9702 · 25.1
Standard candles flashcards
Revision flashcards for Cambridge 9702 Standard candles (syllabus 25.1). Flip, recall, then mark a real past-paper question.
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Define 'luminosity' (L) in astronomy.
Luminosity is the total power of electromagnetic radiation emitted by an astronomical object.
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Define 'flux' (F) in astronomy.
Flux is the power of radiation received per unit area at a specific distance from the source.
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What does the inverse square law for radiation assume about the radiation spread?
It assumes that the radiation spreads uniformly in all directions over a spherical surface from the source.
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What characteristic defines an astronomical object as a 'standard candle'?
A standard candle is an object with a precisely known intrinsic luminosity (total power emitted).
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How is the distance to a standard candle determined using its known luminosity (L) and observed flux (F)?
The distance (d) is calculated using the formula $d = \sqrt{\frac{L}{4\pi F}}$.
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What is a 'blackbody' in the context of stars?
A blackbody is a theoretical object that perfectly absorbs and emits all wavelengths of electromagnetic radiation. Stars are often approximated as blackbodies.
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State Wien's Displacement Law in terms of peak wavelength and temperature.
Wien's Displacement Law states that the peak emission wavelength ($\lambda_{max}$) of a blackbody is inversely proportional to its absolute temperature (T), given by $\lambda_{max} T = \text{constant}$.
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How does the temperature of a star relate to its peak emission wavelength according to Wien's Law?
Hotter objects emit radiation with a shorter peak wavelength ($\lambda_{max}$), appearing bluer.
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Which physical quantities does the Stefan-Boltzmann Law relate to a star's luminosity (L)?
It relates luminosity (L) to the star's surface area (proportional to $r^2$) and its absolute temperature (T) to the fourth power, as $L = 4\pi r^2 \sigma T^4$.
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Why are laws like Wien's and Stefan-Boltzmann important for the concept of standard candles?
These laws help astronomers characterize and calibrate the intrinsic luminosity and temperature of celestial objects, essential for identifying them as reliable standard candles.
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What is the primary role of standard candles in the 'cosmic distance ladder'?
They provide accurate distance measurements for objects far beyond our solar system, forming a crucial rung in the cosmic distance ladder.
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How do standard candles contribute to validating Hubble's Law?
Their accurate distance measurements are fundamental for determining the expansion rate of the universe and verifying Hubble's Law.
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State the Stefan-Boltzmann Law as a formula.
$L = 4\pi r^2 \sigma T^4$, where L is luminosity, r is radius, T is absolute temperature, and σ is the Stefan-Boltzmann constant.
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State Wien's Displacement Law as a formula.
$\lambda_{max} T = b$, where $\lambda_{max}$ is the peak wavelength, T is the absolute temperature, and b is Wien's displacement constant (approx. $2.9 \times 10^{-3} \text{ m K}$).
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What are two main types of standard candles used for measuring large cosmic distances?
Cepheid variable stars and Type Ia supernovae.
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How does the luminosity of a Cepheid variable relate to its pulsation period?
The period-luminosity relationship states that the longer the pulsation period of a Cepheid variable, the greater its intrinsic luminosity.