Ch 4 · 6.2.2 Stars · O Level Physics 5054
Topic 4 of 5 in Space Physics — O Level Physics 5054 — 29 lessons.
Introduction
Stars are born, shine for millions or billions of years, and die, some quietly and some in enormous explosions. The atoms in your body, apart from hydrogen, were made inside stars that exploded long before the Sun formed. (Syllabus section 6.2.2.)
Key ideas — Galaxies and distances
1\ light-year = 3.0 × 10^8\ m/s × (365 × 24 × 3600\ s) ≈ 9.5 × 10^15\ m
🧑🏫 Teacher's explanation
Imagine looking at a massive city at night from a high-altitude airplane. You see thousands of tiny lights. Each light is a house, but from your height, they look like single points. In space, a galaxy is like that city, but instead of houses, it contains billions of stars. Our Sun is just one single 'house' in the vast 'city' of the Milky Way.
📖 Definitions
🔍 Worked example — Easy
Calculate the distance in meters that light travels in one year, given the speed of light c = 3.0 × 10^8\ m/s.
🔍 Worked example — Standard
A star is located 4.2\ light-years from Earth. Calculate this distance in meters.
🔍 Worked example — Exam-style
The Sun is approximately 8\ light-minutes from Earth. Calculate the distance in meters. (Use c = 3.0 × 10^8\ m/s)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — The life cycle of a star
cloud arrow protostar arrow stable star arrow begincases red giant arrow planetary nebula + white dwarf & (less massive stars)\\ red supergiant arrow supernova arrow nebula + neutron star or black hole & (more massive stars) endcases
🧑🏫 Teacher's explanation
Think of a star like a giant, controlled explosion that is constantly fighting against itself. On one side, gravity is trying to crush everything toward the center. On the other side, the intense heat from nuclear fusion in the core creates an outward pressure. As long as these two forces are perfectly balanced, the star stays stable.
📖 Definitions
🔍 Worked example — Easy
A star is formed from an interstellar cloud. If the cloud's radius decreases, what happens to the temperature of the protostar? Explain using the concept of gravitational attraction.
🔍 Worked example — Standard
A star is at a stable stage when the inward force of gravity is F_g and the outward pressure force is F_p. If F_g = 5.0 × 10^30 N and F_p = 5.0 × 10^30 N, is the star stable?
🔍 Worked example — Exam-style
A star is 4.5 light-years away. Calculate the distance in meters. (Speed of light c = 3.0 × 10^8 m/s, 1 year = 365.25 days)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the unit used to measure such astronomical distances. [1] (b) Describe the changes that occur in a star when the hydrogen in its centre is exhausted. [3] [4 marks]
Worked example
The gravitational attraction of its own mass pulls the star inwards. The very high temperature at the centre (from fusion) produces an outward force. In a stable star these two forces are balanced, so its size stays the same.
🔍 Worked example — Easy
A star is in a stable state. Identify the two opposing forces acting on the star's structure.
🔍 Worked example — Standard
A star is located 4.2 light-years from Earth. If light travels at 3.0 × 10^8 m/s, calculate the distance in meters. (Use 1 year = 365.25 days and 1 day = 86400 s)
🔍 Worked example — Exam-style
A star has exhausted its hydrogen fuel and has begun fusing helium. Describe the change in its size and the reason for this change.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) The distance travelled by light in a vacuum in one year. (2) An interstellar cloud of gas and dust collapsing and increasing in temperature because of its internal gravitational attraction. (3) It will expand into a red giant, then form a planetary nebula with a white dwarf at its centre. (4) A neutron star or a black hole. (5) Heavier elements were made by nuclear fusion inside the massive star and in the supernova explosion.