In a number of posts I will be studying the composition of the Milky Way, with the goal of better understanding the sources of the light from objects in the galaxy observed here on the surface of this planet we call Earth. I will focus on radiation that I can capture in my telescope –the visible spectrum.
Figure 1: Horsehead and Flame Emission, Reflection and Dark Nebulae in the Orion Region. Imaged at Lac Teeples with the RASA-8 telescope on 2020-10-11.
Nebulae
In parts 1 and 2 of this series we have learned that the Milky Way’s interstellar medium is approximately 15% of the total mass in the galaxy. It is richer in heavy elements than the galaxy in general, with small but important amount of Carbon, Nitrogen and Silicate dust. Of the hydrogen gas found in the Milky Way’s interstellar medium, roughly 1/3 is in the form of molecular hydrogen, H2, and 2/3 is in the form of neutral hydrogen atoms, H.
Looking more closely, the interstellar medium is broken down into the following types:
· Molecular Clouds (dominated by molecular Hydrogen, H2),
· Neutral gases (dominated by neutral hydrogen, H),
· Ionized gases (ionized neutral hydrogen, source of Hydrogen-Alpha radiation), and
· Coronal gases (near stars - very hot).
These gassy/dusty regions can produce three general types of nebulae: reflection nebulae, emission nebulae, and dark nebulae.
Dark Nebulae
Dark nebulae are dense interstellar clouds containing dust that are capable of blocking the passage of light. Such clouds exist in many places, but they form a dark nebula only when they block light in a noticeable pattern as seen from Earth. Dark nebula may block light from background stars or bright nebulae and are noticed when they produce a pattern that we see superimposed on that background light.
There is no special consideration regarding wavelength of radiation since the dark nebulae represent an absence of radiation. Dark nebulae visible from Earth will be located nearby on the scale of the Milky Way galaxy, and some are visible in nearby galaxies such as Andromeda.
Figure 2: Dark Nebula B150, imaged at Lac Teeples on 2020-12-18 with the RASA-8 telescope.
Reflection Nebulae
The clouds that form dark nebulae, cold and dusty molecular clouds, as well as neutral gas regions, do not radiate in the visible spectrum. However, if they contain a sufficient amount of dust they are capable of reflecting light from nearby stars. This reflected light will scatter off the dust particles in a manner similar to what we see in the Earth’s atmosphere. The scattered light will tend to be blue because the scattering is more efficient at blue wavelengths. This is the same process which makes our Earthly sky blue.
Reflected light will have a continuous spectrum nearly identical to the stars that are the light source (biased to the blue because of scattering efficiency there). The energy for this reflected light can be from one or many stars. For the reflected light to be a potential target for Earth-bound astrophotographers there must be a great deal of light and the star and surrounding gases must be fairly close to us.
Perhaps the most famous reflection nebula is the blue light surrounding the Pleiades star cluster, a nebula which also involves some emission. It is 450 light-years away. Another is the Witch’s head nebula illuminated by the bright star Rigel, 1000 light-years away. The “ghost of Gamma Cassiopeia” is a reflection nebula 550 light-years away. The great Orion nebula is a complex blend of emission and reflection and is found ~1350 light-years away. These distances are around 1% of the diameter of the galaxy (106,000 light years)- right in our neighborhood.
Figure 3: The Ghost of Gamma Cassiopeiae reflection nebula, imaged at Lac Teeples on 2021-10-05 with the RASA-11 telescope.
I noted the odd fact that stars are known to be distributed along the plane of the Milky Way but appear in all directions from the Earth. This is because our attention is drawn to the nearby stars which are indeed in all directions. The Nebulae involving star formation (emission and/or reflection) are generally associated with the spiral arms of the galaxy, so they are more likely to be found along the path of the milky way. “Planetary Nebulae” may appear to be distributed in all directions since our observations are biased to nearby bright objects and these are caused by the death of individual stars. A check on these distributions is at the bottom of this post in an addendum.
Emission Nebulae
Emission nebulae result from the release of energy at very specific frequencies due to energy from an external source being absorbed and then emitted. The emitting gases are ionized, meaning that the energy source has stripped one or more electrons from the gas atoms. The energy required to ionize and drive emission at optical wavelengths must be very powerful.
The source of energy will be the radiation from an exceptionally bright star or from the process whereby a star is dying (e.g. a supernova). Nebulae that are not being bombarded by energy in this way will radiate energy, but at lower energies not in the visible wavelengths.
Stars emit energy due to their high temperature and this at nearly all frequencies (continuous spectrum). This energy strikes the gases near the star, raises its temperature, ionizes it and may excite the gases to higher energy states. If we observe a star with a gas cloud in the foreground we may see absorption lines, representing the loss of that energy that has excited the gas to higher energy states. The gas, in turn, radiates at specific frequencies.
Stars that are cooler than around 25,000K don't give off enough ultraviolet radiation with wavelengths shorter than 91.2nm (the wavelength needed in order to ionize Hydrogen atoms). This results in the reflection nebulae around these stars giving off more light than the emission nebulae. Stars that are hotter than 25,000K generally give off enough ultraviolet radiation to cause the emission nebulae around them to be brighter than the reflection nebulae. Such bright stars are extremely rare and do not live long, so they are generally seen near the location where they were born – amid a gas cloud. In many emission nebulae, an entire cluster of young and extremely hot stars is contributing energy.
An emission nebula's colour depends on its chemical composition and degree of ionization. Given the makeup of the universe the interstellar gas clouds are primarily composed of Hydrogen. It is no surprise then that the main spectral line we see in all nebulae, both in the Milky Way and in other galaxies, corresponds to a transition in energy level for Hydrogen. This is the red colour of the Balmer series.
If more energy is available, other elements will be ionized, and green and blue spectral lines become possible. By examining the spectra of nebulae, astronomers infer their chemical content. Most emission nebulae are about 90% hydrogen, with the remaining helium, oxygen, nitrogen, and other elements.
Categories of Emission Nebulae
The main categories of emission nebulae are H-II regions (“H two”), planetary nebulae, and supernova remnants. As noted above a very powerful energy source is needed for these types of nebulae.
H-II is singly ionized hydrogen. Since Hydrogen atoms have only one electron this is the only type of ionized hydrogen possible. An H-II region will be dominated by the Hydrogen-alpha red emission, of the Balmer series. The most famous H-II region is probably the Orion Nebula.
Figure 4: The Cocoon emission, reflection and dark nebula, a star-formation region. Imaged at Lac Teeples on 2022-08-05 with the RASA-11 scope.
“Planetary” nebulae are associated with the end of life of a bright star that has shed its outer layers leaving an extremely intense star core with a surface temperature (100,000K) much higher than a typical star (our sun’s surface temperature is 5,800K). The energies are so high that other emission lines will become visible. Emission from O-III (doubly ionized Oxygen) may be common, particularly at the greenish-blue “forbidden line” at 500.7 nm. The ejected gas shell radiating by the hot core will be visible to us as a ring, disk or similar but more complex shape.
Figure 5: The Crab supernova remnant nebula M1, imaged at Lac Teeples on 2021-12-26 with the RASA-11 telescope.
Supernovae are large stars which have exploded in a spectacular fashion. They may be brighter than a galaxy for a short period of time. The smoking ruins left over, known as supernova remnants, have an emission-line optical spectrum.
Depending on the energy remaining in the remnants the emission may be the three most popular with amateur astrophotographers, namely Hydrogen-alpha, Oxygen – II and Sulphur II (S-II). Several other emissions lines may appear as well.
For emission nebulae there will also be blackbody continuum radiation, but this is of no interest for the optical realm (unless something is extremely hot, like a star). Perhaps most emission nebulae will also involve some reflection of light, making them also reflection nebulae.
We have now clarified the general types of nebulae including the three main types of bright emission nebulae. In the final part of this series we will look more closely at the emission lines and how we use filters to image these.
Addendum: Sky Distribution of Nebulae as seen from Earth
We assess below the hypotheses that:
1) Planetary nebulae will probably be distributed as are the bright nearby stars from which they arise. That is, they will appear in nearly all directions equally, and
2) Bright/Emission nebulae will be distributed closer to the plane of the milky way because they are typically associated with regions of star formation located in spiral arms.
There are too few supernovae to have a reasonable sample. Dark nebulae are associated with the other nebulae. We ignore reflection nebula, as they are usually combined with bright emission nebulae.
Notable Planetary Nebulae and their galactic latitude
Nebula | Galactic Latitude (degrees) | Notes on Direction |
NGC7293 Helix Nebula | -57 | Away from galactic plane |
M27 Dumbbell Nebula | -3 | Toward galactic plane |
NGC3242 | +32 | Away from galactic plane |
NGC7009 | -34 | Away from galactic plane |
NGC6543 | +30 | Away from galactic plane |
NGC6572 | +12 | Close to Galactic Plane |
NGC7662 | -17 | |
M1 | -6 | Toward galactic plane |
NGC7027 | -3 | Toward galactic plane |
M57 Ring Nebula | +14 | Close to Galactic Plane |
| | |
A galactic latitude of 0 is in the direction of the galactic plane. The plane has a varying width (wider at the galactic centre in Sagittarius). We consider here anything with a latitude of 10 degrees or less to be close to the galactic plane.
Recall that roughly 40% of the sky will be within +/- 23 degrees (corresponding to the tropics): If evenly distributed over the sky we should still see more objects near zero and very few near 90 degrees.
In this table we see that three Planetary Nebulae are within 10 degrees of the galactic plane, and six are within +/-23 degrees where we’d expect four to be found. Overall there appears to be a fairly even scattering over the sky with a small amount of clustering along the galactic plane.
Entries in the table above are a sample derived from Telescopius.com. Objects were selected by type and listed in order of apparent magnitude. The process will identify only those observable at Lac Teeples observatory. This is intended to be illustrative and is not a true random sample.
We assume that no significant bias results from the northern latitude of observation since the galactic plane is offset from the equatorial plane by 27 degrees. The remaining bias is expected to lead to a false increase of objects near the galactic plane.
Notable Bright Nebulae and their galactic latitude
Nebula | Galactic Latitude (degrees) | Notes on Direction |
M45 Pleiades | -23 | |
NGC2244 Rosette | -2 | Toward galactic plane |
NGC2175 | 0 | Toward galactic plane |
M42 Orion | -19 | |
NGC2467 | 0 | Toward galactic plane |
NGC7023 | +14 | |
IC348 | -18 | |
NGC7000 | -1 | Toward galactic plane |
M78 | -14 | |
NGC6853 | -3 | Toward galactic plane |
| | |
In this table we see that five “Bright” Nebulae are within 10 degrees of the galactic plane, and all ten are within +/-23 degrees where we’d expect four to be found if it were random. Overall, there appears to strong clustering of such objects along the galactic plane.
Entries in the table above are a sample derived from Telescopius.com. Objects were selected by type and listed in order of apparent magnitude. Planetary nebulae were removed. The process will identify only those observable at Lac Teeples observatory. This is intended to be illustrative and is not a true random sample.
Both of the hypotheses have proven correct (to the extent that these samples are representative).
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