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Cambridge IGCSE Physics · 0625

Chapter 6: Space physics — Part 4

Topic 6.2.2 · Stars

Galaxies and light-years

A galaxy is made of many billions of stars. The Sun is a star in the Milky Way. Other stars in the Milky Way are much further from Earth than the Sun is.

Astronomical distances can be measured in light-years. One light-year is the distance light travels in vacuum in one year.

1 light-year = 9.5 × 1015 m.

Scale sketch: the Sun as one star in the Milky Way, other stars much further from Earth than the Sun, with a light-year defined as the distance light travels in one year (9.5 × 10¹⁵ m).
Diagram 1: Scale sketch: the Sun as one star in the Milky Way, other stars much further from Earth than the Sun, with a light-year defined as the distance light travels in one year (9.5 × 10¹⁵ m).

Life cycle of a star

Learn this sequence:

  • A star forms from interstellar clouds of gas and dust that contain hydrogen.
  • A protostar is a cloud collapsing and heating because of its own gravity.
  • It becomes a stable star when inward gravity is balanced by outward pressure from the high temperature in the centre.
  • All stars eventually run out of hydrogen as fuel.
  • Most stars expand to red giants; more massive stars become red supergiants when most hydrogen in the centre has become helium.
  • A red giant from a less massive star forms a planetary nebula with a white dwarf at the centre.
  • A red supergiant explodes as a supernova, forming a nebula of hydrogen and new heavier elements, leaving a neutron star or a black hole.
  • A supernova nebula may form new stars with orbiting planets.
Life cycle of a star: gas cloud → protostar → stable star → red giant or red supergiant → planetary nebula + white dwarf, or supernova → nebula + neutron star or black hole.
Diagram 2: Life cycle of a star: gas cloud → protostar → stable star → red giant or red supergiant → planetary nebula + white dwarf, or supernova → nebula + neutron star or black hole.

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