1. The Expanding Universe Additional reading: Higher Physics for CfE, p.77-80. Notes p.56 The radiation emitted by stellar objects can give us information about the temperature of the objects. When heated, at first objects emit invisible infrared. On further heating they glow … red Hotter again we get … orange And even hotter gives … yellow Then … then 4. Temperature Considerations

2. In fact, hundreds of years ago, potters recognised that the colour their materials glowed in the kiln gave them an actual measurement of the temperature of the kiln: Dark Red … 550 o C … 823 K Cherry Red … 750 o C … 1023 K Orange… 900 o C … 1173 K Yellow… 1000 o C … 1273 K White… 1200 o C … 1473 K

3. Black-Body Radiation Very hot solids and liquids usually emit continuous spectra of radiation as opposed to the line spectra we detect from gases. A surface that can emit and absorb all wavelengths of the em spectrum is called a black-body. Its continuous spectrum is called black-body radiation. Intensity (Wm -2 ) solids & liquids gases

4. A black-body spectrum can be identified by the following shape characteristics: 1. A bell shape that rises steeply, at shorter wavelengths, to a peak intensity, then falls off gently at longer temperatures. 2.As the object temperature increases, the overall shape remains the same, but the peak wavelength shifts to a shorter value. 3.As the object temperature increases, the intensity for all emitted wavelengths, increases.

5. Typical Black Body Spectra a)Determine the peak wavelength for each of the temperatures in the diagram. Temperature (K) Peak wavelength (nm) 3500 4000 4500 5000 5500 800 700 650 580 510 intensity

6. b) Stars behave quite like “black bodies”. Describe what happens to the peak wavelength emitted by a star, as its temperature increases. intensity For higher temperatures, the peak wavelength shifts to a lower value.

7. Categorising Stars – Additional Info The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color (spectral type or surface temperature) vs. its luminosity (intrinsic brightness or absolute magnitude). On it, astronomers plot stars' color, temperature, luminosity, spectral type, and evolutionary stage. This diagram shows that there are 3 very different types of stars: Most stars, including the sun, are "main sequence stars," fueled by nuclear fusion converting hydrogen into helium. For these stars, the hotter they are, the brighter. These stars are in the most stable part of their existence; this stage generally lasts for about 5 billion years.luminositymagnitudeastronomers"main sequence stars,"nuclear fusionhydrogenhelium As stars begin to die, they become giants and supergiants (above the main sequence). These stars have depleted their hydrogen supply and are very old. The core contracts as the outer layers expand. These stars will eventually explode (becoming a planetary nebula or supernova, depending on their mass) and then become white dwarfs, neutron stars, or black holes (again depending on their mass). supergiantsmain sequencedepending on their mass Smaller stars (like our Sun) eventually become faint white dwarfs (hot, white, dim stars) that are below the main sequence. These hot, shrinking stars have depleted their nuclear fuels and will eventually become cold, dark, black dwarfs.white dwarfs

8. Big Bang Expansion

9. Complete Problems from Tutorial IV The Big Bang Theory Q. 1 Answers