
In astronomy, as in all of science, we measure quantities and when we make measurements we present our results with numbers. These measurments relate to sizes of objects, durations, energy of observed processes, and temperature (and more ...). Part of understanding what a number  a numerical value based on some measurments  conveys is actually having a "feeling" for how "big" or how "small" a number is in relation to other numbers. In fact, this is how we understand and relate to the world around us in our daily experiences which involve as varied activities as running, driving, working, and even using our language to talk and relate information to others. Sizes of objects and duration of events make sense only in relation to other sizes and other durations which "we have in mind".
Suppose someone tell you that Mars is about 80 million kilometers from the Earth. What does this number mean? Well, from your daily experiences this number should sound "very large". Part of this has to do with the fact that in our experience  limited here on the surface of the Earth  we don't travel that large of a distance in a day or two. We go 10 or 20 or 30 or ... few hunderd kilometers. So, 78 million kilometers is very large compared to our dialy distances on the Earth. On the other hand, if someone tell you that the size of an atom is about 0.000000000001 kilometers, you would say that "wow, that is so tiny!". And yes compared to our expereinces it is small.
Notice that "large" and "small" only make sense in relation  or by reference  to other things that we do have a sense of. If you were told that the size of our Galaxy is about:1,000,000,000,000,000,000 kilometers, then, comparatively this dwarfs the 78 million kilometer distance from Earth to Mars ... all of a sudden, Mars feels like the next door neighbor (which, on the astronomical scale of distances it actually is!).
So, the basic point of all this is that in order for you to interpret numbers in astronomy, in order to understand what someone means when they give you a distance or a time or an energy that seems "incomprehensible", you should have a feeling of scales of distance and time and energy that are present in the universe. For instance, how large is a typical star compared to the size of the city of Denver? How much energy does the Sun put out in one day, compared to ... say, how much electricity we use in one day? These are the kinds of things that allow us to anchor a reference of number to hold on to, and it is only in relation to such an anchor that astronomically measured quantities will begint to make sense to you.
You might have already noticed that the numbers involved in astronomy range over a very large scale of sizes. The smallest sized objects you'll hear about (the electron and the atom )is much much much smaller than the Earth which is in turn itself much much much smaller than the Size of the Galaxy, and the scales continue up like this to the size of the Universe.
Since these numbers range over such small and such large values we get tired of writing out so many zeros, as was being written in the pervious section. We introduce something called powers of 10 notation. This is just a very simple introduction; you will get more and more experienced with this notation as you study astronomy and as you go along to the secions and examples on scientific notation and algebraic powers.
Perhaps it is best to illustrate this with an example. Consider the size of our Galaxy again ...:
Example: We said that the size of our Galaxy is 1,000,000,000,000,000,000 kilometers. To get away from this cumbersome notation we count the number of zeros, which in this case is 18 zeros (count this yourself and maker sure you agree). We notice that 10 has one zero and 100 has two zeros and so on ... thus, each zero reprsents one power of 10, or one order of magnitude. So, instead of writing 18 zeros in front of 1, we write 10 and realize that if we multiply 10 by itself 18 times we will get a number with 18 zeros in front of it. Thus we write: 10^{18}. We say that out galaxy is "10 to the power of 18" kilometers long!
This illustrates the basic idea of powers of 10. Each zero represents a power of 10, and if the zero is to the right of a number it represents a power of 10 larger and if a zero is to the left of a number it represents a power of 10 smaller (in this case we use negative powers). So, we can say that:
Length of our Galaxy is: 10^{18} kilometers
From Earth to Mars it is 10^{7} kilometers
From School to my house it is: 10 kilometers
The size of an atom is: 10^{12} kilometers
Powers of 10 thus allow us to compare sizes (and durations ... see scales below). We can say that the size of our galaxy is "11 orders of magnitude larger than the distance from here to mars and it is 17 orders of magnitude larger than the distance from school to my house". Basically, powers of 10 allow us to talk about degrees of "largness" and "smallness". This might seem confusing at this point, but it will clarify as you see more and more examples  which are abundant in astronomy.
Here are some examples of the typical quantities that are measured in astronomy. As you can see, distances and sizes, time, and energy are some of the most important measurments that are made, and thus they constitute some of the most relevant scales and orders of magnitude that you should be familiar with. The following tables should give you some feeling of the "typical" numbers that are associated with each measured quantity. You need not worry about the details of each number  just the orders of magnitude.
Distance and Size Scales
Object or Distance 
Size ( km ) 

Universe 
10^{23} 

Milky Way Galaxy 
10^{18} 

Nearest Star 
10^{13} 

Size of Solar System 
10^{9} 

The Sun 
10^{6} 

The Earth 
10^{3} 

A Mountain 
10^{0} 

Humans 
10^{3} 

A Cell 
10^{8} 

An Atom 
10^{12} 

The Nucleus 
10^{17} 
Time Scales
Duration/Age 
Time (s) 

Age of The Solar System 
10^{17} 

Last Ice Age 
10^{12} 

Human Life Time 
10^{9} 

A Day 
10^{4} 

A Lecture 
10^{3} 

A Moment/A Second 
10^{0} 

(light) ??? 
10^{} ? 

Lifetime of a decaying atomic particle 
10^{} ? 
Typical Energy Scales
Energy Source 
Energy (joules) 

Big Bang 
10^{68} 

Radio galaxy 
10^{55} 

Supernova 
10^{43} 

Sun's radiation for 1 year 
10^{34} 

Volcanic explosion 
10^{19} 

Hbomb 
10^{17} 

Thunderstorm 
10^{15} 

Lightening flash 
10^{10} 

Baseball pitch 
10^{2} 

Hitting keyboard key 
10^{2} 

Hop of a flea 
10^{7} 