Physical Science: The Universe: Galaxies, Stars, and the Solar System
This astronomy domain covers stars, galaxies and cosmology, and the solar system.
Stars
Stars form from collapsing nebulae and ignite hydrogen fusion when the core reaches roughly 10 million K. On the H–R diagram (luminosity vs. temperature), the main sequence runs diagonally, giants and supergiants sit above, and white dwarfs below. Spectral classes hot to cool are OBAFGKM. Low-mass stars end as white dwarfs; massive stars may leave neutron stars or black holes after supernovae. Distance comes from parallax nearby and standard candles (Cepheids, Type Ia supernovae) farther out. Redshift indicates recession.
Galaxies and cosmology
The Milky Way is a barred spiral; galaxies are spiral, elliptical, or irregular. Hubble's law: v = H0d, using H0 = 70 km/s/Mpc as an illustrative value rather than a universally agreed measurement - the farther the galaxy, the faster it recedes. Expansion, the cosmic microwave background, and light-element abundances support the Big Bang about 13.8 billion years ago. Dark matter and dark energy dominate the mass-energy budget.
Solar system
The Sun fuses hydrogen in its core, surrounded by photosphere, chromosphere, and corona. Terrestrial planets (Mercury, Venus, Earth, Mars) are small, rocky, dense; the giant planets are large and ringed, with Jupiter and Saturn usually called gas giants and Uranus and Neptune ice giants. Kepler's laws: elliptical orbits, equal areas in equal times, and P² ∝ a³. Asteroids lie mainly between Mars and Jupiter; comets come from the Kuiper belt and Oort cloud. Earth's seasons come from its 23.5° axial tilt, not distance from the Sun.
Worked example
A galaxy recedes at 7000 km/s. With H0 = 70 km/s/Mpc, d = v/H0 = 7000/70 = 100 Mpc. Reasoning: divide velocity by the Hubble constant; the units cancel to megaparsecs. Holding the other cosmological-model assumptions fixed, a larger H0 corresponds to a shorter expansion timescale.
Traps
Explaining seasons by Earth's distance from the Sun; confusing moon phases with eclipses; assuming main-sequence stars fuse helium (they fuse hydrogen); thinking hotter stars are redder (they are bluer); treating redshift as motion through space rather than expansion of space.