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Subject: Cambridge International A2 Level Physics (9702)
By the end of these notes, you should be able to:
A star is a giant ball of hot gas, made mostly of hydrogen. Inside a star, hydrogen atoms fuse (join) together to form helium. This nuclear fusion reaction releases a huge amount of energy, which the star sends out into space as radiation — light, heat, and other forms of electromagnetic radiation.
A galaxy is a collection of billions of stars held together by gravity. Our own galaxy is called the Milky Way.
Luminosity is the total power of radiation emitted by a star — in other words, it tells you the total amount of energy the star sends out into space every second, in all directions combined.
Think of it like a light bulb: a 100 W bulb releases more energy per second than a 60 W bulb. A star with higher luminosity releases more energy per second than a star with lower luminosity.
Important: Luminosity is the total energy output of the star itself. It does not depend on how far away you are from the star — it is a property of the star alone.
When a star emits radiation, that radiation spreads outward in all directions through space. By the time it reaches Earth, it has spread over a much larger area, so the amount of radiation hitting each square metre of Earth is much smaller than the total amount leaving the star.
Radiant flux intensity (F) is the power of radiation received per unit area at a certain distance from the star. In simpler terms, it is the brightness of the star as measured on Earth — how much radiation is hitting each square metre of your detector.
Key distinction:
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