Stars, Galaxies, and the Universe
Domain VII asks you to use stellar properties, the H-R diagram, galaxy structure, and cosmological evidence to explain the origin and evolution of the universe. Items focus on luminosity and temperature relationships, stellar end states, distance measurement, and redshift.
Stellar properties and evolution
Spectral classes O, B, A, F, G, K, M run hot to cool; the H-R diagram plots luminosity against temperature. Main-sequence stars fuse hydrogen to helium in their cores; mass sets luminosity, temperature, and lifetime, so massive stars burn brighter and die younger. Aging stars leave the main sequence as giants or supergiants. End states depend on mass: white dwarfs up to the Chandrasekhar limit of about 1.4 solar masses, then neutron stars or black holes. Elements heavier than iron form in supernovae and neutron-star mergers.
Distance
Parallax measures nearby stars: one parsec is the distance at which a star shifts one arcsecond, about 3.26 light-years. Farther out, astronomers use standard candles such as Cepheid variables and Type Ia supernovae.
Galaxies and cosmology
Galaxies are spiral (including barred), elliptical, or irregular. The Milky Way is a barred spiral roughly 100,000 light-years across, with the Sun about 26,000 light-years from the center. Rotation curves and cluster dynamics imply dark matter. Hubble's law, v = H₀d, together with redshift, shows that space itself expands; the cosmic microwave background at about 2.7 K is the relic of recombination, and dark energy appears to accelerate the expansion.
Worked example
A galaxy recedes at 7,000 km/s. With H₀ = 70 km/s/Mpc, d = v/H₀ = 100 Mpc. Since 1 Mpc is about 3.26 million light-years, d is roughly 326 million light-years.
Traps
Saying galaxies fly through space from a center instead of space expanding; calling the Big Bang an explosion at a point; using parallax for distant galaxies; and assuming heavy elements were forged in main-sequence cores.