Fusion is the main energy source for stars.
E.2.2. Explain that in a stable star (for example, the Sun), there is an equilibrium between radiation pressure and gravitational pressure.
A stable star is a star in which there is an equilibrium between the radiation pressure and the gravitational pressure. The reason why the powerful reactions in the Sun have not forced away the outer layers of the Sun is because of this balance between the outward pressure and inward gravitational force.
E.2.3. Define the luminosity of a star.
Luminosity is the total power radiated by a star, it is measured in Watts (W). It depends on both the surface temperature of the star and its radius or surface area. If the radius of the two stars is the same, the one with the higher temperature will have the greater luminosity. If the temeperature of the stars is the same, the one with the larger radius will have the great luminosity.
E.2.4 Define apparent brightness and state how it's measured.
Apparent brightness: The power received per unit area. The SI units are Wm^-2.
E.2.5 Apply the Stefan-Boltzmann Law to compare the luminosities of different stars
The radiation from a perfect emitter is known as black body radiation. The graph below shows a spectrum of radiation from black-body emitters at different temperatures. A hot object emits radiation across a broad range and there is a peak in intensity at a particular wavelength. For a hotter body, the peak is at a higher intensity and shorter wavelength.
The peak wavelength (at which the maximum amount of energy is radiated) is related to the surface temperature by Wien's displacement law.
E.2.6 State Wien's (displacement) law and apply it to explain the connection between the colour and temperature of stars.
Questions 5 & 6
5.
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