Standard candle
Astrophysics & Cosmology - OCR A-Level Physics
Key Definition
Standard candle
An astronomical object whose absolute luminosity is known (or can be determined independently), allowing its distance to be calculated from its observed intensity.
An astronomical object whose absolute luminosity is known (or can be determined independently), allowing its distance to be calculated from its observed intensity.
- If you know L and measure I, you can calculate d = $\sqrt{L/(4\pi I)}.$
- Type Ia supernovaeThermonuclear explosions of white dwarfs that all reach the same peak luminosity — standard candles for cosmological distances. are excellent standard candles: they all have approximately the same peak luminosityThe total powerThe rate of energy transfer. Measured in watts (W). radiated by a star across all wavelengths. Measured in watts (W). (\(\sim 10^{36}\) W) because they occur when a white dwarfThe remnant of a low-mass star after it has shed its outer layers as a planetary nebula. Supported against gravitational collapse by electron degeneracy pressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻².. reaches the Chandrasekhar limit (1.4 $M_{\odot}$).
- Cepheid variable starsStars whose luminosity varies periodically; the period–luminosity relationship makes them standard candles for distance measurement. are also standard candles: their periodThe time taken for one complete oscillation or wave cycle. Measured in seconds (s). of pulsation is directly related to their luminosityThe total powerThe rate of energy transfer. Measured in watts (W). radiated by a star across all wavelengths. Measured in watts (W). (periodThe time taken for one complete oscillation or wave cycle. Measured in seconds (s).-luminosityThe total powerThe rate of energy transfer. Measured in watts (W). radiated by a star across all wavelengths. Measured in watts (W). relationship).
- Standard candles are essential for measuring distances to other galaxies (too far for parallax measurements).
- Type Ia supernovae can be seen in very distant galaxies, providing evidence for the accelerating expansion of the universe.
Examiner Tips and Tricks
- The key idea: standard candles bridge the gap between intrinsic brightness (luminosity, which we calculate) and apparent brightness (intensityThe power transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared., which we measure).
- This is how we build the cosmic distance ladder.