StarsCambridge IGCSE Physics: Revision notes
Section 1
What are galaxies and where is the Sun?
A galaxy is a massive collection of billions of stars held together by gravitational attraction. Our Sun is just one star among billions in the Milky Way, which is our own galaxy. The Sun is relatively close to Earth compared to other stars in the Milky Way, but other stars are vastly more distant. The distances between stars are so enormous that we cannot measure them effectively in kilometres, so astronomers use a different unit called the light-year.
Key fact: Other stars in the Milky Way are much further away from Earth than the Sun is.
Think of a galaxy like a city: the Sun is one streetlight among billions of others spread across the entire city, each incredibly far apart from its neighbours.
Section 2
What is a light-year and how do we measure cosmic distances?
A light-year is a unit of distance, not time. It represents the distance that light travels through the vacuum of space in exactly one year. The value is:
1 light-year = 9.5 × 10¹⁵ m
This enormous number reflects just how vast space is. Light travels at 3 × 10⁸ m/s, so even at this incredible speed, it takes a year to cover a light-year of distance. Using light-years allows astronomers to express astronomical distances in manageable numbers rather than huge powers of ten in metres.
Example: If a star is 4 light-years away, the light we see from it today actually left that star 4 years ago. We are literally looking back in time when we observe distant stars.
Examiners expect you to know and use the value 9.5 × 10¹⁵ m for one light-year. You may need to convert between light-years and metres, or use this value in calculations about stellar distances.
Section 3
How are stars formed from interstellar clouds?
Stars form through a multi-stage process that begins in vast clouds of gas and dust floating between existing stars:
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Initial formation: A protostar begins to form when a region of an interstellar cloud (made mostly of hydrogen) collapses under the influence of gravitational attraction. The cloud is pulled inward by gravity.
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Heating during collapse: As the protostar collapses, gravitational potential energy is converted into thermal (heat) energy. The temperature at the core rises dramatically.
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Reaching stability: Eventually, the protostar becomes a stable star when gravitational attraction pulling inward is balanced by the outward force from the high central temperature (pressure). This equilibrium is crucial — without it, the star would either collapse further or explode outward.
Once this balance is achieved, the star can remain relatively stable for billions of years, fusing hydrogen in its core.
Examiners want to see that you understand the balance between gravity pulling inward and pressure pushing outward. This equilibrium is what defines a stable star and keeps it from collapsing or exploding.
Students often think gravity alone holds a star together. In fact, a star is a dynamic balance—gravity and pressure are constantly opposing each other. Without the outward pressure from heat, gravity would collapse the star.
Section 4
What happens when stars run out of hydrogen?
All stars eventually exhaust their hydrogen fuel supply after billions of years. What happens next depends critically on the mass of the star. Stars follow different evolutionary paths based on their initial mass:
| Star Mass | Final Stage | Remnant |
|---|---|---|
| Less massive stars (like the Sun) | Expand to form a red giant | White dwarf (after shedding outer layers as a planetary nebula) |
| More massive stars | Expand to form a red supergiant | Neutron star or black hole (after supernova explosion) |
Red giants and red supergiants: When hydrogen runs out, the star's core contracts and heats up further. The outer layers expand dramatically, and the star becomes larger and cooler on the outside (appearing red). Red supergiants are much larger and more luminous than red giants.
What causes different outcomes: More massive stars reach higher temperatures and pressures, allowing them to fuse heavier elements beyond hydrogen. Eventually they explode as supernovae. Less massive stars gradually shed their outer layers more gently.
A less massive star ends like a candle—it gradually burns down and fades away. A massive star ends like a firecracker—it explodes violently and dramatically.
Section 5
How do supernovae create new elements and new stars?
Supernovae are catastrophic explosions that occur when massive red supergiants reach the end of their lives. These explosions are crucial to the chemistry of the universe:
The supernova process:
- A red supergiant collapses catastrophically
- The core rebounds, triggering a violent explosion
- Extreme temperatures in the explosion create new heavier elements (iron, nickel, silicon, oxygen, etc.) that did not exist before
- The explosion ejects a vast cloud of gas and dust called a nebula, enriched with these new elements
- This leaves behind either a neutron star or black hole
Creation of new stars and planets: The nebula ejected from a supernova can eventually collapse under gravity to form new stars. Crucially, these new stars form with orbiting planets made from the enriched material. This means:
- New stars contain heavier elements created in the supernova
- Planets around these stars contain the same heavy elements (carbon, oxygen, iron, etc.)
- Life as we know it depends on these heavy elements
This recycling process has occurred many times in the history of our galaxy, meaning the atoms in your body were likely created inside ancient stars and distributed by supernovae.
Examiners expect you to understand the cycle: supernovae create heavy elements, which are distributed in nebulae, which collapse to form new stars with planets. This is a complete cycle showing how the universe builds complexity over time.
Our own solar system formed from a nebula enriched by previous supernovae. The iron in your blood, the calcium in your bones, and the carbon in your cells were all created inside dying massive stars billions of years ago.
Must Know
- Galaxies contain billions of stars; the Sun is in the Milky Way; other stars are much further away
- One light-year = 9.5 × 10¹⁵ m — this is the distance light travels in one year and must be memorised for calculations
- Star formation: Protostars form from collapsing interstellar clouds of hydrogen; they become stable stars when gravitational inward force balances outward pressure from the hot core
- Star endings depend on mass: Less massive stars → red giant → white dwarf (+ planetary nebula); more massive stars → red supergiant → supernova → neutron star or black hole
- Supernovae create heavy elements (iron, carbon, oxygen, etc.) that are ejected in nebulae; these nebulae collapse to form new stars with planets
- The cycle repeats: Supernovae distribute elements → new stars and planets form → these contain the elements needed for life
That's the notes covered.
Carry on to the next subtopic.