Emission Nebulae: The Final Frontier

Emission nebulae derive their name from the light that is emitted from gas excited by energetic radiation from nearby hot stars. The gas is always rich in hydrogen, which happens to be the most abundant element. Nebulae therefore radiate with a distinctive red hue. The hydrogen often mixes with oxygen, which provides green light.  Green and red combine to give off yellow, while dust, which both absorbs starlight and reflects it, provides blue reflection nebulae. All these effects are seen in the beautiful Orion nebula, Crab nebula, Trifid nebula, and the Lagoon nebula.   The nebulae in Orion feature extensively in this collection because, at 1500 light years distant, they are among the closest and best studied of the Galaxy's star-forming regions.  Scientists give them the name HII regions today.

The HII regions (emission nebulae) are so named because they are composed mostly of plasma of ionized hydrogen (HII) and free electrons. The hydrogen atoms of the interstellar medium are ionized by the ultraviolet radiation from a nearby star or stars. Only very hot stars, typically young stars, have enough radiation in the ultraviolet region at wavelengths necessary to ionize the hydrogen. The excess energy beyond that needed to ionize the hydrogen goes to kinetic energy of the ejected electrons. Eventually, by collision, this energy is shared by other particles in the gas. Equilibrium is established in a typical emission nebula when the temperature equivalent of this kinetic motion is between 7000 K and 20,000 K. For a typical emission nebulae, the density of ions (and electrons) is 1.0E8 to 1.0E10 particles per m^3.

As the ions de-excite to lower energy levels, in most cases after recombination of ions with electrons, they emit their characteristic spectral lines. The most prominent of these in the visible spectrum is the red line of hydrogen, giving most emission nebulae a characteristic red glow. There also exist "forbidden lines" (ones not normally seen in earth-bound laboratories) in the spectra from nebulae. The most prominent are green lines from doubly ionized oxygen, giving some nebula green shading.  Interspersed within the glowing gas of nebulae are lanes of dark dust, which can give nebulae their dramatic appearance. Some of the most famous and beautiful of the emission nebulae visible from the Northern Hemisphere are the Orion Nebula (M42, the Messier catalog number), the Lagoon Nebula (M8), and the Trifid Nebula (M20). The stars of the Trapezium illuminate the Orion Nebula, located in the sword of Orion. The Lagoon Nebula is a very large nebula, which has distinct bright rims and small dark clouds projected onto its brightest parts. Dust lanes that divide it into three distinct parts characterize the Trifid Nebula in Sagittarius.

Other nebulae can be found virtually all over the sky.  The constellation of Sagittarius contains nine in itself.  These include: UKS 3, UKS 5, UKS 22, UKS 26, AAT 12, AAT 31, AAT 69, AAT 85, and AAT 86.  Not all of these are visible with low power microscopes, but many can be seen easily using them.  Two of the objects most commonly seen are called the Trifid and Lagoon nebulae.  These are both full of color and enjoyable to discover on a clear night.

Perhaps the most well known and studied nebula lies in the constellation of Orion.  It is therefore known as the Orion nebula.  The Orion nebula (pictured to the left) is very beautiful to observe and are easily seen with a telescope of small magnitude.

The beautiful colors of the emission nebulae have made them very popular to study.  At visible wavelengths, as in the image above, the Orion nebula is lit by the pinkish glow of excited hydrogen and the greenish glow of ionized oxygen. The four apparently bright stars in the center are the Trapezium. The optical images of the Orion Nebula contain glowing hydrogen (pink) and ionized oxygen (green). They also reveal the existence of dust. In order to see through the dust, you must look at near-infrared wavelengths (about 1500 nanometers). Just as red light is scattered less by dust than blue light, infrared light is scattered even less than red light. At near-infrared wavelengths,

you can see straight through the dust.

            Constellations are havens for emission nebula.  You can find them in Scorpius, Monoceros, Carina, Cygnus, Cassiopia, and Norma.  A star or group of small stars begin to draw the interstellar gases off of a larger star and begin to burn the hot gases that are extracting forming the colors that are generally seen in the nebulae.  In heated interstellar gas, electrons and protons combine to form hydrogen atoms, usually with the electron initially in an excited state. It then drops down to the lowest state, emitting photons at each step. The strongest emission at visible wavelengths corresponds to the jump from level 3 to level 2 and has a wavelength of 6563. This is why emission nebulae appear red.

            Diffuse nebulae are the larger of the two types. Some diffuse nebulae contain enough dust and gases to form as many as 100,000 stars the size of the sun.  A diffuse nebula may occur near an extremely hot, bright star. The intense ultraviolet light from the star energizes the gas atoms of the nebula and enables the mass to emit light. A diffuse nebula of this kind is called an emission nebula.  Astronomers believe some emission nebulae are places where new stars are forming. Gravity causes a portion of a nebula's dust and gases to contract into a much smaller, denser mass. Pressure and temperature build up within the mass of dust and gases as contraction continues for millions of years. In time, the mass becomes hot enough to shine--and forms a new star.

The California Nebula is a large emission nebula in Perseus. It is huge in size, spanning over 145 x 40 arcminutes of the sky. It was named for its slight resemblance in shape to the US West Coast State. The main emission line of the nebula is H alpha, that is Hydrogen emitting at the wavelength 656.3 NM, at the red end of the visible light spectrum. Therefore the nebula appears in a deep rich red color. Because of its low surface brightness the California Nebula is primarily a photographic object.    

The Crab Nebula, remnant of the 1054 supernova in Taurus and is also a very popular nebula to study.  It is shown below in a three-color reconstruction from BVR CCD images taken in 1993 with the 1.1m Hall telescope at Lowell Observatory. The field of view is 4.7 arcminutes; north is at the top. The red image is dominated by H-alpha and [N II] emission, while the B and V filters include substantial mixtures of continuum and line emission. The pulsar is visible as the southwestern (lower right) of the two stars just southeast of the brightest part of the nebula. The Crab Nebula’s location in Taurus,

makes it an easier nebula to find, though it is not as bright in the sky as Orion.  It is a lot more faint than some of the other nebulae, yet beautiful if one happens upon it.  The Crab Nebula is just another stop along the exciting realm of nebulae

Another well-known emission nebula is the Trifid nebula.  It is located in Sagittarius and gets its name from the three distinct lanes that surround it.  This is another beautiful example of the awesome nature of these nebulae.  It too is composed of mainly hydrogen and is powered by four stars that feed it its beautiful colors.  The blue colors come from an O-type star and the reds are composed of three different stars combined together.  Its neighbor is a much larger nebula named the Lagoon nebula.

The Lagoon nebula, also located in Sagittarius, is much larger and even more diverse in color than the Trifid nebula and is located only one and a half degrees away from it.  It is a bright, diffuse nebula in the southern constellation Sagittarius; cataloged as M8 or NGC 6526. It is visible to the naked eye and has an angular area larger than that of the full moon. The central parts are extremely bright, and some stars can be seen embedded in the nebulosity. Because of the nebula’s large size, light from its stars cannot illuminate all of the associated interstellar gas and dust.  Thus, parts of it appear blacker than the surrounding sky. The Lagoon Nebula is an excellent example of a star-forming region.  The main part of the nebula is about 60 light-years across, although the faint streamers seen stretching off to the left and right make the full extent of the Lagoon over 110 light-years.  A cluster of bright stars appears to the left of the brightest white portion of the nebula; these are the stars forming from the gas in the Lagoon.

Another famous nebula is called the Horsehead nebula. The black indentation to the red emission nebula seen just to the right center of the above photograph is one of the most famous features in any nebulae on the sky. Because of its shape, it is known as the Horsehead Nebula. The bright star near the center located in the belt of the familiar Constellation

of Orion.  The horse head feature is dark because it is really a dense dust cloud, which lies in front of the bright nebula and blocks the light. Like clouds in our sky, this cosmic cloud has chanced to assume a recognizable shape. After thousands of years, the internal motions of the cloud will alter its appearance. The emission nebula's red color is caused by electrons recombining with protons to form hydrogen atoms. Also visible in the picture are blue reflection nebulae. This type of nebula contains dust, which preferentially reflects the blue light of nearby stars.

            Yet another nebula called the Eagle nebula can be found and it is just as colorful as the others are. Bright blue stars are still forming in the red pillars of the Eagle Nebula. Made famous by a picture from the Hubble Space Telescope in 1995, the Eagle Nebula shows the dramatic process of star formation. To the upper right of the nebula lies the heart of the open cluster M16. The Hubble’s picture closely depicts the true colors of the stars and nebula. The bright blue stars of M16 are continually forming from the Eagle Nebula gas, most recently in the famous gas and dust pillars seen below the photo's center. Of all the young stars in M16, the most massive shine the brightest and the bluest. A typical age for a star in this cluster is about 5 million years, making them only 1/1000
the age of our Sun. Light takes about 7000 years to reach us from M16.

Ten thousand years ago, before the dawn of recorded human history, a new light must suddenly have appeared in the night sky and faded after a few weeks. Pictured above is the west end of the Veil Nebula known technically as NGC 6960 but less formally as the Witch's Broom Nebula. The rampaging gas gains its colors by impacting and exciting existing nearby gas. The supernova remnant lies about 1400 light-years away towards the constellation of Cygnus. This Witch's Broom actually spans over three times the angular size of the full Moon. The bright blue star 52 Cygnus is visible with the unaided eye from a dark location but unrelated to the ancient supernova.

The Omega Nebula contains glowing gas, dark dust, and some unusually massive stars. Also known as the M17 and the Swan Nebula, the Omega Nebula is about 5000 light-years away, 20 light-years across, and visible with binoculars in the constellation of Sagittarius. A recent epoch of star formation has created some very massive stars that haven't yet had time to self-destruct. Until then, these stars appear very bright and emit light so energetic it breaks up the surrounding gas and dust. It is objects like these that would have thrilled the ancients.

I have laid out a broad horizon of emission nebulae and explained how they have come about.  They are beautiful objects to view and some can even be seen with the naked eye.  Pictures can be seen on the Internet, or more excitingly found with a telescope.  I can do no more to convince you of the exciting world of emission nebulae, so I leave you this final photograph, please enjoy!