Supernovae as Standard Candles (AQA A Level Physics): Revision Note
Exam code: 7408
Type 1a supernovae as standard candles
A standard candle is defined as:
An astronomical object of known brightness that can be used to calculate galactic distances
The most common examples of standard candles are:
cepheid variable stars
type 1a supernovae
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
This critical mass means the explosion is the same each time, hence it produces a very consistent light curve
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
Worked Example
An astronomer observes a type 1a supernova in a distant galaxy with an apparent magnitude of +11 at its peak of brightness.
At its peak, it is known that a type 1a supernova has an absolute magnitude of about −19.
Show that the galaxy is about 10 Mpc away.
[2]
Answer:
Step 1: Write down the magnitude equation
Where:
Apparent magnitude,
Absolute magnitude,
Step 2: Substitute in the values of and and rearrange
[1 mark]
Step 3: Take logs of both sides to determine the distance
[1 mark]
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