Astronomical Nebula & Nebulae Simplified


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I have often been asked to provide information about Planetary Nebulae and nebulae in general using terms and language that the non-astronomer, and those fortunate folks, young and old, in school with no previous background in astronomy, can more easily understand.  Well, I'll give it a shot!
So this page will be devoted to making an attempt to do just that.  If, after you've read over this page, you still find the matter somewhat confusing, welcome to the club!   But feel absolutely free to email me with your questions and if I can't answer them, I'll look for a source that can, or attempt to further clarify the information on these pages.  One Internet resource that is just the cream of the crop is NASA's Astronomy Picture Of the Day web site. It has a searchable archive of definitions, images, and celestial objects.

Okay; first, lets define the term 'nebula'.   The word 'nebula' is from the days of old (and I mean like, from the Roman Empire and the medieval ages) when Latin was the principal language in the western world, and also used to define most scientific phenomena and species, etc.  I'm not a language expert, so I can't or won't dwell on this point.  In Latin, the word nebula means "a cloud, or a mist, or a vapor".  Nebulae is just the plural (more than one) of nebula.  In ancient times, before the invention of the telescope, people would look up at the night skies  and see various areas of the sky that instead of having a star or several stars close together, they would instead see a small faint glow or 'mist' with their unaided eyes.  So, not being able to determine what they were seeing, to them it looked like a cloud, a mist or a vapor. So these objects were then called nebulae.  Again, this was well before the invention of the telescope. 

Word of the invention of the first crude telescopes in the very early 1600's led to small improvements  in those instruments.  Galileo Galilei was very intrigued by word of these devices and set to work on making his own telescope.  Soon he had created one that would show objects with a magnification of 8 times, and turned his telescope to the night skies.   His observations of stars and planets, and especially the discovery of the four major moons of Jupiter, changed the world.  Immediately after Galileo's discoveries, scientists, astronomers, and anyone with the curiosity to explore the new vistas and undiscovered sights in the Universe had a tool in which to, they hoped, unlock the secrets of the night sky.  And many new discoveries were made as telescopes improved those first few years after being invented.  But guess what? The few objects that were known as 'nebulae' before the invention of the telescope soon became hundreds, maybe thousands!  Why? Because the early telescopes were very poor in quality compared to the telescopes of today and because the telescopes could not 'resolve', or make clear, just what the astronomers were viewing (This is still happening today, but at further limits of the Universe).  Many of the 'nebulous', or cloudy objects that man had viewed for thousands of years with their unaided eyes, became clearer to the early telescopic astronomers, but many still could not be 'resolved' into stars or regions of gas.  So the list of objects classified as nebulae grew and grew as astronomers continued to search the night skies with better and larger telescopes.

Okay, now jump forward to the invention of photography and the application of it to astronomy.  This first occurred in 1845, but it was not until the development of suitable film plates that astro-photography really made its impact, starting around the 1870's.  The introduction of photography into observatories around the world once again changed the world.  Why?  Because when taking a photograph, it's 'eye' (shutter) can stay open for much longer than a persons eye, and continually record, and build up, over a determined period of time (exposure length)  the light that is entering the camera.  The human eye cannot do this to any great extent ; our 'persistence of vision' is very short (well less than a second of time), while a camera's can be as long or as short as we want to make the exposure.  The resulting picture may not be what we desired (under-exposed, over-exposed, blurry, etc), but as long as the shutter is open, the camera will let the light 'fall' onto the film and depending on the film properties, the incoming image will build up on the film for some period of time.   But I digress.....

Now that photography and astronomy were united, another new world opened up for astronomers.  Telescope design changed dramatically, the lenses, mirrors, and other optics of the instruments became a science into itself, and film greatly suited for astronomical work came into being.  Now, many of the objects that when viewed through a telescope with the human eye appeared  'nebulous', became wondrously distinct, resolved celestial objects and they didn't just appear as a cloud, or a mist, or a vapor.  They had form, structure, and character and could be more easily classified as to their particular type of object (in many cases, but not all!).  Exploration into the deeper regions of our visible Universe became possible, and with it, came discoveries that amaze us to this day.

Objects that had been described as nebulae for centuries were soon found to be a certain type of astronomical object other than nebulous, and could be further classified.  For example,  the magnificent Globular Cluster Omega Centauri in the constellation Centaurus, visible to the unaided eye, was found, along with over a hundred similar objects (most of them definitely not visible to the unaided eye), not to be nebulous at all. All the objects that had the characteristics of these clusters, a spherical grouping of from tens of thousands to millions of stars, could now be classified as globular clusters rather than by the general term of nebulae.  So object by object, the classification process evolved, removing, or in some cases, retaining, the term nebula.  Several objects that we can see with unaided eyesight under dark skies are called galaxies in our present day.  There is the Andromeda Galaxy in Andromeda and the Pinwheel Galaxy in the constellation Triangulum.   These and all other galaxies seen through telescopes, and even after photography was being used, were still called nebulae up until the mid 1920's (and in some cases, even after that).   They were called nebulae and spiral nebulae because for the longest time, there were no methods of 'resolving' these objects into what they actually were - island universes like our own Milky Way Galaxy.   It took astronomer Edwin Hubble, using the 100 inch telescope on Mt. Wilson (California), to finally provide enough data and observations to convince the world of astronomy that these objects were external galaxies to our own galaxy.   Thus every galaxy previously known as a 'nebulae' correctly became known only as galaxy such-and-such, or such-and-such galaxy.

Now where are we in this nebula simplified thingy?  Okay, objects have been found and observed to actually be nebulous!  Great.  Now, with the use of photography, spectroscopy, and more recently CCD imaging (CCD= Charge Coupled Device), the objects that do exhibit nebulosity can be classified more specifically.   All right, you caught me on that word spectroscopy.  It is pronounced 'spect-tros-ka-pe' and is basically just the study of the spectrum of light.  The spectrum of sunlight is what you see in a rainbow.  All the colors in a rainbow make up the 'spectrum' of our Sun.  When you allow the white light of  the Sun to pass through a glass prism, it takes each 'color' a different length of time to travel through the glass, and therefore will emerge at a different point outside the prism.  All the colors combined though, make up the white light of the Sun.  Each star, and almost every astronomical object that can be observed, has its own light spectrum. Now understand that when I speak of spectrum, or light, I am also referring to parts of the spectrum that lies outside our visual world; in other words, energy that we cannot see such as ultraviolet, X-ray, and infrared.  For the astronomer, these are just as important regions of an objects spectrum. This is the fundamental study of astronomy -   to determine what makes up the light, what is contained in the light from any object and what physical properties are or have affected the light.  Why? Because there is no other property, of any astronomical body or object outside of our Solar System, that we can work with!  We don't have a table made on the star Sirius, or a can of peas that was sent to us from somebody in the Andromeda Galaxy, so we only have their light, or spectrum, to work with.    From our own Solar System, we do have meteorites that have fallen to Earth, interplanetary dust, Moon rocks, some rocks blown over here from Mars, but as far as I know, that's it (If you know of other E.T. physical matter that has been collected in some way for study and research here on Earth, please let me know!).  The vast, vast amounts of gas and dust in our Universe can also be studied in this manner - by spectrum analysis.  The most common element in our known Universe is the gas Hydrogen. Hydrogen, and many other elements, including Helium, Nitrogen, Carbon, etc. show themselves within the spectrum of objects (if they're present).  That allows us to study not only the light from these objects, but what elements are also present in that objects' spectrum. 

Finally! On to the different types of nebulae that are truly (at this current time in history) classified as such. Some folks in the astronomy community (amateur and professional) may slightly disagree with my following classifications, but disagreement within astronomy has always been a boon to its advancement! I have the following five types of nebulae, and others may say there are only three or four types, while others may insist on six types.  Often times reflection nebulae and emission nebulae are together called 'Bright Nebulae'.

1) Reflection Nebulae; (2) Emission Nebulae; (3) Dark Nebulae; (4) Planetary Nebulae; and (5) Supernova.
Each of these types is covered below, next to beautiful representative images of the nebulae type.   I hope the information provided both above and below help to clarify, and simplify, the understanding of Nebulae.  But its a nebulous situation!

Nebulae Simplified - Five Types of Nebulae and Their Characteristics


The Merope (Reflection Nebula) Nebula in Taurus 1. Reflection Nebulae -This is one example of a reflection nebula.  This is named the Merope (mer-o-pe) Nebula in the constellation Taurus.  It is found among the beautiful star cluster named 'The Pleiades' (plea-ah-dees).  This star cluster is also known as M45 (Messier Object 45), and as 'The Seven Sisters'.
The dust and gas of this nebula does not emit its own light.  It only REFLECTS the light from the nearby star or stars; in this case, from the star Merope.   Under very dark skies, with an optically corrected moderate sized amateur style telescope, you can detect this reflection nebula, as well as additional reflection nebulae regions within the Pleiades cluster.  To detect even a faint bluish hue though is quite a task, requiring larger telescopes, and fine optics. Other nebulae will, if at all, show different hues.  Reflection nebulae abound in the night sky, and many that are found in concert with Emission Nebulae, next on our tour of Nebulae.
The Rosette Nebula (Emission Nebula) in Monoceros 2. Emission Nebulae - Here is an example of an emission nebula.  This is the Rosette Nebula in the constellation Monoceros (mo-nos-er-os).   The nebula surrounds a young star cluster.  This nebula is emitting its own light, due to the presence of ultraviolet radiation from the very hot young stars that make up the cluster.  This radiation, although not visible to the eye, is strong enough to 'excite' the atoms in the interstellar dust and gas so that they 'jump' to different energy levels and emit their own form of radiation which can be seen in the nebula's light spectrum.  It is not simply reflecting the light from the cluster.   Emission nebulae can be found in many areas of the night sky, the most famous being the Orion Nebula.  In astronomical terms, these type of nebulae are also referred to as H II regions (H two). This is because of the hydrogen atoms having been ionized - gaining or losing its single electron due to the strong ultraviolet radiation from a star or stars.
The Horsehead Nebula (Dark Nebula) in Orion 3. Dark Nebulae - Some of the most difficult, but spectacular objects to be seen at night are dark nebulae.  The image to the left is called the 'Horsehead Nebula' and is located in the constellation of Orion.  But believe me, you will probably never see this good of a view through a telescope.  It is often an extremely difficult object to view, because the background is just barely lighter than the foreground object. Dark nebulae are clouds of gas and dust that absorbs some light from behind them. The light that is absorbed heats up the dust particles, and thus these particles re-radiate, or emit, some of the absorbed energy as infrared light (Infrared light is on the opposite side of the light spectrum from ultraviolet).  Some of the background light is also scattered into the surrounding interstellar medium and thus can reduce the contrast between background and foreground, making viewing them even more difficult.
The Eskimo Nebula (Planetary Nebula) in Gemini 4. Planetary Nebulae - Using the Hubble Space Telescope to obtain visual and other spectral data,  our knowledge of the 1000+ known PN's has increased immensely.   The image shown is of the PN known as the 'Eskimo Nebula', and sometimes as the 'Clownface Nebula'.  It is in the constellation Gemini.  This is just one example of many forms that PN's take while expelling their outer layers of gas.
PN's are formed when old stars of a particular size, similar to our Sun's size, have consumed most of their hydrogen fuel after billions of years.  The hydrogen has mostly been converted to helium, and the star has expanded to become a Red Giant.   The star does not so much explode as much as it ejects the gases at much lower speeds and at different times.  As the star evolves, the central core of the star evolves into a White Dwarf, a very hot star. The very high temperature radiation causes the ejected gases to become ionized and glow.
The Crab Nebula (Supernova) in Taurus 5. Supernova Remnant -When a star goes supernova, it is a violent and catastrophic event that more often than not outshines the parent galaxy's total light output.  Stars in external galaxies that have been witnessed as supernovas have been measured to exceed the Sun's brightness by a billion times.  The nebula pictured here is the remains of a star that went SN in the year 1054A.D., as witnessed on Earth.  It was so bright, it was visible during the daytime for several weeks. This is the Crab Nebula, also known as M1 in Taurus. It is estimated to be at a distance of 6,500 light years. The speed of light is approximately 300,000 kilometers per second or 186,000 miles per second. It is believed that the most powerful SN's occur due to a companion star's mass being gravitationally drawn to a white dwarf star. When the dwarf's mass exceeds a certain value, it undergoes rapid Carbon burning and explodes, immediately ejecting the equivalent of 1 solar mass (The Sun's mass).

I wish I could I could say that this is the end, but I know its not!   Each of the nebulae types discussed above could have volumes added to their stories (and do!).  But I hope that these brief outlines provide the information that you are seeking, and will inspire you, and others to investigate further not only these beautiful and mysterious Heavenly bodies, but all the many other varieties of celestial objects that can be found, and enjoyed, either as a great hobby with great people, or as a profession with great people.

If you care to suggest improvements or changes to this page on my web site, please feel free to email me and send in your comments.  Thanks, and enjoy the Universe!


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Doug Snyder
Planetary Nebulae Observer's Home Page
February, 2000