A star like the Sun passes through the following stages, each showing as a movement on the HR diagram.
1. Formation. A cloud of gas and dust (a nebula) collapses under gravity. Gravitational potential energy is transferred to thermal energy and a hot, dense protostar forms.2. Main sequence. When the core is hot enough (about 107 K), hydrogen nuclei fuse into helium. Outward radiation and gas pressure balance the inward gravitational force, so the star is stable, in equilibrium. The Sun will stay on the main sequence for about 1010 years.3. Red giant. When hydrogen in the core runs out, the core contracts and heats up. Hydrogen fusion continues in a shell around the core, and the outer layers expand and cool. The star becomes much larger and brighter but cooler, so it moves up and to the right on the HR diagram. The core becomes hot enough for helium to fuse into carbon and oxygen.4. Planetary nebula. When helium fusion ends, the outer layers are thrown off into space as a glowing shell of gas.5. White dwarf. The hot, dense core remains as a white dwarf, about the size of the Earth. No fusion takes place; it slowly cools and dims. It moves to the bottom left of the HR diagram.Much more massive stars fuse heavier elements, become red supergiants and end in a supernova, leaving a neutron star or black hole.
1. Formation. A cloud of gas and dust (a nebula) collapses under gravity. Gravitational potential energy is transferred to thermal energy and a hot, dense protostar forms.2. Main sequence. When the core is hot enough (about 107 K), hydrogen nuclei fuse into helium. Outward radiation and gas pressure balance the inward gravitational force, so the star is stable, in equilibrium. The Sun will stay on the main sequence for about 1010 years.3. Red giant. When hydrogen in the core runs out, the core contracts and heats up. Hydrogen fusion continues in a shell around the core, and the outer layers expand and cool. The star becomes much larger and brighter but cooler, so it moves up and to the right on the HR diagram. The core becomes hot enough for helium to fuse into carbon and oxygen.4. Planetary nebula. When helium fusion ends, the outer layers are thrown off into space as a glowing shell of gas.5. White dwarf. The hot, dense core remains as a white dwarf, about the size of the Earth. No fusion takes place; it slowly cools and dims. It moves to the bottom left of the HR diagram.Much more massive stars fuse heavier elements, become red supergiants and end in a supernova, leaving a neutron star or black hole.