Flashcards · 9702 · A Level Topic 25

Astronomy and cosmology (A Level) flashcards

15 cards on A Level Topic 25, subtopics 25.1 to 25.3. Pick a subtopic or study them all.

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Luminosity of a star: The total power of radiation it emits. Unit W.
Radiant flux intensity, and the inverse square law: The power received per unit area at the observer: F = L ÷ (4πd²). Unit W m⁻².
Standard candle: An astronomical object of known luminosity. Measuring its radiant flux intensity on Earth gives its distance from F = L ÷ (4πd²).
Why are Type Ia supernovae used as standard candles?: They all explode at the same critical mass, so they reach the same peak luminosity and can be seen across very large distances, giving distances to galaxies.

Wien’s displacement law: λ_max ∝ 1/T: the peak wavelength is inversely proportional to the surface temperature. λ_max T = 2.9 × 10⁻³ m K.
Stefan–Boltzmann law: L = 4πσr²T⁴, where r is the star’s radius, T its surface temperature (in K) and σ the Stefan–Boltzmann constant.
How do you estimate the radius of a star?: Use Wien’s law with the peak wavelength to find T. Then put T and the luminosity into L = 4πσr²T⁴ and solve for r.
A star’s peak emission is at 480 nm. Estimate its surface temperature.: T = 2.9 × 10⁻³ ÷ 480 × 10⁻⁹ = 6.0 × 10³ K.
Two stars have the same radius; one is twice as hot. Compare their luminosities.: L ∝ T⁴, so the hotter star is 2⁴ = 16 times as luminous.

Redshift: Lines in the spectra of distant galaxies appear at longer wavelengths than their known (laboratory) values, because the source is moving away.
Redshift equation: Δλ ÷ λ ≈ Δf ÷ f ≈ v ÷ c, where Δλ = observed − laboratory wavelength and v is the speed of recession (v much less than c).
A line of laboratory wavelength 656.3 nm is observed at 660.4 nm. Find the recession speed.: Δλ = 4.1 nm. v = c × Δλ ÷ λ = 3.00 × 10⁸ × 4.1 ÷ 656.3 = 1.9 × 10⁶ m s⁻¹.
Hubble’s law: v ≈ H₀d: the recession speed of a galaxy is proportional to its distance (SI units: v in m s⁻¹, d in m, H₀ in s⁻¹).
Why does redshift lead to the idea of an expanding Universe?: Almost all galaxies are redshifted, and the redshift increases with distance. So galaxies are moving apart, faster the further away they are: the Universe is expanding.
How does Hubble’s law lead to the Big Bang theory?: If the galaxies are moving apart now, they were closer together in the past. Tracing back, the Universe began in a very small, dense, hot state and has been expanding since.