The Death of a Star
Stars don't last forever. When a star's core begins to run out of hydrogen, gravity gains the upper hand over pressure and the core starts to shrink. Soon, the core temperature rises enough to cause the hydrogen in a shell outside the core to begin fusion. The energy flowing outward increases, causing the outer regions of the star to expand. The expanding atmosphere moves farther from the hot core and cools to red. The star becomes a red giant. Eventually, the collapsing core will grow hot enough for helium fusion to occur, producing carbon, oxygen, and heavier elements. In helium fusion, the star stabilizes and its outer layers shrink and warm up. In this period, the star remains in the upper right part of the H-R Diagram. 
The dwindling supply of fuel in a star's core ultimately leads to the star's death as a white dwarf, neutron star, or black hole. As Figure 17 shows, the final stages of a star's life depend on its mass.
Low-and Medium-Mass Stars
Low-mass and mediummass stars, which can be as much as eight times as massive as the sun, eventually turn into white dwarfs. Such stars remain in the giant stage until their hydrogen and helium supplies dwindle and there are no other elements to fuse. Then the energy coming from the star's interior decreases. With less outward pressure to support the star against gravity's inward pull, the star collapses. The dying star is surrounded by a glowing cloud of gas. Such a cloud is called a planetary nebula, because the first ones found looked like planets when viewed through a small telescope. Figure 18A shows a planetary nebula.
Figure 17 The mass of a star determines the path of its evolution.
Interpreting Diagrams What are two possible end-stages of a high-mass star?
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