Astrophysics option (one possible Unit 2 AoS 2 option): the structure of the solar system, stellar life cycles, the colour-magnitude diagram, distance measurement (parallax, standard candles), and cosmological structure (galaxies, the expanding universe, Big Bang model)

What this dot point is asking

VCE Physics Unit 2 AoS 2 is an option topic chosen by the school from a list (sound, astrophysics, sport science, light, biomechanics, motion in two dimensions). This page covers the astrophysics option as one example.

The solar system

Structure. Sun at centre. Eight planets: terrestrial (Mercury, Venus, Earth, Mars) and gas giants (Jupiter, Saturn, Uranus, Neptune). Dwarf planets (Pluto, Eris, Ceres). Asteroid belt between Mars and Jupiter. Kuiper Belt beyond Neptune.

Distance scales.

• Earth-Sun: 1 astronomical unit (AU) = $1.5 \times 10^{11}$ m.
• Earth-Moon: $3.84 \times 10^8$ m.
• Sun-Neptune: about 30 AU.

Origin. Formed about 4.6 billion years ago from a collapsing molecular cloud (the solar nebula).

Stars and stellar evolution

Stars. Self-gravitating spheres of plasma producing energy by nuclear fusion. Hydrogen fuses to helium in the core (for sun-like stars).

Classification. Spectral types O, B, A, F, G, K, M (hottest to coolest). Sun is G-type. Brown dwarfs are sub-stellar.

Main sequence. Stars spend most of their lives fusing hydrogen. Position on the Hertzsprung-Russell (HR) diagram depends on mass; massive stars are hotter and more luminous but shorter-lived.

Life cycle of a sun-like star.

1. Protostar (collapsing nebula).
2. Main sequence (hydrogen fusion, several billion years).
3. Red giant (hydrogen exhausted in core; helium burning).
4. Planetary nebula and white dwarf.

Life cycle of a massive star.

1. Protostar.
2. Main sequence (much shorter, millions of years).
3. Red supergiant.
4. Supernova explosion.
5. Neutron star (if remnant mass 1.4 to 3 solar masses) or black hole (above about 3 solar masses).

Supernovae. Released energy comparable to a galaxy's luminosity for weeks. Produce elements heavier than iron (which cannot form by fusion in normal stellar burning).

The Hertzsprung-Russell (HR) diagram

A plot of luminosity (vertical) vs surface temperature (horizontal, reversed). Stars cluster into specific regions:

• Main sequence. A diagonal band from hot bright (top left) to cool dim (bottom right).
• Red giants. Upper right (cool but bright).
• White dwarfs. Lower left (hot but dim).

A star's position on the HR diagram reveals its evolutionary state.

Distance measurement

The "cosmic distance ladder" uses different techniques at different scales.

Parallax. As Earth orbits the sun, nearby stars appear to shift against the background. The apparent angular shift (parallax angle $p$) gives the distance:

Works for stars within about 1,000 parsecs (with Gaia satellite up to about 10,000 pc).

1 parsec = 3.26 light-years = $3.086 \times 10^{16}$ m.

Standard candles. Objects with known intrinsic brightness. Compare to apparent brightness to determine distance.

• Cepheid variables: brightness varies periodically; the period-luminosity relation gives intrinsic brightness.
• Type Ia supernovae: very consistent peak luminosity; standard candles for cosmological distances.

Redshift. Light from distant galaxies is shifted to longer wavelengths (the Hubble flow). The redshift gives recession velocity; Hubble's law $v = H_0 d$ gives distance.

Galaxies and structure

Milky Way. Spiral galaxy, around 100,000 light-years across. About 200 billion stars. Sun is in the Orion Spur, about 26,000 light-years from the galactic centre. Galactic centre contains a supermassive black hole (Sagittarius A*, about 4 million solar masses).

Local Group. Cluster of 50+ galaxies including Milky Way, Andromeda, and many dwarfs. About 10 million light-years across.

Observable universe. About 93 billion light-years in diameter (due to expansion). 100 billion to 2 trillion galaxies. Filamentary large-scale structure with voids between.

The expanding universe and Big Bang

Hubble's discovery (1929). Distant galaxies are receding from us, with velocity proportional to distance. The universe is expanding.

Hubble's law. $v = H_0 d$, where $H_0$ is Hubble's constant (about 70 km/s/Mpc).

Big Bang model. Extrapolating expansion backward, the universe was very dense and hot about 13.8 billion years ago. Evidence:

1. Hubble redshift (universe expanding).
2. Cosmic microwave background (CMB): faint radiation from the early universe, observed at temperature 2.7 K.
3. Abundance of light elements (hydrogen, helium, lithium) matching Big Bang nucleosynthesis predictions.

Dark matter. Galaxies rotate as if there is more mass than visible. Dark matter constitutes about 27 percent of the universe's mass-energy.

Dark energy. The expansion is accelerating (discovered 1998 via Type Ia supernovae). Dark energy is the hypothesised driver (about 68 percent of mass-energy).

Common errors

Confusing AU and light-year. AU is solar-system scale (~$10^{11}$ m). Light-year is interstellar scale (~$10^{16}$ m).

Parallax formula. $d (\text{pc}) = 1 / p (\text{arcsec})$. Note units.

Star's mass vs lifetime. Heavier stars are brighter but live shorter. Sun (1 solar mass) lives about 10 billion years; a 10 solar mass star lives about 10 million years.

Big Bang as explosion. The Big Bang is the expansion of space itself, not an explosion in pre-existing space.

Treating dark matter and dark energy as the same. Different things. Dark matter clusters with galaxies (gravitational). Dark energy is smooth and drives expansion.

In one sentence

Astrophysics applies physics to the universe at scales beyond Earth: the solar system (planets, AU scale), stellar evolution (main sequence to red giant to white dwarf or supernova and neutron star / black hole), distance measurement (parallax for nearby stars, standard candles for galaxies, redshift for cosmological distances), and the Big Bang model of the universe (expansion observed by Hubble 1929, cosmic microwave background, light-element abundances; with dark matter and dark energy accounting for the majority of the universe's mass-energy).