The Hertzsprung Russel Diagram

Scientists love to plot to graphs, yay...because its so much fun! So one day Mr. Hertzsprung and Mr. Russel decided to meet up for some tea. 

Mr. Hertzsprung: "Mr. Russel old friend, are you up for a game of putting?"

Mr. Russel: "Actually I was thinking about plotting luminosity and against surface temperature today"

Mr. Hertzsprung: "..."

So anyway, this is what they come up with.

Important things to notice:

1. On the bottom x axis: This can be surface temperature in Kelvins or Spectral Class. 

The surface temperature in Kelvins increases from right to left...opposite to normal convention. This is most likely because we are used to having blue on the left and right on the right to show the visible spectrum. 

If you look at the black body radiation graph 

We see that peak wavelength is inversely proportional to the temperature. Remember that the black body is emitting all wavelenghts, but there is only one peak wavelength at the specific temperature. 

Notice that blue is on the left, red is the on the right and that the temperature increases from right to left.

Spectral class is the classification of stars according to its temperature and chemical composition. If they have the same chemical composition = they will have similar absorption or emission spectra. If they have the same temperature = they will appear to the same colour because they have the same peak wavelength.

2. On the vertical axis we have luminosity or absolute magnitude.

Luminosity is defined by the total amount of energy emitted by the star per second (in watts as it is joules/sec).

Luminosity depends on the temperature of the star and its radius of surface area (in otherwords...its size). Here size and hotness does matter...lol.

Absolute magnitude is the luminosity the star would have if it was observed from 10 parsecs away. Why make this movement of the star? 

The stars in the observable sky are different distances from the sun. We need to move all the stars to the same distance to in order to make comparisons between them. 

3. The line running from the top left to the bottom right is the main sequence. Our sun is right now towards the bottom right of the main sequence.

White dwarfs are located towards the bottom left.

Giants are towards the top right.

Red Giants are located above the Giants.

There is always a 3 marker question in the exam on simply..."label the location of the white dwarfs...etc"