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The Sun’s power output is 4 x 10 26 watts

 If the energy source was gravitational or chemical, the Sun would have formed more recently or depleted its fuel long ago.

 The Sun is powered by thermonuclear fusion reactions in the core where hydrogen is being converted into helium and releasing energy in a process called the proton-proton chain.

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What is the energy source for the Sun?

 Our model of the Sun’s interior must account for the conditions we observe at the surface.

• Our star appears to be static.

 At extremely high temperatures and pressures, four hydrogen atoms can combine to make one helium atom and release energy.

• The Sun consumes 6 x 10 11 kg of hydrogen per second 4/24/2020 APHY101 2

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Hydrostatic equilibrium

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Interstellar Medium

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How is the energy transported in a star?

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What happens when the hydrogen runs out?

 The interior can no longer repel the inward pull of gravity.

 The core heats under pressure, causing the outer layers to expand and swell.

 These outer layers get farther from the hot core and cool, resulting in a red color.

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 An aging star like the Sun may lose as much as 40% of its original mass.

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What happens to large stars at the end of their life?

 Iron is split into (p + , n, e ) by gamma rays • p + + e → n 4/24/2020 APHY101 11

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