Astronomy and Space Science
This domain covers the universe, stars, galaxies, and the solar system, including Earth's motions and the Earth-Moon system. Questions test evidence for the Big Bang, stellar evolution, Kepler's laws, planetary comparisons, and seasonal or eclipse reasoning.
Universe and Stars
The Big Bang model is supported by the cosmic microwave background, the redshift-distance relation (Hubble's law), and light-element abundances. Stars form in nebulae, fuse hydrogen on the main sequence, and evolve according to mass. The H-R diagram plots luminosity against temperature; main sequence stars fuse hydrogen to helium. Massive stars end as supernovae, leaving neutron stars or black holes; low-mass stars become white dwarfs. Spectral classes OBAFGKM run hot to cool.
Solar System and Earth Motions
The nebular hypothesis explains the solar system's flattened, rotating disk. Terrestrial planets are small, rocky, and dense; Jovian planets are large, gaseous or icy, with rings and many moons. Kepler's laws state that orbits are ellipses, equal areas are swept in equal times, and P^2 = a^3 when period is in years and semimajor axis is in AU. Seasons result from axial tilt, not distance. Moon phases depend on the Sun-Earth-Moon angle; eclipses require near-perfect alignment near the nodes.
Worked Example: Kepler's Third Law
An asteroid orbits the Sun with semimajor axis 4 AU. What is its orbital period? P^2 = a^3 = 4^3 = 64, so P = 8 years. Reasoning: use solar units; the law compares orbits around the same central mass. A larger semimajor axis means a longer period.
Traps That Cost Marks
- A light-year is a distance, not a time.
- Redshift indicates recession; blueshift indicates approach.
- Seasons are caused by axial tilt, not proximity to the Sun.
- Lunar phases are not caused by Earth's shadow; that is a lunar eclipse.
- The H-R diagram's temperature axis increases to the left.
- Main sequence stars are not all the same mass or age.