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15.4Chapter 15: The Earth in Space (Universe and Stars)

The life cycle of stars — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 6 2 quizzes

📚 Alhussan Physics - Stage 9 Hub / Topical Revision
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Physics 15.4 The life cycle of stars — Nebulae, Main Sequence, Red Giants, White Dwarfs and Supernovae — Alhussan International Academy — Physics (Stages 7–9) notes: Describe what a nebula is and how stars form and die.Light travel time — Light from some objects has taken million…

  • Different ages
  • Stellar nursery
  • Contraction
  • Nuclear fusion
  • Conditions
  • Stable stage

Section 1 of 8Looking back in time

Light travel time
  • Light from some objects has taken millions or billions of years to reach us.
Different ages
  • Some stars we see are very young and others are near the end of their life cycle.

Section 2 of 8Nebulae and star formation

Nebula
Nebula a cloud of dust and gas.
Stellar nursery
  • A nebula in which gravity pulls the gas together to make a star.
Contraction
  • As the ball of gas contracts it heats up until its centre is hot enough for fusion to begin and a star is born.
Example
  • The Eagle Nebula is a stellar nursery and the Orion Nebula can be seen with the naked eye.

Section 3 of 8The main sequence

1Nuclear fusion
  • In the centre of stars hydrogen atoms fuse to make helium and energy is released, which makes the star shine.
2Conditions
  • The temperatures and pressures needed for fusion are found in the centre of stars.
3Elements
  • Helium then fuses to make other elements, and elements up to iron are made in stars like the Sun.
4Stable stage
  • A star spends most of its life, billions of years, on the main sequence, with the inward pull of gravity balanced by outward pressure.
5The Sun
  • It is a main-sequence star with enough fuel to shine for about another 5 billion years.

Section 4 of 8Life cycle of a Sun-like star

1
A nebula of gas and dust is pulled together by gravity.
2
The star becomes a main-sequence star.
3
When the hydrogen is used up, it grows into a red giant, which may swallow Mercury, Venus and possibly Earth.
4
The outer layers are thrown into space as a planetary nebula, which has nothing to do with planets.
5
The centre shrinks to a hot white dwarf.
6
The white dwarf cools to an invisible black dwarf.

Section 5 of 8Life cycle of a massive star

1
A nebula forms a main-sequence star much bigger than the Sun.
2
The star swells into a red supergiant.
3
It explodes as a supernova, one of the most energetic explosions in the Universe.
4
The remains form a neutron star or a black hole.
1Heavy elements
  • Elements heavier than iron are made only in supernova explosions.
2Recycling
  • The remnants flung into space form nebulae that can make new stars and planets, so we are made of stardust.

Section 6 of 8Comparing the two life cycles

StageSun-like starMassive star
BirthNebulaNebula
Long stable stageMain sequenceMain sequence
ExpandingRed giantRed supergiant
EndPlanetary nebula and white dwarf, then black dwarfSupernova, then neutron star or black hole
  • Both
    • Both types of star produce nebulae that can go on to form new stars.

Section 7 of 8Key terms

TermMeaning
NebulaA cloud of dust and gas in space.
Nuclear fusionThe joining of light nuclei, such as hydrogen into helium, releasing energy.
Main sequenceThe long stable stage of a star's life.
Red giantA large cool star formed when a Sun-like star runs out of hydrogen.
SupernovaThe explosion of a massive star.
White dwarfA small hot dense remnant of a Sun-like star.

Section 8 of 8Exam technique

1
Thinking a planetary nebula involves planets — it only looks like a planet.
2
Thinking the Sun will become a black hole — it will become a white dwarf.
3
Thinking all elements are made in the Sun — heavy elements form in supernovae.
4
Thinking stars burn fuel like fire — their energy comes from nuclear fusion.

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