Aag okay then. Let's see, anyone keen for some introductory astrophysics? Let's have a look at star classification.
With all the gazillions of stars out there it helps to have some sort of a classification system, I suppose this is true for just about any other subject. We have a binomial nomenclature
system for living organisms which forms part of a larger taxonomic ranking
system. So to better make sense of all the different types of stars there is also a classification system for stars. Here's the rub.
All stars can be sorted according to two attributes:
1. Spectral Type - This is basically the colour of the star and it is a direct function of the temperature of the star.
2. Luminosity - This is basically how bright the star is and is mostly a function of the star's size. Take note that this is not how bright the star appears as seen from earth but rather its absolute brightness.
Unfortunately neither of these two attributes are particularly easy to measure as they are both affected by distance and particularly the latter. The colour of a star is affected by distance because all objects in space are moving away from a certain point (the point where the Big Bang is assumed to have taken place) and very distant objects are moving faster than objects closer by. This means that distant objects' colour is affected by "red shift" more so than objects closer by. I could write up a little something about red shift in another post if anyone is interested. The brightness of a star is affected by distance for obvious reasons.
In order to classify a star it is necessary to know the distance to the star. There are various ways to determine the distance to stars and this is also something that might make for an interesting post if anyone is interested.
Now you may have guessed that hotter stars are also brighter stars while cooler stars are dimmer. This is true for the most part but it is not always the case and there are actually a large number of exceptions. If you where to plot all stars on a graph of which the horizontal axis gives their temperature or colour while the vertical axis represents the luminosity, you get what is called the Hertzsprung–Russell diagram
(or HR Diagram for short). Here is a HR Diagram plotted for some 23,000 stars, grabbed from Wikipedia.
Each one of those specks represent a different star and you can clearly see that the vast majority of stars falls in the band called the "main sequence". The HR Diagram holds lots and lots and lots of information and I won't be able to explain all of it here (mostly because I don't understand all of it anyway).
Another very interesting topic is the development and aging of stars. I won't be able to go into that in too much detail either but what I can say at this point is that stars typically begin their life at the bottom right of the main sequence and move towards the upper left as they age. At some stage in the star's life, depending on it's size and composition, it may branch off from the main sequence to become a giant or supergiant. Alternatively, if it does not have enough fuel to burn it can shed its outer layers as a planetary nebula, become a white dwarf and slowly fizzle out, sliding along the white dwarf curve from top left to bottom right (this is in all likelihood how the sun will meet its final demise).
It means the HR Diagram can be used to make predictions as to the future of a star, the age of the star, the relationship to other stars (it becomes an interesting exercise to plot the HR Diagram for all the stars in a specific open cluster galaxy for instance - but that's yet another story).
Just to put our sun in perspective, it can be noted that it falls pretty close to the center of the diagram. It sits on the main sequence at a luminosity of 1 and a colour index of 0.66 (temperature of 5780 Kelvin). This means our sun is a pretty ordinary star. It is still fairly young, has a pretty average temperature and pretty average size.
Betelgeuse on the other hand sits on the very top right of this diagram. It's pretty cold, very big, very old and about to go whallop.