LEARNING OBJECTIVES
What you will be able to do
- describe the Solar System and orbital motion
- relate gravity to orbital speed and period
- explain stellar formation and life cycles
- use redshift and cosmic evidence to describe the Universe
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Explain the Solar System, stellar evolution and evidence for an expanding Universe.
Space physics applies familiar ideas—gravity, energy, waves and nuclear processes—across immense distances and times. Models are tested using light received from objects we cannot visit.
The Solar System is one tiny part of the Milky Way, which is one of billions of galaxies in an expanding Universe.
SECTION 01
Earth, the Moon and the Solar System
The Solar System contains the Sun, eight planets, dwarf planets, moons, asteroids and comets. Planets move in elliptical orbits, while moons orbit planets. The Sun's gravity supplies the centripetal force.
Earth's rotation causes day and night; its yearly orbit and tilted axis cause seasons. The Moon's roughly monthly orbit produces phases because we see different fractions of its sunlit half.
SECTION 02
Orbits and gravity
Gravity provides inward acceleration even when orbital speed is constant, because velocity direction continuously changes. Closer circular orbits generally have greater speed and shorter period.
The gravitational field weakens with distance. A geostationary satellite orbits above the equator in Earth's rotational direction with a 24-hour period, so it remains above one point and suits communications.
| Orbit | Advantage | Limitation |
|---|---|---|
| Low Earth | short delay and detailed imaging | small coverage; moves across sky |
| Geostationary | continuous wide-area coverage | longer delay; weak polar coverage |
SECTION 03
Stars and stellar evolution
Gravity collapses a nebula into a protostar. When the core becomes hot enough, hydrogen fusion begins and a stable main-sequence star forms, with inward gravity balanced by outward pressure.
A Sun-like star expands to a red giant, sheds outer layers and ends as a white dwarf. A massive star becomes a red supergiant, explodes as a supernova and leaves a neutron star or, if sufficiently massive, a black hole. Supernovae create and disperse heavy elements.
SECTION 04
Galaxies and the expanding Universe
The Sun lies in the Milky Way galaxy. Spectral lines identify elements in stars. If the lines shift toward longer wavelengths, the source is receding; more distant galaxies generally show greater redshift and recession speed.
The expansion supports a Universe that began from a very hot, dense state. Cosmic microwave background radiation is cooled remnant radiation, and the observed abundance of light elements also supports the Big Bang model. The Universe's age is about the inverse of the Hubble constant when consistent units are used.
Calculating redshift
- A spectral line emitted at 500 nm is observed at 510 nm.
- Change = 10 nm.
- z = 10 ÷ 500.
Answer: Redshift z = 0.020.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Gravity maintains orbits and determines their properties.
- A star's initial mass controls its life cycle.
- Spectra reveal composition and motion.
- Redshift and background radiation support an expanding Universe with a hot origin.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- describe the Solar System and orbital motion
- relate gravity to orbital speed and period
- explain stellar formation and life cycles
- use redshift and cosmic evidence to describe the Universe