Astronomy and Planetary Science
This section covers astronomy and planetary science. The exam tests whether you can use physical laws to interpret the Solar System, stars and cosmology, and whether you can distinguish observation from inference.
Solar System and stars
- The Solar System formed from a collapsing molecular cloud and a protoplanetary disk; accretion, differentiation and the frost line explain rocky versus icy planets.
- Kepler's laws describe orbits; Newton's law of gravitation supplies the physical basis. For orbits in solar masses, years and astronomical units, P squared equals a cubed.
- Stars are classified by spectral type OBAFGKM and plotted on the Hertzsprung-Russell diagram. Main-sequence stars fuse hydrogen; mass largely sets luminosity and lifetime.
- Parallax distance in parsecs is one divided by parallax angle in arcseconds.
Cosmology
- Cosmological redshift arises from expansion of space, not simply motion through space.
- The cosmic microwave background is relic radiation from recombination; its small anisotropies constrain early density variations.
- Evidence for dark matter includes galaxy rotation curves and cluster dynamics; dark energy is inferred from accelerated expansion.
Worked example
A planet orbits at 4 AU from a 1 solar-mass star. Using P squared equals a cubed, P squared equals 64, so P equals 8 years. If the star were 4 solar masses, P squared equals 64 divided by 4, giving P equals 4 years. Reasoning: the period depends on both orbital radius and central mass.
Traps
- Confusing parallax in arcseconds with milliarcseconds, which changes distance by a factor of 1000.
- Treating apparent magnitude as intrinsic luminosity without distance.
- Describing cosmological redshift as a simple Doppler shift.
- Reading the Hertzsprung-Russell diagram with luminosity increasing downward.