7.16P–7.18PThe life cycle of a star
Physics only — not on the Combined Science specification
A star goes through a life cycle. The life cycle is determined by the size of the star.
You should be able to describe the life cycle of a star for two cases: a star the size of the Sun, and a star much more massive than the Sun.
Fusion processes in stars produce all of the naturally occurring elements. Elements heavier than iron are produced in a supernova. The explosion of a massive star — a supernova — distributes the elements throughout the universe.
You should be able to explain how fusion processes lead to the formation of new elements.
Fusion joins light nuclei to form heavier ones. A main sequence star fuses hydrogen into helium; later in its life it fuses helium into heavier elements, and a massive star continues through progressively heavier elements up to iron. Fusion in a star cannot produce anything heavier than iron, so those elements are formed only in the extreme conditions of a supernova — and the explosion then scatters all of them into space, where they become part of later stars and planets.
Common mistake
Iron is the dividing line. Everything up to iron is made by fusion inside stars; everything heavier than iron is made in a supernova.
7.19PHow we observe the Universe
Physics only — not on the Combined Science specification
Almost everything known about the Universe has arrived as electromagnetic radiation, and the history of astronomy is largely the history of collecting more of it, at more wavelengths.
The naked eye came first, then the optical telescope from the early 17th century, which gathers far more light than an eye can and so reveals fainter and more distant objects. Photography meant an image could be built up over hours rather than glimpsed, and could be measured and shared. Then came telescopes for wavelengths the eye cannot see at all — radio, infrared, ultraviolet, X-ray and gamma — and each one showed something the others could not, from cool dust clouds to the cosmic microwave background.
Some telescopes have to be in space, and there are two reasons, both worth stating:
• The atmosphere absorbs most wavelengths. Ultraviolet, X-rays and much of the infrared never reach the ground at all, so a telescope for them is useless on a mountain and has to be above the air. • The atmosphere blurs and dims what does get through. Moving air makes stars twinkle, which smears fine detail, and light pollution from towns adds a background glow.
Set against that, a space telescope is enormously expensive to launch, hard to repair and has a limited life. Radio waves and visible light do pass through the atmosphere, so telescopes for those can stay on the ground — which is why the largest telescopes of all are built on high, dry mountains rather than launched.
Common mistake
"There is no atmosphere in space" is only half the answer. Say what the atmosphere does that matters — it absorbs whole bands of wavelengths, and it blurs the rest — because the mark is for the consequence, not the fact.
Before you practise
- A star forms when gravity pulls together a cloud of dust and gas — a nebula — into a protostar.
- Fusion begins when the protostar is hot and dense enough, and the star reaches the main sequence.
- It is stable while the outward pressure from fusion balances the inward pull of gravity.
- Sun-sized: main sequence → red giant → white dwarf.
- Much more massive: main sequence → red super giant → supernova → neutron star or black hole.
- Elements heavier than iron are made in a supernova, which then scatters them.
- Some telescopes must be in space because the atmosphere absorbs whole bands of wavelengths and blurs the rest.