NGC 6720



See the color spectrum.

From Jim Kaler's STARS; Return to Planetary Nebulae

PO Ring Ring M 57
The Ring Nebula in Lyra is perhaps the classic planetary nebula. Curtis calls it "well known and remarkably complex." The term, which simply means "disk-like," comes from William Herschel. The Ring Nebula was found in 1779 before Herschel announced his discovery of the first of his "planetary nebulae" (NGC 7009) in 1785, and was added to the class later.

Planetary nebulae are the compressed ejecta of dying stars as they turn from giants into white dwarfs. The photograph on the left gives a good sense of how the Ring looks in a small telescope (minus the central star, which is quite difficult to see). The image in the middle is Curtis's composite drawing made from several photographs, while that on the right is the Hubble view. The Ring is easily found between Beta and Gamma Lyrae.

The distance of the Ring Nebula is measured by direct parallax to be 2300 light years away (accurate to about 40 percent). The angular dimensions of this elliptical object of 86 X 62 seconds of arc (a "second" 1/3600 of a degree) translate to true dimensions of 0.95 X 0.7 light years. The long axis would therefore stretch 20 percent of the way from the Sun to Alpha Centauri. The nebula, expanding at a rate of about 30 kilometers per second, is illuminated by the ultraviolet light of the 16th magnitude (15.7) star at the center, which is now a cooling, but still very hot, nascent white dwarf with a temperature of about 150,000 Kelvin and a luminosity some 500 times that of the Sun. It looks faint only because most of its light is radiated in the ultraviolet. Outer shells produced by mass loss in the giant star that created the nebula extend out almost twice as far as seen here, making the whole system nearly two light years across.

On the right is the spectacular Hubble view. The layered colors reveal radiation from different chemical elements in different stages of ionization, blue from ionized helium, yellow-green from doubly-ionized oxygen, red from ionized nitrogen (see the spectrum below). The central star is barely visible at the center. We might be looking down the mouth of a barrel, or more likely the throat of an hour glass. The structure may be similar to that of the Dumbbell Nebula, just seen from a different perspective.



As discovered by Sir William Huggins in 1864 when he examined NGC 6543 with his spectroscope (visually; there was no photography), planetary nebulae radiate emission line spectra. Each kind of atom or ion will radiate at one or more particular wavelengths depending upon the atomic structure. In the spectrogram above, the light from the Ring Nebula has been passed into a spectrograph that has no spatially defining slit or aperture. Each emission line therefore produces a picture of the nebula in the light of its associated atom or ion. The above slitless spectrogram runs nearly the entire length of the spectrum visible to the human eye. Each emission is labelled with its ion and the emission wavelength in Angstroms. The major problem with slitless spectrograms is that the atomic/ionic images overlap one another. A longer exposure, which would require a defining slit or small aperture to isolate a portion of the nebula, would reveal with clarity many more lines, as illustrated by the modern digital spectrum of BV-1.

At far right is a blend of the Hydrogen-Alpha line (the first and strongest line of the hydrogen Balmer series) and a pair of forbidden lines (indicated by brackets) of singly ionized nitrogen, [N II]. They are clearly separated in the spectrum of BV-1, which used a narrow slit to admit the light into the spectrograph. Forbidden lines are not really "forbidden," just unlikely in a laboratory setting; in the low density nebulae they can reach great strength. Going from right to left, we see the next three members of the Balmer series, Hydrogen Beta, Gamma, and Delta, the latter glowing faintly in the far violet. The brightest emissions are the forbidden lines of doubly ionized oxygen ([O III]) at 5007 and 4959 Angstroms. These, along with hydrogen-beta, were the emissions seen by Huggins that proved nebulae to be gaseous. The oxygen lines were not actually identified as such until 1928. Ions are produced when atoms are hit with the energetic light from the central star, which then creates a sea of free ejected electrons. The lines of hydrogen are produced by the recombination of electrons with hydrogen ions (protons), those of He I by the recombination of electrons with ionized helium ions, those of He II by the recombination of electrons with doubly ionized helium ions. Forbidden lines are created by the collisions of electrons with atoms or ions.

As seen in the Hubble image, different lines have different structures. The blue He II line (from ionized helium) at 4686 Angstroms concentrates around the hot central star, where energies are highest, whereas the neutral helium line at 5876 Angstroms, and especially the red neutral oxygen [O I] lines at 6363 and 6300 Angstroms, which take much less energy to produce, are formed in an outer ring far from the star where stellar energies are lower. The effect, called stratification, is also seen in the Hubble image. The spectrum extends to the right into the infrared, and to the left into the ultraviolet. The horizontal streaks are the spectra of unrelated stars that happen to fall within the field of view.

The spectrum provides the means for the calculation of nebular parameters and chemical composition. The temperature of the radiating gas is sensitive to the ratio of the strength of the [N II] line at 5754 Angstroms to the strengths of those near H- Alpha. We can also use the strength of the 4363 line of [O III] (buried in H-Gamma and unlabelled, but seen in the spectrum of BV-1) to the strengths of the oxygen lines near H-Beta. Other line ratios (for example that of the [O II] lines at 3726 and 3729 Angstroms, which are blended even for BV-1) are sensitive to density. Atomic theory applied to the strength-ratios of lines from different ions then leads to the abundances of various elements relative to hydrogen. Planetary nebulae are commonly enriched in helium, nitrogen, and carbon as a result of nuclear processes that took place in the parent evolving star, and thus provide a means for testing theories of stellar evolution and element creation.

See the spectrum at full resolution.

Left image: University of Illinois Prairie Observatory. Middle image and quote by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right image: The Hubble Heritage Team (AURA/STScI/NASA). Spectrum: Y. Norimoto, Okayama Astrophysical Observatory, NAOJ.