🧰 You need: pen, pencil, ruler, calculator, coloured pencils if you have them. Time: 45 minutes. Course: Separate Physics only.
Information – read this first
- Our solar system has one star (the Sun), eight planets, dwarf planets such as Pluto, and natural satellites (moons). It is part of the Milky Way galaxy.
- The planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Memory aid: My Very Easy Method Just Speeds Up Naming.
- A star forms from a nebula – a cloud of dust and gas pulled together by gravity. This forms a protostar. When it is hot enough, hydrogen nuclei fuse to make helium and it becomes a star.
- A main sequence star is stable because the inward force of gravity is balanced by the outward pressure from fusion energy.
- A star about the size of the Sun: main sequence → red giant → white dwarf → black dwarf.
- A star much bigger than the Sun: main sequence → red supergiant → supernova (explosion) → neutron star or black hole.
- Elements up to iron are made by fusion in stars. Elements heavier than iron are made in a supernova, which spreads them through the universe.
- Gravity keeps planets and moons in orbit. In a circular orbit the speed stays the same, but the direction keeps changing, so the velocity changes.
What to do
- (2 min) Write the title The Life Cycle of a Star and today’s date. Underline both with a ruler.
- (12 min) Draw the life cycle flowchart:
• At the top, draw a cloudy shape labelled nebula. Arrow down to a small circle labelled protostar. Arrow down to a circle labelled main sequence star.
• From the main sequence star, draw two arrows splitting left and right.
• Left path, headed “About the size of the Sun”: big circle red giant → small circle white dwarf → small dark circle black dwarf.
• Right path, headed “Much bigger than the Sun”: very big circle red supergiant → starburst shape supernova → then split into tiny circle neutron star and black circle black hole.
• Colour the stars if you can (red giants red, white dwarf white or yellow). - (5 min) Next to the main sequence star, draw two arrows: one pointing in, labelled gravity, and one pointing out, labelled pressure from fusion. Write: “The forces are balanced, so the star is stable.”
- (6 min) Draw the solar system: a Sun on the left edge of the page and the eight planets in order in a line. Make Jupiter and Saturn the biggest. Label each planet.
- (5 min) Draw an orbit diagram: a circle for the Earth in the middle and a larger circle for the Moon’s orbit. Put the Moon on the orbit. From the Moon, draw one arrow pointing towards the Earth labelled gravity, and one arrow along the orbit (at a right angle to the first) labelled velocity.
- (12 min) Write the heading Calculations. Copy the worked example, then do Q1–Q4.
- (3 min) Check your answers and correct in a different colour.
Calculations
Use time = distance ÷ speed and speed = distance ÷ time. For a circular orbit, distance = circumference = 2 × π × radius.
Worked example: The Sun is 150 000 000 000 m away. Light travels at 300 000 000 m/s. How long does sunlight take to reach us?
t = 150 000 000 000 ÷ 300 000 000 = 500 s (8 minutes 20 seconds)
Q1 Light from the Moon travels 384 000 000 m to reach Earth. How long does it take?
Show answer
t = 384 000 000 ÷ 300 000 000 = 1.28 s
Q2 The International Space Station orbits at a radius of 6 800 000 m. Calculate the distance it travels in one orbit.
Show answer
2 × π × 6 800 000 = 42 700 000 m (about 42 700 km)
Q3 One orbit takes the ISS 92 minutes. Convert this to seconds, then calculate its speed.
Show answer
92 × 60 = 5520 s. Speed = 42 700 000 ÷ 5520 = about 7700 m/s (7.7 km/s)
Q4 The Earth orbits the Sun at a radius of 150 000 000 km and takes 365 days. Calculate its speed in km/s. (1 day = 86 400 s)
Show answer
Distance = 2 × π × 150 000 000 = 942 000 000 km. Time = 365 × 86 400 = 31 536 000 s. Speed = 942 000 000 ÷ 31 536 000 = about 30 km/s
⭐ Challenge
The Moon moves at a steady speed around the Earth. Explain why it is still accelerating.
Show answer
Velocity has a direction as well as a size. The Moon’s direction is always changing as it goes round, so its velocity is changing. A changing velocity means it is accelerating. The acceleration is caused by the Earth’s gravity, which pulls the Moon towards the centre of its orbit.
