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Astronomy 1020 Stellar Astronomy Spring_2017 Day-29 Course Announcements • Mar. 31 (Fri) – Last day to drop a class with W, F, FA • • • • • • • SW chapter 15 – Due Monday 4/3 SW chapter 16 – Due Monday 4/10 SW chapter 17 – Due Friday 4/14 SW chapter 18 – Due Wednesday 4/19 SW chapter 19 – Due Wednesday 4/26 SW chapter 20 & 21 – Due Thursday 5/4 – 10:30 am – no late accepted The LAST 1st Quarter night is Monday 4/3 at 7:30pm -- On campus • Reports are due at Class Time on Monday April 24. • NO LATE REPORTS ACCEPTED! • EXAM-3: Wed. 4/5 – Chapters 13, 14, 15, 16 Main-sequence stars fuse hydrogen to helium in their cores. Eventually, much of the core H is converted to He. A core of He ash is built up (does not fuse at this point). Helium Core Is Degenerate H fusion only takes place in a shell around the 100 percent He core: hydrogen shell burning. If H fusion is not happening in the core, the star is no longer main sequence. Since the He is not fusing, gravity begins to win over the pressure, crushing the He. The core becomes more dense, and becomes electron-degenerate. This means pressure is not from moving atoms, but from a quantum mechanical effect: There’s a limit to how tightly electrons can be packed together. When the fuel runs out of the core, the luminosity increases. Why? When the core shrinks, its gravitational pull gets stronger. Weight of the outer layers increases. This results in increased pressure: Fusion in the shell goes faster. Faster nuclear reactions release more energy. This leaves the star’s surface at a higher rate (higher luminosity). Increase in pressure and luminosity results in increased size and decreased surface temperature: red giant. H-R diagram: Star moves up and to the right. iClicker Questions Stellar Evolution: Main Sequence Lifetimes Core Stages 1 – option 1 He core is small, dense, electron-degenerate. Outer envelope is greatly expanded, cooler. Fusion of H in shell creates more He, making He nuclei in core denser and hotter. Once hot enough, fusion of He begins in the degenerate core. He fuses to carbon (C) via the triple-alpha process starts suddenly in the helium flash. Star shrinks and heats up. After the helium flash, the star is on the horizontal branch of the H-R diagram. At first, He C in the core, H He in a shell around the core. Star is smaller and hotter. Helium is then used up in the core. He fusion in an inner shell and H fusion in an outer shell all surrounding a C core. Star gets more luminous and cool, and enters the asymptotic giant branch (AGB). As an AGB star, the star expands even more than as a red giant, and cools. H-R diagram: moves up and to the right again. Dense, electron-degenerate carbon core. iClicker Questions Stellar Evolution: Core Stages 1 – option 4 After the AGB: Planetary Nebula The star is very thinly spread. Cannot hold on to the outer layers easily. Outer layers are ejected into space, due to instabilities in the interior. After the AGB: Planetary Nebula The ejected material creates a planetary nebula. The core shrinks and first gets very hot, but eventually cools into a compact white dwarf. If the conditions are right, the star will ionize the gas in the expanding outer layers. Will last for about 50,000 years before the gas expands too far and disperses. iClicker Questions Stellar Evolution: Sun Stages Planetary Nebula Metals Planetary Nebula Spectra Leftover core of star remains as white dwarf. They are hot, but not very luminous. Masses 0.6–1.4 M, size like Earth. Density: a ton per teaspoonful! Core is mainly carbon and is electrondegenerate. Cools off in millions of years (not making new energy). All low-mass stars become white dwarfs. As the star burns its hydrogen, it accumulates a helium ash Because energy flow in the core of the star is by radiation, the helium ash isn’t being stirred out. Eventually a dead helium core starts to form at the center of the star The central helium core is not fusing. It’s just being squeezed by gravity and added to by the hydrogen fusing above it Because the helium ash gets in the way, the star gets brighter To continue to burn hydrogen with all that helium in the way, the core of the star gets a little hotter, the surface gets a little bigger and the star gets a little brighter .