4.3 After the main sequence
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Whatever the mass of a star, sooner or later its supply of hydrogen for nuclear reactions will run out. When this happens, the equilibrium between gravity and energy production in the core will no longer be sustainable and the star will reach the end of its stable main-sequence lifetime. The HR diagram is extremely useful in visualising what happens next; you can view the subsequent evolution of the star as a path on the diagram.
If the energy production simply came to an end, the star would eventually simply collapse under gravity. Before this can happen though, further nuclear reactions come into play, converting helium into heavier elements. This results in a burst of energy that causes the outer layers of the star to expand. As the star becomes larger, the outer layers cool – it becomes redder, moving to the right on the diagram. Although cooler, the larger surface area of the star as it expands means that the total luminosity increases, moving it upwards on the diagram. The large yellow-orange arrow on the diagram illustrates this expansion for a star of one solar mass as it expands to become a red giant.
The helium fusion reactions taking place in this stage of a star’s life are far less efficient than the fusion of hydrogen to helium, and so this phase does not last as long as the main sequence part of a star’s life. Eventually, the helium and heavier elements will also run out and the star follows the blue arrow to the left, first becoming hotter as it starts to collapse and eventually fading as it gets even smaller, moving down the diagram to form a white dwarf at the bottom left.
Along the way, some stars pass through the instability strip mentioned in the video from Section 2, becoming pulsating variable stars. You will look at these and other types of variable stars in more detail next week.