Our star and the planets' energy

Curriculum links

Quoted word for word from the official documents.

Step by step

The caption is what the class reads on screen; the talking points are in your notes drawer (N) when you present. Steps marked Pupils only appear only in the pupil lesson.

1. 01

   The Sun's engine

   Presented and for pupils

   Caption

   In the Sun's core, at about 15 million °C, hydrogen fuses to form helium, and that releases the energy that reaches us as sunlight.

   Talking points

   • NASA: the core is the hottest part of the Sun, at about 15 million °C. Nuclear reactions there, where hydrogen is fused to form helium, power the Sun's heat and light.
   • NASA: energy from the core is carried outward by radiation through the radiative zone, taking about 170,000 years to reach the top of the convection zone, where bubbles of hot plasma carry it up to the photosphere, the layer we see.
   • Never look straight at the Sun without proper eye protection (NASA).

   Ask the class

   Where does the energy in sunlight come from?

   1. ANuclear fusion in the Sun's coreRight answer
   2. BBurning gas on the Sun's surface
   3. CLight the Sun collects from other stars

   WhyNASA: nuclear reactions in the core, where hydrogen is fused to form helium, power the Sun's heat and light.

   Sources 1Present from this step →

2. 02

   A star in balance

   Presented and for pupils

   Caption

   Gravity pulls the Sun inwards, the pressure from fusion pushes outwards, and the two are in balance.

   Talking points

   • NASA: the core is hot enough to sustain nuclear fusion, and this creates outward pressure that supports the star's gigantic mass, keeping it from collapsing.
   • AQA 4.8.1.1: fusion reactions lead to an equilibrium between the gravitational collapse of a star and the expansion of a star due to fusion energy.
   • NASA: the Sun is a little less than halfway through its lifetime, with another 5 billion years or so to go before it becomes a white dwarf.

   Ask the class

   Why does the Sun not collapse under its own gravity?

   1. APressure from fusion energy pushes outwards and balances gravityRight answer
   2. BThe Sun has no gravity of its own
   3. CThe planets pull it outwards

   WhyFusion in the core creates an outward pressure that supports the Sun's mass. The outward push and the inward pull of gravity are in equilibrium.

   Sources 1 15Present from this step →

3. 03

   Sunlight at Earth

   Presented and for pupils

   Caption

   The Sun's power spreads out over a growing sphere, and at Earth's distance each square metre facing the Sun gets about 1,361 watts.

   Talking points

   • NASA Sun Fact Sheet: the Sun's luminosity is 382.8 × 10²⁴ J/s, which is 3.828 × 10²⁶ W.
   • NASA Earth Fact Sheet: solar irradiance 1,361.0 W/m². The fact sheet notes define this as the solar energy per square metre at the distance of the planet's semi-major axis, which for Earth is 149.598 million km.
   • That definition makes it the sunlight arriving at Earth's distance from the Sun, in space above the atmosphere, on a surface square on to the Sun.

   Ask the class

   The Sun gives out 3.828 × 10²⁶ W. Earth is 1.496 × 10¹¹ m from the Sun. Divide the power by the area of a sphere of that radius, 4πr², to find the power per square metre at Earth, in W/m².

   Answer1,361 W/m². Answers from 1,351 to 1,371 are marked right.

   WhyArea = 4π × (1.496 × 10¹¹ m)² = 2.812 × 10²³ m². Power per square metre = 3.828 × 10²⁶ ÷ 2.812 × 10²³ = 1,361 W/m², NASA's figure for Earth.

   Sources 2 3 4Present from this step →

4. 04

   Twice as far, a quarter as bright

   Presented and for pupils

   Caption

   Twice as far from the Sun, the same light covers four times the area, so each square metre gets a quarter as much.

   Talking points

   • Edexcel 1AS0 13.9: the inverse square relationship between distance and brightness or intensity.
   • NASA's solar irradiance for each planet, W/m²: Mercury 9,082.7, Venus 2,601.3, Earth 1,361.0, Mars 586.2, Jupiter 50.26, Saturn 14.82, Uranus 3.69, Neptune 1.508. Each is 1,361 ÷ r² (r in au, from the fact sheets) to within about 1%.
   • True scale: the labels mark the bodies.

   Ask the class

   Mars is on average 1.524 au from the Sun. Earth, at 1 au, gets 1,361 W/m². How much does each square metre at Mars get, in W/m²?

   Answer586 W/m². Answers from 582 to 590 are marked right.

   Why1,361 ÷ 1.524² = 1,361 ÷ 2.323 = 586 W/m². NASA's Mars Fact Sheet gives 586.2 W/m².

   Sources 3 6 7 8 9 10 11 12 16Present from this step →

5. 05

   Out at Jupiter

   Presented and for pupils

   Caption

   Jupiter is 5.2 times as far from the Sun as Earth, so its sunlight is spread over 5.2 × 5.2 = 27 times the area.

   Talking points

   • NASA Jupiter Fact Sheet: semi-major axis 5.20336 au, solar irradiance 50.26 W/m². 1,361 ÷ 50.26 = 27.1.

   Ask the class

   Jupiter is 5.2 au from the Sun. Compared with Earth, how much sunlight does each square metre at Jupiter get?

   1. AAbout a fifth as much
   2. BAbout a twenty-seventh as muchRight answer
   3. CAbout a hundred-and-fortieth as much

   Why5.2² = 27, so about 1/27 as much. NASA gives 50.26 W/m² at Jupiter, and 1,361 ÷ 50.26 = 27.1. A hundred and fortieth would be 5.2³, the wrong power.

   Sources 3 9Present from this step →

6. 06

   Why is Venus hotter?

   Presented and for pupils

   Caption

   Venus gets less than a third of Mercury's sunlight, yet it is far hotter, because of what its atmosphere does with the energy.

   Talking points

   • NASA fact sheets: solar irradiance Mercury 9,082.7 W/m², Venus 2,601.3 W/m². Mean temperature Mercury 167 °C, Venus 464 °C.
   • NASA defines the black-body temperature as the surface temperature a body would have in radiative equilibrium with no atmosphere but the same albedo. Mercury's is 439.6 K (about 166 °C), within a degree of its mean of 440 K (167 °C): NASA describes its atmosphere as essentially a vacuum, so the surface itself absorbs and emits. Venus's is 226.6 K (about −47 °C), because its Bond albedo of 0.77 means it reflects 77% of the sunlight that reaches it.
   • NASA: Venus's thick atmosphere traps heat in a runaway greenhouse effect, making it the hottest planet, with a surface about 467 °C.
   • Earth too: its black-body temperature is 254 K (about −19 °C), and NASA says the natural greenhouse effect keeps its average at 15 °C.
   • AQA 4.6.3.2 (HT): a body at constant temperature absorbs radiation at the same rate as it emits it.

   Ask the class

   Venus reflects 77% of the sunlight that reaches it, and still averages 464 °C. Why?

   1. AIts thick atmosphere traps the heat in a runaway greenhouse effectRight answer
   2. BIt is closer to the Sun than Mercury
   3. CNuclear fusion in its core heats it

   WhyWith no atmosphere and the same albedo, Venus would settle at about −47 °C (NASA's black-body temperature). Its thick carbon dioxide atmosphere traps heat, so the surface warms far more before it gets rid of energy as fast as it absorbs it.

   Sources 3 4 5 6 7 13 14 15Present from this step →

7. 07

   Find the hottest planet

   Pupils only

   Caption

   Your turn: find the planet that is hotter than Mercury, and click on it or step to it with the arrows.

   Talking points

   • NASA: Venus, the second planet from the Sun, has the hottest surface in the solar system, hotter even than Mercury.

   Pupil task

   Focus on the planet whose surface is hotter than Mercury's.

   HintIt is the second planet from the Sun, the next one out after Mercury.

   The viewer checks the task as the pupil works and says when it is done.

   Sources 5 13Open this step as a pupil →

8. 08

   Sunlight at Neptune

   Presented and for pupils

   Caption

   At Neptune each square metre gets about 1.5 watts, around 900 times less than at Earth.

   Talking points

   • NASA Neptune Fact Sheet: solar irradiance 1.508 W/m², and 1,361 ÷ 1.508 = 902.5.
   • A good answer: Neptune is about 30 times as far from the Sun as Earth, so the same power is spread over a sphere 30² = 900 times the area.

   Ask the class

   Explain why each square metre at Neptune gets about 900 times less sunlight than at Earth.

   AnswerWritten in the pupil’s own words, up to 400 characters. It is not marked.

   Sources 3 12 16Present from this step →

Sources

Every fact in this lesson comes from these sources, and each step lists the ones it uses.

1. Sun: Facts, NASA Science
2. Sun Fact Sheet, NASA NSSDCA
3. Earth Fact Sheet, NASA NSSDCA
4. Notes on the fact sheet parameters (solar irradiance, Bond albedo, black-body temperature), NASA NSSDCA
5. Planetary Fact Sheet (metric), NASA NSSDCA
6. Mercury Fact Sheet, NASA NSSDCA
7. Venus Fact Sheet, NASA NSSDCA
8. Mars Fact Sheet, NASA NSSDCA
9. Jupiter Fact Sheet, NASA NSSDCA
10. Saturn Fact Sheet, NASA NSSDCA
11. Uranus Fact Sheet, NASA NSSDCA
12. Neptune Fact Sheet, NASA NSSDCA
13. Venus Facts, NASA Science
14. What is the greenhouse effect?, NASA Science
15. AQA GCSE Physics (8463) specification, version 1.1
16. Pearson Edexcel GCSE (9-1) Astronomy (1AS0) specification, issue 3