Pearson Science Double Award Physics 8: Astrophysics

Astrophysics uses gravity and light to describe orbital systems and stellar life cycles. This note follows the 4SD0 boundary through point 8.10. Stellar spectra, luminosity diagrams, redshift, expansion and Big Bang evidence at points 8.11 to 8.18 belong to separate Physics and are excluded here.

Structure of the universe

The universe contains billions of galaxies. A galaxy is a gravitationally associated collection containing billions of stars, along with gas and dust. The Solar System lies within the Milky Way galaxy. A star is not the same as a galaxy, and the Solar System is only one planetary system within the Milky Way.

Gravitational field strength $g$ is force per unit mass. It varies between planets and the Moon because their masses and sizes differ. An object's mass remains constant, but its weight $W=mg$ changes with local field strength.

Gravity and orbits

Gravity supplies the inward resultant force for moons orbiting planets, planets and comets orbiting the Sun, and artificial satellites orbiting Earth. Orbiting objects are accelerating because velocity direction changes even when speed is constant.

Moons orbit planets, while planets orbit the Sun in paths that are nearly circular. Comets orbit the Sun in much more elongated paths, so their distance and speed vary strongly. Artificial satellites can be placed in different orbits for communication, observation or navigation.

For a circular orbit, distance travelled in one revolution is circumference. Orbital speed is

$$v=\frac{2\pi r}{T},$$