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HW7-3: (260) RQ 2, 3, 6; (261) Prob 1; (261) Learning to Look 1-2; Supp. Q. 4 (260)RQ 2: (a) Why is there a lower end to the main sequence? (b) Why is there an upper end? (a) Stars less than 0.08M☉ (8% of M☉) are not hot enough in their cores to run fusion. These are brown dwarfs. (b) Very massive stars have so much fusion that the outward forces overwhelm gravity and they are unstable. Stars greater than 100M☉ cannot even form. Stars more than 25 or 30M☉ engage in mass loss and lose weight down to 25 or 30M☉. Sunday, November 25, 12 1 HW7-3: (260) RQ 2, 3, 6; (261) Prob 1; (261) Learning to Look 1-2; Supp. Q. 4 (260) RQ 3: What is a brown dwarf? A brown dwarf is a “failed star.” They are balls of gas without fusion. The upper end of brown dwarfs is well defined: 8% M☉ = 80 Jupiters. There is a not-so-welldefined line between small brown dwarfs and large planets. (260) RQ 6: Why do expanding stars become cooler and more luminous? As they expand they have more surface area to shine from, so they are more luminous. The expansion and extra light output sap energy and lower the surface temperature. Sunday, November 25, 12 2 HW7-3: (260) RQ 2, 3, 6; (261) Prob 1; (261) Learning to Look 1-2; Supp. Q. 4 (261) P 1: In the Figure 12-2, how much of the sun’s mass is hotter than 13,000,000 K? About 7.3%. Probably a bit more. Sunday, November 25, 12 3 HW7-3: (260) RQ 2, 3, 6; (261) Prob 1; (261) Learning to Look 1-2; Supp. Q. 4 (261) P 1: In the Figure 12-2, how much of the sun’s mass is hotter than 13,000,000 K? About 7.3%. Probably a bit more. L2L 1: In the photograph of the Pleiades on page 255, there are no bright red stars. Use the H-R diagram to explain why the brightest stars are blue. Have there ever been bright red stars in this cluster? This is a young cluster. The brightest stars are blue giants: still on the main sequence. They haven’t expanded yet, so they’re surfaces haven’t cooled off yet. There have not yet been any red giants or red supergiants. Sunday, November 25, 12 4 HW7-3: (260) RQ 2, 3, 6; (261) Prob 1; (261) Learning to Look 1-2; Supp. Q. 4 (261) P 1: In the Figure 12-2, how much of the sun’s mass is hotter than 13,000,000 K? About 7.3%. Probably a bit more. L2L 1: In the photograph of the Pleiades on page 255, there are no bright red stars. Use the H-R diagram to explain why the brightest stars are blue. Have there ever been bright red stars in this cluster? This is a young cluster. The brightest stars are blue giants: still on the main sequence. They haven’t expanded yet, so they’re surfaces haven’t cooled off yet. There have not yet been any red giants or red supergiants. Sunday, November 25, 12 5 HW7-3: (260) RQ 2, 3, 6; (261) Prob 1; (261) Learning to Look 1-2; Supp. Q. 4 (261) L2L 2: Look at the photograph of the star cluster M67 on page 255. Why are there no bright blue stars in this cluster? This is a old cluster. The bright blue giants have already evolved off the main sequence and have probably died in a supernova. Sunday, November 25, 12 6 HW7-3: (260) RQ 2, 3, 6; (261) Prob 1; (261) Learning to Look 1-2; Supp. Q. 4 SQ 4: Look at Table 12-2 on page 245. Larger mass stars have more fuel to burn. Why do they run out of fuel so quickly in comparison to low mass stars? The high mass stars have extremely high luminosities, so they burn through their fuel very quickly. Sunday, November 25, 12 7