1. (i) What, in the atmosphere, is responsible for absorbing infrared radiation?
(ii) The spectrum of light from a star can be used to determine its temperature. Explain why
this absorption can lead to errors in the value. ..
(i) water vapour 1 (or carbon dioxide)
(ii) longer wavelengths absorbed 1
shifts peak of graph to shorter wavelengths 1
star appears hotter [or reference to appropriate equation] 1

2. (iii) What is responsible for [ultra Violet] absorption?
(iv) What effect can this absorption have on the measured temperature of a star? Explain your
answer.
(iii) ozone !
(iv) lower temperature, hits peak (λmax) to longer wavelengths !
λmaxT = constant !

3. From the Specification
PRINCIPLES OF THE USE OF STELLAR SPECTRAL CLASSES
Description of the main classes, O B A F G K M
Temperature required: need for excitation
Helium absorption (O): need for higher temperature
Hydrogen Balmer absorption lines (B, A): need for atoms in 2 = n state
Metals absorption (F, G): occurs at lower temperature
Molecular bands (K, M): occur at lowest temperature

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 left and high mass stars at the right.
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

11. 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.

12. 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.

15. 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