Spec links: 4.8.1.1 Our solar system (Physics only); 4.8.1.2 The life cycle of a star (Physics only); 4.8.1.3 Orbital motion, natural and artificial satellites (Physics only, with HT content) · Reading time: about 4 minutes
The star that faded
Look at the constellation Orion on a winter night and you will see a bright orange-red star at one shoulder. It is called Betelgeuse. Late in 2019, astronomers noticed it had become much dimmer than usual. News reports asked if it was about to explode. It was not. A cloud of dust it had thrown out was blocking some of its light. But one day, Betelgeuse really will explode. To see why, we need to follow a star’s life from start to finish.
Born in a cloud
Every star begins as a nebula, a huge cloud of dust and gas, mostly hydrogen. Gravity pulls the material together. As it falls inwards it gets hotter, forming a protostar. When the centre is hot and dense enough, hydrogen nuclei start to join together to make helium. This is nuclear fusion, and it releases huge amounts of energy.
The star is now a main sequence star, like our Sun. It is stable because two forces are balanced. Gravity pulls inwards, and the outward pressure from fusion energy pushes outwards. Our Sun is about 4.6 billion years old and is roughly halfway through this stage.
Two different endings
What happens next depends on the star’s mass. When a star about the size of the Sun runs low on hydrogen, it swells into a red giant. Later it throws off its outer layers and its hot core becomes a white dwarf. Over time this cools and fades into a black dwarf.
A star much more massive than the Sun swells into a red supergiant, which is what Betelgeuse is now. It then collapses and explodes in a supernova. What is left becomes a neutron star, or, if there is enough mass, a black hole.
Fusion inside stars makes elements up to iron. Elements heavier than iron are made in supernovae. The explosion scatters all these elements through space, where they can form new stars and planets. The iron in your blood was made inside stars.
Going round in circles
Planets orbit the Sun and moons orbit planets. Artificial satellites orbit the Earth. In each case, gravity provides the force that keeps the object moving in a circle. The speed can stay the same, but the direction keeps changing. So the velocity changes, which means the object is accelerating. For a stable orbit at a smaller radius, a satellite must move faster.
The International Space Station orbits about 400 km above the Earth. In one orbit it travels about 42 500 km, and this takes about 92 minutes. We can find its speed using distance travelled = speed × time, s = v t, so v = s ÷ t:
s = 42 500 km = 42 500 000 m; t = 92 × 60 = 5520 s
v = 42 500 000 ÷ 5520 ≈ 7700 m/s
That is nearly 8 kilometres every second. Astronauts on board see about 16 sunrises a day.
Key words
- Nebula: a cloud of dust and gas from which a star forms.
- Nuclear fusion: the joining of two light nuclei to form a heavier nucleus, releasing energy.
- Main sequence star: a stable star in which the inward force of gravity is balanced by the outward pressure from fusion energy.
- Supernova: the explosion of a massive star at the end of its life.
- White dwarf: the hot, dense core left behind when a star about the size of the Sun throws off its outer layers.
- Satellite: an object that orbits a larger body; moons are natural satellites.
Questions
- What is a nebula? [1 mark]
- Which element is mainly fused in a main sequence star, and which element does it form? [2 marks]
- Where are elements heavier than iron made? [1 mark]
- Explain why a main sequence star stays the same size for billions of years. [2 marks]
- (HT) A satellite moves in a circular orbit at a constant speed. Explain why it is accelerating. [3 marks]
- Light travels at 300 000 000 m/s. The Sun is about 150 000 000 000 m from the Earth. Calculate the time for light to travel from the Sun to the Earth. Give your answer in seconds. [3 marks]
- A navigation satellite travels about 167 000 km in one orbit. One orbit takes 12 hours. Calculate its orbital speed. Give your answer in m/s to 2 significant figures. [4 marks]
- Describe the life cycle of a star much more massive than the Sun, from its formation to the end of its life. [6 marks]
Show answers
- A cloud of dust and gas (from which stars form) (1)
- Hydrogen (1); helium (1)
- In a supernova (1)
- The inward force of gravity (1); is balanced by the outward pressure from fusion energy (1)
- Its direction is continuously changing (1); so its velocity is changing (because velocity has direction) (1); a changing velocity means it is accelerating (1)
- t = s ÷ v (1); = 150 000 000 000 ÷ 300 000 000 (1); = 500 s (1)
- s = 167 000 000 m and t = 12 × 60 × 60 = 43 200 s (1); v = s ÷ t (1); = 167 000 000 ÷ 43 200 = 3866 (1); = 3900 m/s to 2 significant figures (1)
- Indicative content:
• the star forms from a nebula, a cloud of dust and gas
• gravity pulls the dust and gas together, forming a protostar
• the temperature rises until hydrogen nuclei fuse to form helium
• the star becomes a main sequence star
• it is stable because gravity inwards is balanced by outward pressure from fusion energy
• when the hydrogen runs out, the star expands to become a red supergiant
• heavier elements, up to iron, form by fusion
• the star collapses and explodes as a supernova, forming elements heavier than iron and scattering elements through space
• the remains form a neutron star or, if massive enough, a black hole
Level 3 (5–6 marks): all main stages in the correct order with explanation of at least one process, such as fusion or the balance of forces; Level 2 (3–4 marks): most stages in the correct order with limited explanation; Level 1 (1–2 marks): some stages named, possibly in the wrong order.
More reading: See all 30 GCSE Physics comprehension articles