The Solar System & Lifecycles of Stars Notes

Edexcel GCSE Physics: Revision notes

Key facts

  • The Solar System is the Sun, eight planets and their moons, dwarf planets, asteroids and comets.
  • In a circular orbit gravity provides the centripetal force; speed is constant but velocity changes.
  • A star is stable while gravity (inwards) balances thermal expansion from fusion (outwards).
  • A star like the Sun goes nebula, main sequence, red giant, white dwarf.
  • A much more massive star goes nebula, main sequence, red supergiant, supernova, then neutron star or black hole.
  1. 1

    Mercury

  2. 2

    Venus

  3. 3

    Earth

  4. 4

    Mars

  5. 5

    Jupiter

  6. 6

    Saturn

  7. 7

    Uranus

  8. 8

    Neptune

Planets in order from the Sun

What is in our Solar System?

The Sun is at the centre, with eight planets, moons, dwarf planets, asteroids and comets orbiting it.

The Solar System contains the Sun, eight planets with their natural satellites (moons), dwarf planets, asteroids and comets.

Planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

  1. 1

    Mercury

    closest to the Sun

  2. 2

    Venus

  3. 3

    Earth

  4. 4

    Mars

  5. 5

    Jupiter

  6. 6

    Saturn

  7. 7

    Uranus

  8. 8

    Neptune

    furthest from the Sun

Planets in order from the Sun

Geocentric model

  • Earth at the centre
  • Sun and planets orbit Earth
  • Early idea

Heliocentric model

  • Sun at the centre
  • Planets, including Earth, orbit the Sun
  • Supported by observational evidence

In the heliocentric model, what is at the centre?

How orbits work

In a circular orbit, gravity is the centripetal force: it changes the direction of velocity but not the speed.

Moons, planets, comets and artificial satellites move in orbits held by gravity.

In a circular orbit, gravity provides a centripetal force towards the centre. It changes the direction of the velocity, not the speed.

For a stable orbit, a faster orbital speed needs a smaller radius, and a slower speed a larger radius. Weight differs between bodies because the gravitational field strength gg depends on a body's mass and radius; mass stays the same.

gravityOP
Circular orbit: gravity always points to the centre

A satellite orbits the Earth at constant speed. Is its velocity constant?

Stars like the Sun

A Sun-like star runs from nebula to main sequence to red giant to white dwarf, held steady by a balance of gravity and fusion.

Gravity pulls matter inwards; thermal expansion from the energy of fusion pushes outwards. While these balance, the star is stable on the main sequence. When fusion can no longer balance gravity, the star changes stage.

  1. 1

    Nebula

    Dust and gas pulled together by gravity.

  2. 2

    Main sequence star

    Hydrogen fuses into helium; stable for billions of years.

  3. 3

    Red giant

    Hydrogen runs low; the star expands, cools and glows red.

  4. 4

    White dwarf

    Outer layers shed; a small, dense, hot core cools.

Life cycle of a star like the Sun

What two effects balance in a stable main sequence star?

Massive stars

Massive stars live shorter lives and end in a supernova, leaving a neutron star or a black hole.

Stars much more massive than the Sun fuse fuel faster, so their lives are shorter. They end in a violent explosion rather than gently shedding their layers.

After a supernova, the remnant becomes a neutron star, or a black hole if the remaining mass is large enough.

  1. 1

    Nebula

  2. 2

    Main sequence star

  3. 3

    Red supergiant

    Expands even larger than a red giant.

  4. 4

    Supernova

    Core collapses rapidly in a massive explosion.

  5. 5

    Neutron star or black hole

    Black hole if enough mass remains.

Life cycle of a much more massive star

Which stage follows a red supergiant?

Observing the Universe

Telescopes in space avoid the atmosphere, which absorbs or distorts much of the radiation we want to study.

Early astronomy used the naked eye, then optical telescopes on Earth. Modern telescopes detect radio, infrared, ultraviolet, X-rays and gamma rays, and some are placed outside the atmosphere.

  1. 1609

    Galileo

    First use of an optical telescope for astronomy.

  2. 1932

    Karl Jansky

    Detects radio waves from space, starting radio astronomy.

  3. 1990

    Hubble Space Telescope

    Launched above the atmosphere for clearer images.

Telescope on Earth

  • Atmosphere absorbs many wavelengths (X-rays, ultraviolet, infrared)
  • Atmosphere distorts visible light ("twinkling")

Telescope in space

  • Above the atmosphere
  • Receives clearer, fuller signals

Why are some telescopes placed in space?

Try an exam question

Describe the stages in the life cycle of a star like the Sun.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.