1. Looking for patterns If we plot a graph of luminosity against temperature for lots of stars we get a graph like this…

2. This graph was first drawn up by two astronomers - Hertzsprung and Russell - independently at around the same, so is called a Hertzsprung-Russell (HR) diagram!

3. Things to notice: 1 The temperature increases from R  L. Why? The high temps correspond to small wavelengths, so wavelength increases L  R along the x-axis.

4. Things to notice: 2 The y-axis - luminosity - doesn’t increase linearly: for equal intervals it increases by the same multiple. This is a logarithmic scale - useful for plotting values with a huge range.

5. Things to notice: 3 The y-axis - luminosity: the values are actually multiples of some value L  - the luminosity of our Sun. So they are all relative values.

6. Things to notice: 4 The x-axis - temperature - is also logarithmic. The range of values isn’t as big as luminosity, but it just makes it easier.

7. Things to notice: 5 Instead of temperature, you could label the x- axis with the spectral class of the star... Oh be a fine girl, kiss me

8. Things to notice: 6 Most stars lie on a broad diagonal band - the main sequence. Oh be a fine girl, kiss me

9. 1.Some stars are towards the right (coolish) but high up (bright). Coolish … colour? Cool, so how come they’re so bright? Can you guess what they’re called? 2.Some stars are in the middle (fairly hot) but pretty dim. What must they be like? They’re giving out all colours. So they are…? very big - gigantic red giants very small white dwarfs red

10. Where are we? Hertzsprung-Russell diagram The Hertzsprung-Russell diagram our Sun