1. The HR diagram
The Herztsprung-Russel diagram plots the spectral class O------M of stars against their Absolute Magnitude .
This is equivalent to plotting the temperature of stars against their power output (sometimes called their Luminosity (L) )

4. Points to Note The result is a diagram not a graph.
When temperature values are given we can see that temperature increases to the LEFT.
The temperature scale is NOT linear.
Absolute Magnitude decreases upward on the diagram.
As the absolute magnitude gets smaller the power output of the stars actually increase. This is a logarithmic scale

5. Absolute Magnitude
When absolute magnitude is plotted against spectral class most stars are found to lie in a narrow band. These are called the main sequence stars
Very hot large blue stars
-10 -5 +5
Very cool dim red stars
+10
+15
O B A F G K M
Spectral type

6. Here the diagram is replotted showing the power output of the star against temperature

7. Mass of a star and the HR diagram
The position of a star on the main sequence depends entirely on its mass. Low mass stars are found at the right and high mass stars at the left.
The mass of a star controls its radius, its temperature, its life expectancy and its eventual fate.

8. Points to note on HR diagram
At the top left of the main sequence the stars have very large radius and high surface temperatures. This is what makes them highly luminous.
They are using up their hydrogen fuel tremendously quickly. So they are extremely short lived (c years)
These high mass stars finish their lives in massive supernova explosions
At the bottom right the stars are cool. These low mass stars are very long lived as they use their fuel so slowly. Very low mass M stars live many billions of years and will simply run out of fuel without dramatic events.

9. Surface temperature K
Red giant stars
Red giants have large radius and therefore are very luminous ( therefore have high absolute magnitude). However they are much cooler than would be expected for stars of large radius.
White dwarf stars
White dwarfs are high temperature stars but their small radius (some around the size of Earth) is abnormal for such high temperatures

10. Surface temperature K
Red giant stars
These classes of stars off the main sequence have finished burning hydrogen as their fuel. Red giants are “using helium and later heavier atoms as fuels. Stars here are reaching the end of their lives
White dwarf stars
White dwarf stars are “remnants” They are essentially the central cores of what were main sequence stars like the Sun

12. Red giants Stars of around solar mass will become red giants
Red giants are stars which are excessively luminous for their temperature.
This is because they have large radius.
They are stars that have ceased burning hydrogen as a fuel.
Their outer layers have expanded and cooled.

13. White dwarfs Red giants are unable to hold onto their outer layers
This material is lost into outer space
The dense core of the dead star is exposed. This is a WHITE dwarf star.

16. Absolute Magnitude
Very hot large blue stars
The Fate of our Sun
-10 -5
Red giants
Sun today +5
White dwarfs
Very cool dim red stars
+10
+15
O B A F G K M
Spectral type