Students should be familiar with the position of the Sun on the HR diagram.

The idea

The Hertzsprung-Russell diagram plots absolute magnitude against decreasing temperature. The Sun is a typical main sequence star, meaning it is currently fusing hydrogen into helium in its core. On the HR diagram, the Sun is located near the centre of the main sequence band, which runs diagonally from top left to bottom right. Specifically, the Sun has a surface temperature of approximately 5700 K, placing it in spectral class G. Its absolute magnitude is approximately +4.8, though +5 is often accepted in exam sketches.

What earns the marks

• Identify the Sun's position within the main sequence band.
• State the Sun's absolute magnitude as approximately +4.8 or +5.
• State the Sun's surface temperature as approximately 5700 K.
• Identify the Sun's spectral class as class G.

Common mistakes and how to put them right

• Placing the Sun at an absolute magnitude of 0.

  Put it right: Place the Sun at an absolute magnitude of +5, as 0 represents a much more luminous star.

• Plotting the Sun on the right-hand edge of the main sequence.

  Put it right: Plot the Sun centrally, aligning with 5700 K, not at the cooler 3000 K extreme.

• Assuming the Sun's luminosity is 0 on a logarithmic scale.

  Put it right: If the y-axis is luminosity relative to the Sun, the Sun's position is at 1 (or 10⁰), not 0.

Exam tips

• Always check the axis scales carefully; temperature decreases to the right and absolute magnitude decreases (gets brighter) upwards.
• Mark the Sun's position with a clear, unambiguous cross rather than a large dot to ensure accuracy within the required tolerance.
• Memorise the values 5700 K and +4.8 for the Sun, as you may need to add them to unlabelled axes.