Planetary Nebulae
Click icon to view a planetary nebula from Messier’s catalog
>> Messier’s Planetary Nebulae;
The icon shows the Helix Nebula NGC 7293,
the nearest planetary nebula.
When a star like our Sun comes to age, having longly burned away all the
hydrogene to helium in its core in its main sequence phase, and also
(in the consequent red giant stadium), the helium to carbon and oxygene, its
nuclear reactions come to an end in its core, while helium burning goes on in a
shell.
This process makes the star expanding, and causes its outer layers to pulsate
as a long-periodic Mira-type variable, which becomes more and more unstable, and
loses mass in strong stellar winds.
The instability finally causes the ejection of a significant
part of the star’s mass in an expanding shell.
The stellar core remains as an exremely hot, small central star, which emits high
energetic radiation.
The expanding gas shell is excited to shine by the high-energy radiation emitted
from the central star; the material in the shell is moreover accelerated so that
the expansion gets faster by the time. The shining gas shell is then visible as
a planetary nebula. In deep exposures, the matter ejected in the Mira-variable
state can be detected as an extended halo surrounding many planetary nebulae.
The first planetary nebula ever seen by a human was the
Dumbbell Nebula M27 in Vulpecula, which was
discovered by Charles Messier
on July 12, 1764. The comparison to a “fading” planet was first pronounced by
Antoine Darquier, the discoverer of the second of these objects,
the Ring Nebula M57 in Lyra; he found it shortly
before Messier when both were tracing the same comet in January, 1779.
Following were the subsequent discoveries of the
Little Dumbbell Nebula M76 in Perseus in September 1780, and the
in Ursa Major in February 1781 by Pierre Mechain.
These four planetaries are the only ones which found their way into Messier’s
catalog, and all which where known to summer 1782, before
William Herschel started his
comprehensive scanning the of the deep sky with large telescopes.
One of his first findings within this survey was that of another famous
planetary nebula, the
Saturn Nebula NGC 7009 (his H IV.1) in Aquarius, in September 1782.
William Herschel eventually invented the name
“Planetary Nebula” for these objects in his classification of nebulae
in 1784 or 1785, because he found them to resemble the planet newly
discovered by him, Uranus.
On November 13, 1790, Herschel found the planetary nebula NGC 1514
(his H IV.69), which has a very bright central star; thus he became convinced
that the planetary nebulae were nebulous material (gas or dust) associated
with a central star, and not unresolved clusters as he and others had thought
previously.
The radiation emitted by the planetary nebula is remarkable because of its
peculiar spectrum, as was discovered for the planetary nebula
(also known as Cat Eye Nebula, not in Messier’s catalog) by the
English amateur astronomer and pioneer of astronomical spectroscopy,
William Huggins, on August 29, 1864 and published in the
Philosophical Transactions of the Royal Society of 1864
and later in the Nineteenth Century Review of
June 1897 (according to Hynes):
As expected for gaseous emission nebulae, the spectra of planetaries consist
of emission lines, but 90 to 95 % of the visible light are emitted in one
single emission line only ! This `Chief Nebular Line’ occurs at
500.7 nm (5007 Angstrom), in the green part of the spectrum.
It is this circumstance that planetary nebula brightnesses differ
significantly if determined with various methods: These objects are often
considerably brighter (up to 2 magnitudes, a factor of more than 6) visually
than photographically, because the 5007 Angstrom line lies close to the
highest sensitivity of the human eye. Also, as films are often less sensitive
in the green part of the spectrum, it is difficult to get a good “true color”
image of planetary nebulae.
As this spectral line at 5007 Angstrom could not be assigned to a known
element at the time of its discovery, Huggins suspected it must be emitted
from a previously unknown substance, which was called “nebulium”.
It was not before 60 years later that the “nebulium” spectrum was identified
(by the American astro-physicist Ira S. Bowen) to be caused by
forbidden lines of double ionized “normal” oxygene, “[O III]”
(with the square brackets).
Besides the “nebulium” [OIII] lines, other emission lines occur in the planetary
nebula spectra in weaker intensity. These include more forbidden lines of
ionized oxygene, neon, nitrogene, and other abundant elements, as well as
permitted lines of hydrogene and helium, as well as fluorescence O III lines
in case of strong He II emission. Also, a very week continuum underlies the
line spectrum, which is due to interactions of electrons with ions.
Our Sun will probably reach this state of evolution at an age of about 10-13
billion years; as it is now only about 4.7 billion years old, we have
probably some time left until this event happens.
The planetary nebula has only a short life compared to the time scales in
stellar evolution, being visible only a few thousands or 10,000s of years,
and then fading out as its matter is spread in the cosmic environment,
enriching the interstellar matter with carbon, oxygene, and other elements.
Its central star cools down to a white dwarf.
This is the reason that, although there are very many sunlike stars among
the hundreds of billions in our Milky Way galaxy,
which now come into age (especially in the globular clusters), there are only
about 10,000 planetary nebula (of which only about 1,500 could yet be
detected, the other being hidden behind obscuring interstellar dust);
of the 150 globular clusters with each several
100,000 stars, planetary nebulae have been discovered only in 4
(or perhaps 5) of them, namely Pease 1
in M15 (which may even contain a second one
according to Peterson, 1976, but this one was never confirmed since),
M22,
the probable member Peterson 1 lying 3 arc minutes from globular cluster
NGC 6401 (Peterson 1977),
the recently discovered planetary in NGC 6441
(Jacoby and Fullerton 1997; also see
George Jacoby’s Planetary Nebula gallery), and
a recently found planetary nebula in the faint globular cluster Palomar 6.
As planetary nebulae occur only late in the life of a star, they are usually
absent in open star clusters, because these stellar
swarms tend to dissolve in times much shorter than that needed for a star to
evolve in a planetary nebula:
Only few open clusters live longer than a billion years, while planetary
nebulae occur only for stars of less than 3 solar masses (the more massive
explode as supernovae). Those low-mass stars however have considerably more
than 1 billion years of lifetime on the mean sequence alone while they burn
up their hydrogene.
These arguments are however questionable, as a number of white dwarf stars
has been discovered in young clusters, as
the Pleiades, M45: These stars must have started
their life with a high mass so that they evolved rapidly, but lost a
significant portion of their mass during their lifes, probably in the form
of strong stellar winds, and must have gone through a planetary nebula stage.
It seems that because of the short lifetime of this stage, there is only one
planetary nebula, NGC 2818, which was discovered to be a member of an
inconspicuous, rather old open cluster, NGC 2818A. The more wellknown case
of the planetary nebula NGC 2438 which is
observed in the same direction as M46 is apparently
a chance alignment.
The cooling process of the white dwarf goes on until all thermal energy is
radiated, and the star approaches a stable “end state” as “black dwarf” after
many billion years – the universe is probably still much too young to contain
any “cooled-out” black dwarf.
Planetary nebula are often typized for their appearance, according to the
Vorontsov-Velyaminov scheme:
1 Stellar Image 2 Smooth disk (a, brighter toward center; b, uniform brightness;
c, traces of a ring structure)
3 Irregular disk (a, very irregular brightness distribution; b, traces of ring structure)
4 Ring structure 5 Irregular form, similar to a diffuse nebula 6 Anomalous form
More complex structures are characterized by combinations such as
“4+2” (ring and disk), or “4+4” (two rings).
All individual planetary nebulae mentioned in this page, including the four
Messier objects, are members of our
Planetary nebulae have also been discovered in other galaxies with large
telescopes, including the Large and the
NGC 6822 as well as other galaxies in the
Local Group and beyond.
It is very probable that they are common in all galaxies.
Messier‘s Planetary Nebulae:
M27,
M57,
M76,
M97.
Links
Find beautiful image collections of planetary nebulae, including many which
are not in Messier’s catalogs, at the following sites:
-
Bruce Balick’s Planetary Nebulae page
- Bill Arnett‘s collection of
planetary nebula images by Bruce Balick,
index or as
- Planetary Nebulae Observer’s Homepage
by Doug Snyder; includes an extensive list of
links to Planetary Nebulae Web Resources
-
Planetary Nebulae at University of Oregon
(Galaxy Gallery Page III) - Planetary Nebula
gallery
by George Jacoby (KPNO); -
C.Y. Zhang’s Planetary Nebulae Images
(H Alpha, O III and N IIimages compared to models)
-
Michael Horn’s Planetary Nebula Gallery
- Planetary Nebulae Catalogs List
-
The Abell Planetary Nebulae
presented by -
ARVAL Catalog of Bright Planetary Nebulae
- (dead link!)
- Exact Positions of Planetary Nebulae
by Brian Skiff
- Look at Planetary Nebulae in Messier’s Catalog
- Also look at our small collection
of significant non-Messier Planetary Nebulae
(notify me if you know further cool pages)
References
- Hynes, Steven J.,
Planetary Nebulae, Willmann-Bell, Inc., 1991.
Last Modification: 13 May 1999, 10:00 MET