of all shapes and sizes. Our own galaxy is the Milky Way and it’s known as a barred spiral. It’s quite tricky to measure how big our own galaxy is, because we sit inside it and a lot of it is hidden behind dust. Estimates range from about 100,000 to 180,000 light years in size, which is about the middle of the range as far as galaxies go. There are probably around 100 to 400 billion stars in our galaxy. The Milky Way is part of a group of galaxies known as the Local Group which is itself part of the Virgo Supercluster of galaxies. This supercluster is part of the even bigger Laniakea Supercluster, which has about 100,000 other galaxies. This picture shows what the Milk Way is thought to look like and the position of the Sun and our Solar System.

About 4.5 billion years ago, in a Milky Way that probably looked quite different than the one above, a star was born that would eventually be known by a number of names including; Te Ra, Soarele, Die Sohne, Słonce, Amaterasu and many more, but most commonly now known, in the English speaking world, as the Sun. This unremarkable star (but very remarkable to us) is the basis of our Solar System and marks its centre. Our planet, the Earth, doesn’t really stand out in the Solar System because it is quite small and from 6 billion kms is hardly noticeable at all, as shown by this famous image from Voyager 1 that Cark Sagan was instrumental in producing.

From the Earth the centre of the Milk Way is visible at the top of the sky (the zenith) in New Zealand, Australia, South Africa, Chile and Argentina at certain times of the year. Though there are up to 400 billion stars in our galaxy (roughly four times the amount of neurons in our brains) only about 6000 of them are visible to the naked eye (not the neurons, the stars). So here in New Zealand we get one of the best views on the planet to see the magnificence of the Milk Way’s central bulge. Of course, with the naked eye the central bulge looks like a cloud rather than individual resolvable stars – in that bulge there are billions of stars.

The farthest that the naked eye can see is the Andromeda galaxy, about 2.5 million light years away. The light from that galaxy set out on it’s journey 2.5 million years ago and travelled at a blistering speed of 300,000 km per second, considerably faster than the average Ford Fiesta. So the distance to Andromeda Galaxy is about 31,536,000 (seconds in a year) X 2,500,000 (years) X 300,000 (kilometres every second) – a very big number.