The core becomes so hot that it may pulsate (vary in brightness).
At the same time, any remaining hydrogen in regions farther out from the core fuse into helium. Using helium as a fuel, the star's core continues to burn normally for a while, although the star shines less brightly than it did in its expanded state. At the same time, the core shrinks and the star becomes bluer and smaller. Finally, pressure alone forces the atoms to fuse, forming carbon and oxygen. Since the helium initially is not hot enough to undergo fusion (the process by which two atoms combine, releasing a vast amount of energy), it becomes denser and denser. While the star is in a puffed-up state, helium continues to accumulate at its core. The star continues in a variable state until it completely runs out of fuel. It becomes alternately brighter and dimmer, generally spending about one year in each phase. This change marks the start of a dynamic process in which the star develops into a variable star. White dwarf: Cooling, shrunken core remaining after a medium-sized star ceases to burn.
Variable star: Star that varies in brightness over periods of time ranging from hours to years. Neutron star: Extremely dense, compact, neutron-filled remains of a star following a supernova. Nebula: A cloud of interstellar dust and gas. Words to Knowīlack hole: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.įusion: Combining of nuclei of two or more lighter elements into one nucleus of a heavier element the process stars use to produce energy to support themselves against their own gravity. The star may slowly shrink and expand more than once as it evolves into a red giant. Consequently, the outer layers cool down and the color of the star (which is a function of temperature) becomes red. The core's contracting releases gravitational energy into the surrounding regions of the star, causing it to expand. With no hydrogen left at the core of a star to fuel the nuclear reaction that keeps it burning, the core begins to contract. The star takes on a reddish color, which is what gives it its name. In this phase, a star's surface temperature drops to between 3,140 and 6,741 ☏ (1,727 and 3,727 ☌) and its diameter expands to 10 to 1,000 times that of the Sun. An average-sized star like our Sun will spend the final 10 percent of its life as a red giant. A red giant is a star that has exhausted the primary supply of hydrogen fuel at its core.
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