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10/21/2005
I have attempted to make available on this site a mixture of wonderful images from amateurs, professional astronomers, and observatories both large and small. There are over 200 images on line, and with my CCD imaging setup, I am adding a few more as time and sky permits. They won't be great works of art like some of the images to be found here, but I do hope they will help in answering someone's question. I have combined two data bases into a single one with data on 1,143 planetaries. The data base is linked to the 1200+ observing reports contributed by expert observers like Steve Gottlieb, Kent Wallace, Rich Jakiel, Yann Pothier and many others. There are many gaps of course, but that's part of the challenge - fill in that missing information.
There are pages on Seasonal Best Planetaries- a page for each season with 25 planetaries, and contained within these 25 are objects that are easy, difficult, and almost impossible. More challenges, if you are looking for objects that may defy eye and telescope. I have created 10 pages on 'featured' planetaries - each page devoted to one planetary with images, data, a finder chart, and other information.
Are you looking for observing reports - thanks to observers like Steve Gottlieb and Kent Wallace, two of the best and most prolific deep sky observers, there are over 1200 individual reports on file, and most are associated (hyper-linked) with an entry in the SEC database. This database contains all of the 1,143 planetaries in the Strasbourgh-ESO Galactic Planetary Nebulae Catalog (SEC).
I have information on nebula filters, observing tips, and a page of astronomy links that contains both links to other planetary nebula sites, and great general astronomy sites. Check it out. To add variety, there are pages on 'astro' poetry, and a large selection of MIDI files for your listening pleasure, which you can easily mute if you don't want to listen.
Wow, there is a 'ton' of stuff here, and I hope it helps you in some small way - if you can't locate something, or find errors, or feel something should be added, DO NOT hesitate to let me know. I aim to please!
I have added a desperately needed Search Page - I'm trying to find a better one that will adequately fit the bill and find items that anyone is looking for, including myself.
What follows is a very simplistic explanation of how these objects are
formed and how they evolve. If you are interested in learning more about the concepts of
planetaries, please call up my Internet Links page where I have
connections to several outstanding web sites created by professional astronomers presently
researching all aspects of PNes. In the meantime, I do hope that my explanations
suffice. If not, let me know!
Our Sun, as a quite ordinary star in the cosmos, will (hopefully) remain a stable hydrogen
burning beacon for the next several billion years as it has been for the past several
billion years (roughly 4,500,000,000 years). At a certain stage of our stars life,
approximately after 10 billion years, when the core hydrogen is almost depleted and has
been converted to helium, the star will then start to vigorously consume this repository
of helium. However, this conversion of helium to heavier elements (such as
Nitrogen [atomic no.7], Oxygen[8], and Neon[10] ) at higher temperatures and subsequently
greater energy levels at or near the core of the star cause the star to inflate to many
times it normal size. During this stage of its life, the star is considered a Red
Giant. It is believed that our Sun, during its Red Giant phase, will extend in size beyond
Earth's orbit! As the star is so immense during this phase, it must eventually reach
a stage where the gravitational and expansion forces cannot support the given star mass,
volume, and temperature conditions. At this point, Red Giants begin to expel their
outer layers of hydrogen gas into the interstellar medium, and this process may continue
for many thousands of years. What we observe as 'planetary
nebula' are the hydrogen shells expelled from these stars. The stars temperature
rises dramatically and most of its energy release is in the form of ultraviolet radiation,
which is the major contributor in 'illuminating' the nebula shells of the star.
Consider that the Sun's surface temperature is presently around 5,000 degrees
Kelvin. Some central stars of planetary nebula have been measured greater than
100,000 degrees Kelvin and are classified as White Dwarfs. The Kelvin temperature scale
has its zero degree point at -273 degrees Celsius, which is absolute zero, so 100 °
Celsius = 373° K.
It is being increasingly determined that a good number of planetaries are suspected binary
systems, in which one star has reached the end of its main sequence life, and as it
evolves through the early stages of helium consumption, the hydrogen gas shells cast off
are greatly influenced by the orbiting companion. Professional astronomers are
finding vastly different shapes and structures in distant
planetary gas shells which do not fit the standard model for planetaries. An
orbiting companion star, with its gravitational affects on both the dying star and the
ejected gas, can be modeled to fit these conditions.
Planetary nebulae (PN or PNe), although unique as telescopic astronomical objects,
can be found to occur in a variety of shapes, visibilities, brightnesses, sizes,
structure and central star visibility. Using a typical amateur telescope (4
inch and larger), there are planetaries quite visible from light polluted cities, and
planetaries that can usually only be found by utilizing nebula filters or crafty observing
techniques. The largest planetary in the night sky, the Helix Nebula in
Aquarius (NGC 7293) extends about 15’ (arc minutes) across the night sky, but is
certainly not the easiest to view due to its low surface brightness of approximately
magnitude 13, in spite of a photographic magnitude of approximately 6.3. In
contrast, there are PN’s that are smaller than 10" (arc seconds) in which
detection of the nebulosity and the central star are possible in amateur telescopes.
Planetaries are one of the few deep sky objects in which visual colors can be detected
through the eyepiece when the surface brightness of the object exceeds roughly 8
magnitudes per square arc minute. For example, NGC 3242, the Ghost of Jupiter
planetary, located in Hydra, has a surface brightness of about 7.7, and has a distinct
bluish glow. There are quite a number of planetaries scattered around the sky in
which color is discernable and adds to the mystique of these heavenly bodies.
The term "planetary nebula" is attributed to the astronomer
William Herschel (1738-1822), the discoverer of the planet Uranus and perhaps the greatest
visual astronomer of all time. His observations between 1777 and 1802 amounted to
over 2,500 objects being catalogued, including numerous planetaries. He created this
term and this class of object after observing numerous astronomical bodies in which their
appearance resembled the greenish disk of the planet he had discovered in 1781.
There was some prior reference to ‘nebula’ and ‘planet’ in an
earlier discovery made by a French astronomer, Antoine Darquier de Pellepoix. In
1779, this astronomer described what is now known as M57, the Ring Nebula, as "A very
dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading
planet." The first discovery of a PN was made by Charles Messier in 1764,
and is now known as M27, the Dumbbell Nebula. In fact, the present Messier list
includes four planetaries, the additional two being M76, the Little Dumbbell Nebula, and
M97, the Owl Nebula. At present, included in a multitude of different catalogs,
there are approximately 1,500 known planetaries, and some astronomers theorize that there
may be upwards of 10,000 within the boundaries of our galaxy, but most remain
obscured by interstellar gas and dust. Planetaries are most commonly found along the
galactic plane, and summer viewing for the northern hemisphere astronomer provides a
plethora of tantalizing PN’s. But many interesting ones are also to be
found away from the clutter of the Milky Way, so pleasurable planetary nebulae
hunting and observing can certainly be undertaken any part of the year.
Similarities to HII regions
Although in a few particular cases there is some uncertainty about whether a particular object is a planetary nebula or an HII region there are a number of general observational differences:
(a) Planetary nebulae are usually much more symmetric in
appearance than HII regions.
(b) They occur isolated from each other and from other interstellar
clouds and star clusters.
(c) They have a much smaller mass of gas (about 0.2 solar
mass) than most HII regions.
The optical spectrum of a planetary nebula is similar to that of an HII region in that it consists mainly of bright emission lines such as the Balmer series of hydrogen and the forbidden lines of ionized oxygen, nitrogen, and other such elements. The central star of a planetary is generally hotter than that of an HII region, with a temperature in the range 50,000 - 100,000 K rather than 25,000 - 50,000 K; the higher stellar temperatures mean that many more highly excited ions, such as Ne II and in particular, He II are seen in the ionized shell.
The radio and infrared properties of planetary nebula are rather like those of H II regions. Free-free emission produces a radio-wavelength continuum, while dust grains absorb ultraviolet photons from the star and from the nebula, and re-emit the energy as infrared radiation. These dust grains appear to have a different chemical composition to normal interstellar grains, but their concentration and method of formation is uncertain. Emission from carbon monoxide molecules has been detected from some planetary nebula; this result indicates that there must be molecular hydrogen associated with at least some planetary nebulae, but we do not yet know whether this neutral gas exists outside the ionized region or in small, dense globules within it.
Expansion of the shell
Almost all planetary nebulae have a remarkably symmetric structure, often
in the shape of a ring, but sometimes more like an hourglass. The differences in
shape are presumably mainly due to variations in the way the gas shell was initially
expelled from the star. The speed of rotation of the star, its radiation pressure
and its magnetic field are probably also very important, though as yet no adequate theory
exists for the dynamics of PN shells.
There are two pieces of evidence to show that planetary nebulae are
expanding away from their central stars. Firstly, photographs of the Ring Nebula
taken 40 years apart show that it is slowly increasing its diameter. Secondly, the
Doppler shifts of the light emitted from different parts of the shell can be
measured. In a symmetrical nebula, light comes partly from the side of the shell
nearer the Earth, and partly from the side beyond the central star; if the shell is
expanding there will be a difference in wavelength in the light from t he front and
back. This difference can be measured, and it shows that in many cases the gas is
expanding away from the star at a speed of about 20 km ¹ ( kilometers per
second). Since a typical planetary is about 0.5 pc (parsec) in diameter the shell
must have been produced by the star some 10,000 years ago. A 'parsec' represents the
distance light travels in 3.26 years, roughly about 206,000 times the distance between our
Sun and Earth, which is 93,000,000 miles (150,000,000 km)
The variations of color seen across the faces of some nebulae are due to changes in the degree of ionization in different regions. For example, the green color of the inner parts of the Ring Nebula is an indication that in these regions, oxygen is doubly ionized. In the outer regions, oxygen is only singly ionized and the dominant green spectral lines of O III are absent. The red color here and in many other nebulae is due to a mixture of the H-alpha line of hydrogen and of a forbidden line of ionized nitrogen.
Planetary Nebulae and stellar evolution
Well over 1000 planetary nebulae are known. The best-studied ones
have angular diameters between about 10 arc seconds and several arc-minutes, but
planetaries can range in size from giant objects such as NGC 7293, the Helix Nebula, which
is about one fourth of a degree in diameter (about half the size of a full Moon), to
objects so small that they appear starlike even on the best optical photographs.
These so-called stellar planetaries were discovered by searching the sky with a very low
dispersion spectrograph and looking for objects which, though otherwise resembling stars,
give out most of their light in narrow emission lines.
Most planetary nebulae lie within a few degrees of the galactic plane, with the greatest concentration of objects lying towards the Galactic Center. Planetary nebulae have a typical disc distribution rather than that of Population I or II; they show concentrations towards neither spiral arms, interstellar clouds nor young stars, but occupy instead the region of the Galaxy populated by evolved stars such as novae and RR Lyrae variables. The implication of this distribution is that planetary nebulae are associated with the later rather than the earlier stages of stellar evolution.
It is now generally believed that planetary nebulae are a
normal stage in the late evolution of single stars with masses of between 1 and 5 solar
masses, although there are known examples of binary star systems that are exhibiting
planetary nebula characteristics (Among these examples are NGC 1514, NGC 1360 and IC
4406). Pulsations of such a star during its helium-burning, red-giant stage can lead
to the swift expulsion of essentially the whole of its hydrogen-rich envelope to form the
shell of the nebula. This gas expands outwards at 20 - 30 km s-1, leaving behind the
extremely hot burnt-out core as a very complex blue-hot star. This star is the
source of the ultraviolet ionizing radiation for the nebula, but cools over a period of
100,000 years, becoming an ever-fainter white dwarf and, finally, a black dwarf
star. Long before the star has cooled this far, however, the ionized gas shell will
have become so diffuse that it has blended with the general interstellar medium,
eventually to become embroiled in a subsequent round of star formation. The total
amount of matter returned to the interstellar medium by all the planetary nebulae in the
galaxy is about 5 solar masses per year, which amounts to perhaps 15 percent of all the
matter expelled by all sorts of stars. Planetary nebulae therefore play a
significant role in the evolution of the
whole galaxy.
III. Files Available (as of July 2005):
You can directly download your choice of files by RIGHT CLICKING on the desired URL and selecting 'Save Target As...'; the file will be downloaded to the desired folder.
http://www.blackskies.org/PN_Files/SECGPNv7.xls
Kent Wallace's digital
form of the SEC database, containing data on 1,143 PN's. This data includes
common name, PK#, PN G designation, Right Ascension, Declination, Size,
Magnitude of the planetary, Constellation, where to locate within several popular sky atlases,
and positive and negative sightings by Kent, Steve Gottlieb, and other talented
deep sky observers. This is Revision 7 (2005), and the file name is
SECGPNv7.xls (427KB). Be sure to download the NOTESv7.doc file too.
http://www.blackskies.org/PN_Files/SECGPN_V6DS.xls
Kent's Version 6 database, but MODIFIED to include
Distance, Type, Magnitude of Central Star, and Surface Brightness. This
download duplicates the data in the 23 tables found in the PN Data section of
this site. Currently, this version of Doug's modifications apply to version 6,
but will be updated, as applicable in the near future. Be sure to download this NOTES.doc file too:
http://www.blackskies.org/PN_Files/SECGPN_NOTESv7.doc
For Kent's observation reports, enter:
http://www.blackskies.org/PN_Files/monster.doc
Kent Wallace's
observing reports for 300 PN's in WORD format. These are reports Kent has
written down during the observing time on his 8" f/10 SCT. The file
name is monster.doc (300KB)
For Jay McNeil's files:
http://www.blackskies.org/PN_Files/mcneilpn.txt
and
http://www.blackskies.org/PN_Files/pnreadme.txt
THE MOST COMMONLY SOUGHT AFTER PLANETARY NEBULAE
(KNOWN, POSSIBLE, AND MISCLASSIFIED) By: Jay McNeil
This file, with all the PN data on Jay's list, is available as a comma-delimited
text file. It can be downloaded and imported into a
database program such as Microsoft ACCESS or FileMaker Pro. There are two
files: a readme file (10KB), and the data file (64KB).
Download the files and save as
unformatted text files.
Balmer series
The frequencies of many of the lines in an element are often simply
related, depending on the nature of the transitions. Thus in a hydrogen atom, the
set of all photon frequencies that may be emitted by an electron dropping to the lowest
energy level from higher levels in the atom is called the Lyman series. The set
emitted by an electron falling to the second lowest energy level is called the Balmer
series, to the third lowest energy level, the Paschen series, and so on. The
most familiar of these is the Balmer series, since the frequencies of the emitted photon
fall in the optical part of the spectrum.
(To return to the Introduction, please use your browsers BACK button)
Forbidden Lines
Due to the very low density of interstellar gases and the high temperatures
associated with stars evolving into white dwarfs, certain atomic interactions can occur
within nebulae which are not reproducible in Earths environment. Within these nebulae and
also within certain regions of active galactic nuclei, when an electron transitions from
an upper energy level to a lower energy level, the probability is that the electron will
not collide with another particle (again, due to the low densities associated).
Since a collision is not probable, this means that the decay of the electron will occur
spontaneously and thus allow the production of a photon of light. In higher density
environments, the collision of the electron with another particle would result in the loss
of energy, but without the emission of a photon. Once a photon has been emitted at one of
these 'forbidden' wavelengths, the probability is again low that it will be absorbed by
another atom. Forbidden lines are not found above a critical density of about 108
atoms/cm3.
I have linked a diagram illustrating a typical planetary
nebulae spectrum, showing where in the spectrum these forbidden lines occur.
(To return to the Introduction, please use your browsers BACK button.)
(To return to the Introduction, please use your browsers BACK button)
http://www.blackskies.org
Revised 10/30/97 douglas@blackskies.org
