New View of the Ring Nebula
Planetary nebulae are like cosmic butterflies – they’re all the same thing, yet the variety of colors and forms can hardly be overlooked. The Ring Nebula in the constellation Lyra (also the logo for Redshift 7) is the most well known planetary nebula of all – and probably the most frequently photographed object in the sky. Now an exceptional new photograph joins the endless Ring Nebula gallery.
The photograph is unusual because it is a compilation of six different spectral ranges, and through its great depth shows not only the well-known gas ring, but also the extremely faint halo within the nebula. The photograph was taken through different telescopes in the Calar Alto Observatory in Spain, with a total of 20 hours exposure time. Much can be learned about the nature of planetary nebulae through the great detail in this photograph.
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.
Many, but not all, planetary nebulae are round or have a ring-like form; some are structured completely differently. The variety of shapes comes from the fact that most stars don’t release their layers evenly, but rather in two different directions. The result is a nebula with a bipolar structure. The Hantel nebula in the fox constellation is an impressive example of such a bipolar planetary nebula. The Ring Nebula probably has a bipolar structure too, but since the axis of symmetry points almost directly toward Earth, it appears to us as a ring.
Through an amateur telescope the Ring Nebula is one of the prettiest deep-sky-objects in the heavens. Of course one cannot see it as it looks in the photograph – the outer halo is far too faint for the eye to see and instead of showing its full variety of color the nebula appears as pale, smoky spots. Yet it is for many amateur astronomers a true gem. The oval ring structure can even be seen through small telescopes.
Of course one must first know where to find the Ring Nebula in the night sky. With a planetarium program like Redshift, that’s no problem. The Ring Nebula is located in the small constellation Lyra, whose main star Vega is the brightest member of the so-called summer triangle. It is midway between Sulafat and Sheliak, the lower two corners of the Lyra rectangle. One can recognize it with just a wide angle eyepiece, as it is slightly diffuse compared to the stars. Right now the Ring Nebula is a fascinating object in the early evening sky.
Jan Hattenbach works as a physicist and science journalist in Aachen. Translated by Beth Field.