Black Body Radiation (Edexcel International A Level Physics)

The Sun is a main sequence star.

State what is meant by a main sequence star.

The Sun will eventually leave the main sequence and become a red giant star.

luminosity of Sun = 3.83 × 10²⁶ W

i) Show that the current radius of the Sun is about 7.0 × 10⁸ m.

surface temperature of Sun = 5780 K

(2)

ii) As a red giant star, the radius of the Sun will be about 150 times its current radius. Its luminosity will be about 1600 times its current luminosity. λ_max is the wavelength at which peak power radiation occurs.

Show that when the Sun is in its red giant stage, λ_max is about 1 × 10⁻⁶ m.

current radius of Sun = 7.0 × 10⁸ m

(3)

iii) At this stage of its evolution the Sun will be classified as a red giant star.

The wavelength of red light is between 6.2 × 10⁻⁷ m and 7.8 × 10⁻⁷ m.

Comment on the value for λ_max and this classification of the Sun.

(3)

As the Sun continues its evolution, its mass will decrease. It is estimated that at its largest radius, the mass of the Sun will have decreased significantly.

Explain how the orbital period of a planet would change as the mass of the Sun decreases.

Assume that the radius of the planet’s orbit about the Sun does not change.

A graph of relative intensity against wavelength for a star is shown.

Determine the surface temperature of the star.

Surface temperature of star = ...........................................

Radiation from the star is analysed. The radiation from the star has spectral lines at different wavelengths from the same spectral lines produced by a source in the laboratory, as shown.

Explain what can be concluded about the star from these results.

Your answer should include a calculation.