Our sun is a star.
If you observe the night sky you see a lot of other stars.
But they are not all the same.
Some are brighter than other, and the colour differs also.
Some are bluish, some are white, some are yellowish and some are reddish.
So it seems that you can see two different characteristics.
The color of the star and the luminosity.
Let us first talk about luminosity.
One big problem in Astronomy is to find out how far away the star is from us.
The problem is, you can not say the brighter stars are more close than the not so bright ones, because there are different kinds of stars.
Some shine brighter than others.
If you found out the real brightness of the star you call it the absolute magnitude or luminosity.
How you do that, I will not explain now.
The second quality is the colour.
The colour of the star depends on the size.
Big stars, they have more mass, are hotter, so they shine bluish.
Small stars hold little mass, they are cooler and they shine reddish.
So the colour gives you information how much energy the star produces per time. you might thing more mass would also mean longer life.
But it is vice versa.
A massive star burn hotter, so its mass is gone earlier.
If you take the absolute magnitude of a lot of different stars and plot this against the temperature in an log-log diagram, you get the Hertzsprung-Russell-Diagram (HRD).
Usually you plot the temperature from high to low temperature values and the absolute magnitude from low to high luminosity values.
The interesting think is - the dots are not random, you get a pattern.
The most prominent is called the main sequence.
It is a line, going from the upper-left to the lower-right.
In the lower-right you find the white dwarfs.
In the upper-right you find the giants and super-giants.
The most stars are in the main-sequence.
Every born star started in the main-sequence.
The place in the main-sequence depends only on its starting-mass.
Our sun is also located in the main-sequence.
We think, she has lived a little bit more then half of its lifetime in the main-sequence.
In 4.5 billion years she will become a red giant and in the end of her live she will be a white dwarf.
So with the HRD, you can understand the evolution of stars.
Absolute magnitude and temperature depends on the nuclear processes in the star.
Maybe that comes next.....
February 2016
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