What you’ll learn3 learning objectivesChoose one objective for a focused lesson, or study the complete topic.6.2.2.1(a) galaxies contain many billions of• State: (a) galaxies contain many billions of stars (b) the Sun is in the Milky Way (c) other Milky Way stars are much farther from Earth than the Sun (d) astronomical distances can use light-years: distance travelled by light in space in one yearSyllabus objective6.2.2.2One light-year is equal to 9.5 × 1015• Know: one light-year is equal to 9.5 × 1015 mSyllabus objective6.2.2.3Star life cycle: (a) forms from• Describe a star life cycle: (a) forms from hydrogen-rich gas/dust clouds (b) protostar collapses under gravity and heats up (c) stable star forms when inward gravity balances outward pressure from high core temperature (d) stars eventually run out of hydrogen fuel (e) most expand to red giants; more massive stars become red supergiants after central hydrogen becomes helium (f) less massive red giant forms a planetary nebula with a white dwarf at the centre (g) red supergiant supernova forms a nebula with hydrogen/new heavier elements and leaves a neutron star or black hole (h) supernova nebula may form new stars with planetsSyllabus objective