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HERTZ SPRUNG - RUSSELL DIAGRAM HR DIAGRAM Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars HR DIAGRAM Stars can have a range of characteristics, such as mass, diameter, temperature and luminosity Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars HR DIAGRAM Stars that look to us as though they are near each other, may intact be very far away from each other. Distant but very bright stars look similar to close but dim stars. Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars MAGNITUDE AQA Only On a star chart, stars are given an apparent magnitude value, the lower the value the brighter the star appears The true magnitude (absolute magnitude) can be calculated. This is the apparent magnitude of the star if you were 10 parsecs away. Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars MAGNITUDE Magnitude is a logarithmic scale apparent magnitude m = -2.51 log (I / 2.56 x 10-6) I = intensity 2.56 x 10-6 lux = intensity for magnitude 0 star Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars MAGNITUDE m - M = 5log (d/10) M = absolute magnitude d = distance in parsecs Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars WEIN’S DISPLACEMENT LAW T = temperature of star in K λmax T = constant = 2.9 x 10-3 mK A black body is one that absorbs all the EM radiation that falls incident on it Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars STEFAN’S LAW P =σAT4 P = power A = surface area T = temperature (K) σ = stefan’s constant = 5.67 x 10-8 Wm-2K-4 Sometimes referred to as Stefan-Boltzman Law (Stefan arrived at it experimentally, Boltzman did it theoretically) Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars STEFAN’S LAW The ratio of the power output of two stars can be used to calculate the ratio of the diameters of the stars. (If one of those starts has a known diameter e.g the Sun the diameters of the other star can be determined) Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars HR DIAGRAM Plotting the luminosity of stars against the temperature reveals a pattern Spectral class and absolute magnitude can also be plotted to reveal the same pattern. Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars LUMINOSITY Luminosity is the total radiant power output of the star and so is measured in Watts It can also be measured in lux (1 lux = 1 lumen per metre squared ) (1 lumen = measure of total visible light emitted in a defined beam) Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars LUMINOSITY me times HR diagrams plot luminosity as a factor of solar luminos As a result, the scale is usually logarithmic (Temperature is also plotted logarithmically) Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars Main sequence Sun Questions Describe, briefly the sequence of events which occur in the formation of a star, such as our Sun, from interstellar dust and gas clouds. In your answer, you should make it clear how the steps in the process and sequenced. OCR G485 Jan 2013 [4 marks] Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars Questions Any three from: 1. (Interstellar dust and gas) cloud is drawn together by gravitational force / colapse / gravity 2. Loss in (gravitational) PE / KE increases / temperature increases 3. Fusion (of protons / hydrogen nuclei) takes place 4. Energy is released in fusion reactions 5. A stable star is formed when gravitational pressure is equal to internal / gas / radiation pressure QWC: The steps in the process are correctly sequenced Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars B1 x3 B1 Understand and explain the characteristics of stars Questions The final evolutionary stage of the star is a white dwarf. Describe some of the characteristics of a white dwarf. [2 marks] Explain why, in its evolution, the star is brightest when at its coolest. [2 marks] Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars Questions Any two from: • (extremely) dense / (very) hot / low luminosity • no fusion reactions occur • it is a remnant of a low-mass star • correct reference to Fermi pressure / elec B1 x 2 Red giant identified B1 (It is cooler but has) large surface area (and therefore radiates large amounts of energy) B1 Recall how a HR diagram is constructed Use a HR diagram to describe the life cycle of stars Understand and explain the characteristics of stars