Flashcards · 4PH1 · Topic 8

Astrophysics flashcards

20 cards on Topic 8, subtopics 8a to 8d. Pick a subtopic or study them all.

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Unit of gravitational field strength: Newton/kilogram (N/kg).
Units in orbital speed = 2π × orbital radius ÷ time period: Orbital radius in metres (m), time period in seconds (s), orbital speed in metre/second (m/s).
Units in weight = mass × gravitational field strength: Weight in newtons (N), mass in kilograms (kg), g in N/kg.

The universe, galaxies and our solar system: The universe is a large collection of billions of galaxies; a galaxy is a large collection of billions of stars. Our solar system is in the Milky Way galaxy.
Is gravitational field strength the same everywhere?: No. g varies with location, and is different on other planets and on the Moon.
What causes moons, planets, artificial satellites and comets to orbit?: The gravitational force (gravity provides the centripetal force).
How do the orbits of planets, moons and comets differ?: Planets orbit the Sun in nearly circular ellipses. Moons orbit planets. Comets orbit the Sun in highly elliptical orbits.
Where is a comet fastest, and why?: At its closest approach to the Sun. As it approaches, gravitational potential energy is converted into kinetic energy.
A satellite orbits at radius 8000 km with a period of 7200 s. Orbital speed?: v = 2πr/T = 2π × 8 000 000 ÷ 7200 ≈ 6980 m/s.

How are stars classified by colour?: Colour relates to surface temperature: hotter stars appear blue/white, cooler stars red.
Evolution of a star like the Sun: Nebula → (protostar) → main sequence star → red giant → (planetary nebula) → white dwarf.
Evolution of a star much more massive than the Sun: Main sequence → red supergiant → supernova → neutron star or black hole. Mass determines the fate.
Why is a main sequence star stable?: The inward gravitational force balances the outward pressure from the energy released by fusion of hydrogen into helium.
Absolute magnitude: How bright a star would appear from a standard distance, so stars can be compared fairly. (Apparent brightness also depends on distance.) (Paper 2 only)
Hertzsprung–Russell (HR) diagram: A plot of stars by temperature against luminosity (or absolute magnitude), showing patterns such as the main sequence band. (Paper 2 only)

Red-shift: Light from a galaxy moving away is shifted towards longer wavelengths (Doppler effect). (Paper 2 only)
Two red-shift observations, and what they show: Almost all galaxies are red-shifted (receding), and more distant galaxies show greater red-shift (receding faster). So the universe is expanding. (Paper 2 only)
Argument for the Big Bang from an expanding universe: If the universe is expanding, it was smaller, denser and hotter in the past. Space itself expands, so any galaxy sees the same pattern: Earth is not at the centre. (Paper 2 only)
Cosmic microwave background radiation (CMBR): Microwave radiation found uniformly in all directions: the cooled remnant of radiation from the hot early universe. Only the Big Bang model explains it. (Paper 2 only)
Red-shift equation. A 500 nm line is observed at 500.5 nm. Galaxy's speed?: Δλ/λ₀ = v/c, so v = c × Δλ/λ₀ = 3.0 × 10⁸ × 0.5 ÷ 500 = 3.0 × 10⁵ m/s. (Paper 2 only)