Astronomy
This domain tests solar-system formation, orbital dynamics, stellar evolution, cosmology, and Earth–Sun–Moon interactions. Expect items that ask you to interpret the H–R diagram, apply inverse-square and Kepler relationships, sequence events in stellar life cycles, and distinguish evidence for the Big Bang from speculation.
Core rules and concepts
- Kepler's laws. Orbits are ellipses with the Sun at one focus; equal areas are swept in equal times; P² = a³ when P is in years and a is in AU. Newton's form adds mass: P² = a³/(M₁ + M₂) in solar masses.
- Light and spectra. Wien's law, λmax = 2.9 × 10⁻³/T (m·K), gives temperature from peak wavelength; Stefan–Boltzmann, L = 4πR²σT⁴, links luminosity, radius, and temperature. Doppler shifts reveal radial motion; spectral lines identify composition.
- Stellar evolution. Mass sets the path: low-mass stars become red giants then white dwarfs; high-mass stars fuse through iron, then core-collapse as supernovae, leaving neutron stars or black holes. The Chandrasekhar limit (~1.4 M☉) bounds white dwarfs.
- Cosmology. Hubble's law v = H₀d and cosmic redshift imply expansion; the CMB, light-element abundances, and large-scale structure support the Big Bang. The solar-nebula hypothesis explains the frost line, terrestrial versus Jovian planets, and planetesimal accretion.
- Earth–Moon–Sun. Seasons arise from 23.5° axial tilt, not distance; Moon phases from orbital geometry; eclipses require alignment at nodes; tides from differential gravity.
Worked example
A planet orbits the Sun with a semimajor axis of 4 AU. Find its period. Using P² = a³: P² = 4³ = 64, so P = 8 years. The reasoning matters: the law compares periods and distances in AU and years for objects orbiting the Sun, so no conversion to meters is needed. A second check: 4 AU lies between Mars (1.52 AU) and Jupiter (5.2 AU, period ~11.9 years), so 8 years is consistent.
Traps that cost marks
- Claiming seasons are caused by Earth's changing distance from the Sun; the 3% distance variation cannot explain opposite hemispheres.
- Confusing apparent and absolute magnitude, or mixing parsecs and light-years without converting.
- Assuming more massive stars live longer; they fuse faster and have shorter main-sequence lifetimes.
- Treating the H–R diagram as one star's life track rather than a population plot with evolutionary tracks overlaid.