Transcript H-R Diagrams

Looking for patterns
If we plot a graph of luminosity against temperature
for lots of stars we get a graph like this…
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!
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.
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.
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.
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.
Things to notice:
5 Instead of temperature, you could label the xaxis with the spectral class of the star...
Oh
be
a
fine girl, kiss me
Things to notice:
6 Most stars lie on a broad diagonal band
- the main sequence.
Oh
be
a
fine girl, kiss me
1. Some stars are towards the right (coolish) but high up
(bright). Coolish … colour? red
Cool, so how come they’re so bright? very big - gigantic
Can you guess what they’re called? red giants
2. Some stars are in the middle (fairly hot) but pretty dim.
What must they be like? very small
They’re giving out all colours. So they are…? white dwarfs
Where are we?
The Hertzsprung-Russell diagram
our Sun