Astrophysics (Section 8) covers the Solar System, the life cycle of stars from nebula to remnant, the Hertzsprung-Russell diagram, evidence for the Big Bang, and the expanding Universe.
Solar System: Sun (star) orbited by 8 planets, dwarf planets, moons, asteroids, comets. Star formation: nebula (gas cloud) → gravity collapses it → protostar → nuclear fusion begins → main sequence star. Sun-sized star: main sequence → red giant → planetary nebula → white dwarf. Massive star: main sequence → red supergiant → supernova → neutron star or black hole (if massive enough). Hertzsprung-Russell diagram: plots luminosity vs temperature; main sequence diagonal, giants/supergiants upper right, white dwarfs lower left.
Light from distant galaxies is redshifted (wavelength increases) — galaxies are moving away. Hubble\'s law: recession speed ∝ distance (v = Hd). Further galaxies recede faster. This implies the Universe is expanding. Extrapolating backward: all matter was once in a single point → Big Bang (≈13.8 billion years ago). Evidence: (1) redshift of galaxies, (2) cosmic microwave background radiation (CMBR) — uniform microwave radiation from all directions, remnant heat of the Big Bang. Dark matter and dark energy: most of the Universe is not directly observable.
The mass of the star determines its end state. A star with mass similar to the Sun (up to ~8 solar masses) ends as a white dwarf: after the red giant phase, it ejects its outer layers as a planetary nebula, leaving a hot, dense core that slowly cools. A more massive star (roughly 8–25 solar masses) dies in a supernova explosion; the core is compressed so intensely that protons and electrons merge into neutrons, forming a neutron star (~20 km diameter, incredibly dense). Stars above ~25 solar masses also go supernova, but the core is so massive that nothing can stop the gravitational collapse — it becomes a black hole, from which not even light can escape.
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