Most astronomers agree that a planetary nebula is the fate of our Sun in approximately 5 billion years. It's still expanding, but we can no longer see it. So after 10,000 to 50,000 years, the planetary nebula stops glowing. As the shell expands, the white dwarf's energy can excite the gas, but only out to a certain distance. Planetary nebulae have short lives, at least visually. This radiation ionizes the gas of the expanding planetary nebula and causes it to glow. It also emits a large amount of ultraviolet radiation. The star (now called the central star of the nebula) is extremely hot, reaching temperatures of 200,000 Kelvin. The ejected envelope becomes a spherical shell of cool, thin matter expanding into space at 5-25 miles/second (10-40 kilometers/second) - a planetary nebula. This leaves a small star called a white dwarf. After about 10,000 years (a short period, astronomically speaking), the inner core of the star collapses and the star pulsates, ejecting much of its outer envelope. Stars about as massive as the Sun eject mass when they become red giants near the end of their lives. Planetary nebulae A planetary nebula is the most common end product of stellar evolution.
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