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Astronomy 1020 Stellar Astronomy Spring_2014 Day-30

Astronomy 1020 Stellar Astronomy Spring_2014 Day-30. Course Announcements. 1 st Quarter Observing – Mon. 4/7 @8:30pm Archwood parking lot OR atrium of SSB Rain, shine, sleet, snow … it’s on Lunar Eclipse … Mon-Tues. 4/14-15/2014

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Astronomy 1020 Stellar Astronomy Spring_2014 Day-30

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  1. Astronomy 1020 Stellar Astronomy Spring_2014 Day-30

  2. Course Announcements • 1st Quarter Observing – Mon. 4/7 @8:30pm • Archwood parking lot OR atrium of SSB • Rain, shine, sleet, snow … it’s on • Lunar Eclipse … Mon-Tues. 4/14-15/2014 • IF CLEAR, we’ll be at the observatory from about midnight-ish on. • Dark night, 4/23/2014 (Wed.) weather dependent.

  3. 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!

  4. Core is mainly carbon and is electron-degenerate. • Cools off in millions of years (not making new energy). • All low-mass stars become white dwarfs.

  5. Concept Quiz—The Future Sun When will the Sun leave the main sequence? • when hydrogen is exhausted in the core • when all the Sun’s hydrogen becomes helium • when carbon fusion begins • when it becomes an AGB star

  6. Concept Quiz—Stages Which of the following is the correct order for the stages of evolution of the Sun? • main sequence, white dwarf, planetary nebula, red giant • main sequence, red giant, white dwarf, planetary nebula • main sequence, red giant, planetary nebula, white dwarf • main sequence, planetary nebula, red giant, white dwarf

  7. 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.

  8. 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 .

  9. 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

  10. Many stars are in binary systems. • In each pair of low-mass stars, the more massive star evolves first. • It can only expand so much before it fills its Roche lobe and begins to lose material due to the other star’s gravitational pull.

  11. Material can flow from the giant star to the companion. This is called mass transfer. • The star becomes a white dwarf. • When the second star is a giant, it can dump material onto the white dwarf.

  12. CONNECTIONS16.1 • What happens to planets when stars evolve? • Planet migration may allow planets to survive by moving outward, but some may move inward instead. • Planets have been found orbiting red giants and AGB stars. • As the Sun evolves, Earth’s present location will no longer be in the habitable zone. • Earth may move outward, inward, or stay where it is.

  13. Concept Quiz—Binaries A binary begins with a 2 M and a 1 M star. Which of the following states is not possible later on? • two white dwarf stars • a white dwarf and a main-sequence star • two AGB stars • a white dwarf and a red giant

  14. H can collect on the hot outer portions of the white dwarf. • Nuclear reactions can start, and the star gets much brighter temporarily nova. • For a few hours, it can be a half-million times more luminous than the Sun.

  15. The maximum mass for a white dwarf is 1.4 M, called the Chandrasekhar limit. • If material dumped on the white dwarf pushes it over this limit, it will collapse and explode.

  16. What does it mean to be Degenerate? • Electron energy levels crowded together almost continuous • All low energy levels are full according to the Pauli Exclusion Principle • Only place for additional electrons to go is in high energy levels which means they must move very fast…close to the speed of light • Adding more mass decreases the volume • Temperature is same everywhere

  17. If you add mass to a degenerate object it shrinks

  18. This is called a Type Ia supernova. • The explosion is briefly as luminous as 10 billion Suns. • Nothing of that star is left behind; the other star evolves on its own. • This process requires a binary system.

  19. Massive stars have more hydrogen to start with but they burn it at a prodigious rate The overall reaction is still There are 3 gamma ray photons instead of two as in the proton-proton cycle and it consumes hydrogen much faster

  20. MATH TOOLS 16.2 • A star’s escape velocity decreases once it spreads out: • Sun: • Sun as red giant:

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