Warm-up on board. Sketch problem 1 from the HW. Which star: i) Is largest? Is second smallest? Could be the same size as D? Bright A.

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1 Warm-up on board Sketch problem 1 from the HW. Bright A C Which star: L D i) Is largest? ii) iii) Is second smallest? Could be the same size as D? Dim Hot B T = Main Sequence E Cool

2 Why are stars seen in clusters? What makes up a stellar population? Open Star Clusters M35 and NGC 2158 Credit & Copyright: Canada-France-Hawaii Telescope, J.-C. Cuillandre (CFHT), Coelum

3 Goals from Last Time 1) predict trends in stellar radius with L and T in the HR diagram on and off the Main Sequence 2) Recognize how apparent vs. intrinsic brightness creates selection bias we must remove to decide whether the Sun is typical.

4 Think-Pair-Share Both surveys searched for all stars within 10 pc. Which survey can see stars with fainter apparent brightness? A peak at 12 B peak at 14 C: They have the same apparent brightness limit. (Notice that the Sun is not typical in luminosity in fact, there is no typical luminosity.)

5 Goals for Today 1) Recognize clusters as stars born together; relate the origin and age of field stars to open and bound cluster stars. 2) Identify star classes/evolutionary stages within HR diagrams, relating each to its source of energy. 3) Compare the evolutionary paths of high vs. low mass stars. 4) Know which types of stars follow a standard M-L relation, and use it to prove that massive Main Sequence stars live faster and die younger. 5) Analyze cluster HR diagrams in relation to either general field or other cluster HR diagrams to compare ages. 6) Explain why stellar populations may have different colors and metallicities.

6 Clusters of stars are born together Globular clusters are bound for life. Open clusters come unbound as they age and blow away their leftover gas. Their stars disperse to become field stars.

7 Think-Pair-Share luminosity temperature This diagram shows: A) stars born together in a bound cluster B) stars born together in an open cluster C) stars born in several open clusters, each of which produced stars born together at a distinct time

8 The Hertzsprung Russell diagram luminosity temperature This schematic HR diagram shows a snapshot of a stellar population with stars at all stages of evolution

9 Red giants, white dwarfs, and planetary nebulae (not planets!)

10 Fusion Reactions change with temperature and as raw materials run out in the core MINIMUM TEMP. 8 x 10 6 K 20 x 10 6 K 100 x 10 6 K 600 x 10 6 K 10 9 K REACTION proton-proton chain CNO cycle (H fusion) triple alpha (He fusion) carbon-helium fusion carbon burning What fuses on the horizontal branch? A) H in the core B) He in the core C) H in a shell D) He in a shell Massive/hot stars make the heaviest elements stardust! ( metals )

11 Masses along the Main Sequence Normal hydrogen-burning stars (=Main Sequence stars) follow a mass-luminosity relation L~M 3.5 (Is the Sun on it?) Q: How do we know L~M 3.5? A: orbital motions trace mass

12 Lifetimes along the Main Sequence L = km 3.5 f conv = % of Mass converted to energy by fusion (E=f conv Mc 2 ) star s lifetime τ=e/l τ = f conv Mc 2 /(km 3.5 ) measured approximate to be constant by definition of lifetime algebra τ 1/M 2.5 massive stars live fast and die young

13 Evolutionary paths for high & low mass stars When high mass stars explode, stardust disperses

14 Think-Pair-Share Which star: i) Will live longest (where live = fuse)? ii) Bright Will live longest Dim (where live = not Hot T Cool change form)? L A B = Main Sequence C D E

15 stars are born in gas polluted with the stardust of earlier generations of stars A B Label A & B: which was born first? which is bluer? which has higher metallicity? which might be seen in a globular cluster?

16 Think-Pair-Share In the main sequence for clusters, massive stars are often missing why? Real cluster M55 Which cluster is oldest? A B A) They die without leaving a remnant B) They die earlier than other stars C) There aren t as many of them in the population C

17 Think-Pair-Share Which statement is true? A) If these are both open clusters, M67 probably formed earlier B) If these are both open clusters, M67 probably has more white dwarfs C)If these are both open clusters, M67 is probably bluer

18 Think-Pair-Share Which statement is true? A) massive stars produce more heavy elements by fusion, so have higher metallicity B) the metals in the cores of old stars were produced mainly by earlier generations of stars C) the photospheres of young stars reveal lines that come from mainly from elements made by earlier generations of stars

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