E.2: Stellar Radiation and Stellar Types

1. E.2: Stellar Radiation and Stellar Types The following notes were taken primarily from Physics for IB by Chris Hamper and Physics Course Companion by Tim Kirk

2. E.2.1 • State that fusion is the main energy source of stars • Students should know that the basic process is one in which hydrogen is converted into helium. They do not need to know about the fusion of elements with higher proton numbers. E.2.2 • Explain that, in a stable star (for example, our Sun), there is an equilibrium between radiation pressure and gravitational pressure.

3. “H” is Fuel • How does our sun work? • Fusion of hydrogen into helium that provides the energy, for our sun • Happens on the inside of the sun (Yes, there are different layers) • Produces neutrinos that leave the sun and travel to Earth

4. The proton-proton chain • This is the same reaction discussed in Topic 7. Each complete chain reaction produces 26.7MeV.

5. Remember you need 4 H to end up with one He • See simplified equation:

6. Star Stability • Gravity pulls inward • So much the sun should collapse. • Nuclear explosions push outward • These two have to balance out to be at pressure equilibrium • Ex. Balloon. • Rubber is like gravity • Air is like the explosions • If the temp changes the inside pressure will change and won’t be stable

7. E.2.3 • Define the luminosity of a star. E.2.4 • Define apparent brightness and state how it is measured.

8. Luminosity • Light measurements give us information about the temperature, size and chemical composition of a star. • Luminosity(L) is the total amount of energy emitted by the star per second. • Unit is watt (same as power) • Depends on the temp. • Ex. Two stars have same temp, the bigger one will give out more energy • Sun’s luminosity of 3.839 x 1026W

9. Apparent brightness(b) • Some stars appear brighter than others. • Brightness depends on: • How much energy is radiated (luminosity) • How far away it is located • Apparent brightness is the amount of energy per second received per unit area. • Unit is W/m2 • b = (L) / 4πd2 • d is distant to the star

10. E.2.5 • Apply the Stefan–Boltzmann law to compare the luminosities of different stars. E.2.6 • State Wien’s (displacement) law and apply it to explain the connection between the color and temperature of stars.

11. Black Body Radiation • Black bodies absorbs all wavelengths of light and reflects none. It also is a perfect emitter of radiation. • If temp is increased the energy available is increased. • Means the electrons can gain more energy and move into higher energy levels • Means more photons released, and their average energy is greater. • E = hf, Higher energy means higher frequency/shorter wavelength

13. Stefan-Boltzmann • Each peek represents the intensity(apparent brightness) of radiation at different wavelengths. • Total intensity is the area under the curve. • Power per unit area = σ T4 • σ = 5.6 x 10-8 W/m2K4 (Stefan-Boltzmann constant)

14. Stefan-Boltzmann • If a star has a surface area A and temperature T then the total power emitted (luminosity), L is given by: • L = σAT4

15. Stefan-Boltzmann • At the temperature increases, the peak wavelength is shorter • Relationship between peak wavelength and temp is Wien displacement law: • λmax = (2.90 x 10-3km) / T

16. Example • The maximum in the black body spectrum of the light emitted from the sun is at 480 nm. Given that the Sun’s radius is 7.0 x 108m, calculate the temperature of the sun, the power emitted per square meter, and the luminosity. • Answers: 6000K, 7.3 x 107 W/m2, 4.5 x 1026W

17. E.2.7 • Explain how atomic spectra may be used to deduce chemical and physical data for stars. • Students must have a qualitative appreciation of the Doppler effect as applied to light, including the terms red-shift and blue-shift. E.2.8 • Describe the overall classification system of spectral classes. • Students need to refer only to the principal spectra classes (OBAFGKM).

18. Stellar Spectra Remember: • Electrons only exist in certain energy levels • When excited only produce specific wavelengths. (Emission Spectrum) • When white light passes through same gas these wavelengths are absorbed. (Absorption spectrum)

19. Stellar Spectra • Stars emit a continuous spectrum of EM • Peak intensity depends on the temp. • As this EM pass through the outer layer of the star, some is absorbed. • The absorption spectrum of a star tells us what elements are present because of the missing lines.

20. Stellar Spectra • The absorption spectra also helps us to calculate the temperature of the gas. • Hot gas • Most electrons are already in higher energy levels • Meaning they can’t make the biggest jump • See “Energy Levels” diagram • Means the higher energy photons will not be absorbed. • Means a weak absorption line • Which can let us find the temp

21. E.2.8 • Describe the overall classification system of spectral classes. • Students need to refer only to the principal spectra classes (OBAFGKM).

22. Spectral Classification of Stars • The spectrum of a star is related to it’s temp and chemical composition. • Also the color. The peak points to it’s color • Oh Be A Fine Girl Kiss Me

23. Doppler Shift • As objects move, the wave lengths they produce is either pushed together or spread apart. • Called doppler effect. • Applies to all waves including light from stars. • Red shift – longer λ – star moving away • Blue shift – shorter λ – star moving closer