3.9.2.13
Standard Candles
Astrophysics | AQA A-Level Physics
Key Definition
Standard candle: an astronomical object of known brightness that can be used to calculate galactic distances.
Examples of standard candles
- The most common examples of standard candles are:
- Cepheid variable stars
- Type 1a supernovae
Type 1a supernovae as standard candles
- Type 1a supernovae can be used as standard candles because they reach the same peak value of absolute magnitude each time.
- This type of supernova involves an exploding white dwarf in a binary star system. The white dwarf increases in mass as it attracts material from its binary pair.
- Eventually the white dwarf reaches a critical mass, known as the Chandrasekhar limit (about 1.4 solar masses).
- Crucially, this critical mass means the explosion is the same each time, producing a very consistent light curve with a peak absolute magnitude of about $M = -19$.
Measuring distances with standard candles
- Since we know the absolute magnitude $M$ of a Type 1a supernova at peak brightness ($M \approx -19$), and we can measure its apparent magnitude $m$ from Earth, we can use the distance modulus equation to find the distance:
- Another advantage of using Type 1a supernovae as standard candles is that they are extremely bright. This means they can be used to measure the distance to the furthest galaxies.
Common Mistake
In exam calculations involving standard candles, remember that the peak absolute magnitude of a Type 1a supernova is about $-19$ (negative, not positive). A negative absolute magnitude means the object is extremely luminous. If you use $+19$ by mistake, you will calculate a nonsensically small distance. Also ensure you are comfortable rearranging the distance modulus equation with logs.