THE RING NEBULA IN LYRA
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
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
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.
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.