Me & The Good Old Meade LX200 10" with CCD Me and the New 20" Obsession Dob in Arizona

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The Welcome & Introduction Page

12/02/2010

I. Welcome To Doug's Small Corner Of The Internet World

II. Introduction To Planetary Nebulae

III. Obtaining PN Data Files

 Text  Home Page

   

I. Welcome!
And thank you for visiting my web site.   I am truly honored that you are spending some of your precious time in viewing what I have to offer for the global community that is the Internet.   I hope you will be able to find any information that you came here for, and also enjoy the experience.  If neither is the case, please do not hesitate to contact me and let me know!  And on the other hand, I really do enjoy hearing from anyone that visits these pages and finds it a worthwhile experience.  In any case, have a great day and night !!  Get out there and view the night sky with binoculars or a telescope.  Find out where you can join or support a local astronomy club.  Our night skies are in peril of disappearing amid the proliferation of senseless light pollution.  I have recently moved from California to Arizona in the attempt to find darker skies, and am finding that Arizona is suffering the same fate.


This is a rather large, and seemingly always growing site about mainly one thing - the astronomical objects known as Planetary Nebulae, stars that have reached a stage of their solar life in which their primary fuel source (hydrogen) has been mostly depleted, and now will start converting the material that was created by the burning of hydrogen ( mainly helium). Much of this is explained below, so let me go on more about what is found here on my 'digital palace of the stars'.
I enjoy observing many more objects than just planetaries, and I try to do so whenever possible.  But I find such a grand mixture of beauty, color, structure, form, and challenges in these planetaries that I just had to try and place something on the internet that would help others to see what I try to see.  I have learned so very much since starting this web site, and I hope to continue to learn, as we all do, on what splendid views the Heavens provide us with every clear night.  Unfortunately, there are not enough clear nights in a life time to grasp more than just a micro-inch of the Universe that surrounds us.  I feel we have hardly even begun to imagine what beauty, other proliferations of life, and the scope of the unimaginable Universe lies out of our reach. But I do thank the powers to be and the visionary scientists and engineers for the Hubble Space Telescope.  It has extended our vision and our imaginations far beyond any instrument since the beginning of time.   But we have so, so far to go. I hope our ticket doesn't expire before we reach the end of the ride!

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.

INTRODUCTION TO PLANETARY NEBULAE

Note: Be sure to read "Nebulae Simplified",
a new page added in February, 2000

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.
 
 

 Planetary Nebulae

Similarities to HII regions
Like an HII (pronounced H Two) region, a planetary nebula is a cloud of interstellar gas, mainly hydrogen, ionized by a hot star.  The important difference between the two types of nebula is generic; while HII regions are usually a by-product of the birth of highly luminous ‘O’ type stars, planetary nebula are old objects, formed by the expulsion of gaseous material from the outer layers of a dying star.  The name planetary nebula is misleading, and arose historically because of the visual resemblance between certain small circular nebulae and terrestrial planets such as Uranus.

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    ( 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.

 


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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.

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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.

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Classification of stellar populations
Halo 
Population II 
Intermediate 
Population II 
Disc 
Population
Older 
Population I 
Extreme  
Population I
Subdwarfs High-velocity stars with 
velocities greater than 
30 kms/s normal to the 
galactic plane.
Stars of galactic 
nucleus
A-type stars Open clusters and 
associations
Globular clusters with 
large velocities normal 
to the galactic plane
Long-period variables 
with periods less than 
250 days and spectral 
types earlier than M5
Planetary Nebula 
Novae 
RR Lyrae stars with 
periods below 0.4 days
Strong-line stars Supergiants 
Classical Cepheids 

T Tauri stars

RR Lyrae stars with 
periods longer than 
0.4 days
  Weak-line stars   Supernova remnants 

Pulsars

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http://www.blackskies.org
Revised 10/30/97 douglas@blackskies.org