New View of the Ring Nebula
Another somewhat romantic but still accurate term for a planetary nebula is star cemetery – the “final resting place” of a sun-like, mid-sized star. A star does not “live” forever because the hydrogen in its center is not unlimited, and it is through the fusion of that hydrogen with helium that the star gets its energy. And the consumption is enormous: our sun, for example, uses 560 million tons of hydrogen per second. It has sufficient fuel to last at least another four billion years. Then the dying sun will blow up to become a red giant, releasing its outer layer of hydrogen through violent stellar winds. In the case of the Ring Nebula, these very thin gas clouds are visible as an expanded, almost circular outer halo in the light of the molecular hydrogen.
Later, in the midst of its death-throes, the star releases the rest of its outer gas layer. Only now does the bright and therefore easily visible planetary nebula emerge. Along with hydrogen and helium, it contains stronger elements that came from inside the star. Through a sort of cosmic recycling, these materials may at some point create new stars, planets and perhaps even living beings. The fact that we humans are not composed only of hydrogen and helium proves that the atomic nuclei that make up our bodies were once inside a star.
Finally what is left of the star crashes in on itself and shrinks to about the size of Earth – but is still about the same mass as the Sun. Such a white dwarf is therefore very compact and has an enormous material density. During the shrinking process, the temperature rises more then ten thousand degrees, leaving the surface of the white dwarf in the Ring Nebula at more than 70,000 degrees Celsius (126,000 Fahrenheit). The hot dwarf star in the middle causes the surrounding nebula to light up: without its light, the nebula would be nearly invisible. The Ring Nebula’s central star is seen exactly in the middle of the photograph as a faint star with a magnitude of barely 16.
The Ring Nebula and its central star allow us to guess the destiny of our own solar system in the distant future. But even this phase will not last forever: over time the white dwarf star cools and the gases dissipate into the universe – the planetary nebula fades. Only recently have astronomers discovered a giant, faded planetary nebula that had previously escaped telescopic observation.