Just how big is everything

"Space is big, really big, you just wont believe how vastly mind bogglingly huge it is. I mean you may think its a long way down the street to the chemist but that's just peanuts to space. Listen....."

A word of caution this article contains really big numbers so I will use some simple scientific notation, it looks like this 1*10^2 which basically means 1 *10 *10 or 100, another example 5*10^3 would be 5*10 *10 *10 or 5000 (basically it is the first digit followed by a number of zeroes matching the number after the ^) also each of the units below is a thousand times larger than the one before it.

one, thousand, million, billion, trillion

The simplest to expalin unit of measurement used on the scale of galaxies or the universe is the light year, so I will use that. Light (in a vacuum) travels at 3*10^8 meters per second or 300 000 Km per second, for scale the earth is only around 12 700Km across. A light year is the distance travelled by light in a vacuum in one year and is about 10 000 000 000 000 or 10 trillion Km, the nearest star to our sun is around 4.5 light years away, our galaxy 1*10^5 light years across and the visible universe 9.2*10^9 light years. An interesting point to mention here is that obviously the light we receive shows us what these objects were like when the light left them, as such we see things as they were millions or even billions of years in the past. This means very distant galaxies look very different from near ones and the limit of the observable universe is caused by seeing back so far in time that the light at this distance was emitted long before any stars formed at the point when the universe finally became cool and spread out enough for it to be able to transmit light.

The visible universe (as far as we can see) contains a minimum 170 billion(1.7*10^11) galaxies, the smallest of which contain around 10 million(1*10^7) stars and the largest 100 trillion(1*10^14), giving a low estimate of around 1700 000 000 000 000 000 000 stars in the universe.

Take a few minutes to get your head around these numbers and watch this video attempting to show the length scales involved and this one demonstrating the relative sizes of some of the stars in our galaxy. Each of those behemoths is to the naked eye at most a single dot of light or completely invisible due to the shear distance between Earth and it. Some of those minuscule points of light in the sky every night are even entire galaxies, a hundred billion glowing giants so far off that combined they are barely even visible and most galaxies are utterly undetectable without incredibly powerful telescopes. If we then turn that logic on its head we get this, and bearing in mind that the photo was taken from near Saturn, a distance which is less than insignificant on a galactic scale. Understanding just how small our world is is one of the most incredible and humbling experiences I can think of.

Opening quote taken from The Hitchhikers Guide to the Galaxy by Douglas Adams, everyone should go read it.