Abstract
Of all the photographs in large-format “coffee table” astronomy books or posted on the Internet, those of nebulae are often the most spectacular. With their panoramas of glowing red, green and yellow colored gas clouds, and impenetrable blankets of black dust lanes, often intertwined and seemingly swirling around each other, they give a wonderfully vivid impression of the amazing mechanisms at work in the universe.
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Notes
- 1.
A fuller explanation can be found in the author’s book, Astrophysics Is Easy! A Guide for the Amateur Astronomer published by Springer.
- 2.
Our simple model of an atom has a central nucleus with electrons orbiting around it, somewhat like planets orbiting a sun. Not all orbits are allowed by quantum mechanics: in order to move up to higher energy levels, electrons need very specific amounts of energy. Too much or too little and an electron will not move.
- 3.
The time spent before recombining is very short, millionths of a second, but it also depends on how much radiation is present and the density of the gas cloud.
- 4.
In some astrophysical contexts, such as those defining what happens in the center of quasars, conditions exist that can give rise to materials such as Fe23. The amount of radiation is so phenomenal that the atom of iron (Fe) has been ionized to such an extent that it has lost 22 of its electrons!
- 5.
This is often called the Stromgren sphere, named after the astronomer Bengt Stromgren, who did some pioneering work on HII regions.
- 6.
It may however still be glowing as it reflects, or scatters, the low-energy light, giving a bluish tinge. This is a reflection nebula. See later in this chapter for more details.
- 7.
To determine the true visual magnitude of nebulae can be fraught with difficulties. Assigning a magnitude to a nebula (or any extended object, including galaxies) is performed thus; it is treated as if all the light from the object originates from a single point – the integrated magnitude. So an object that has, say, an integrated magnitude of 5 will not look as bright as a 5th magnitude star.
- 8.
This signifies it is the 741st object in the Lynd’s Catalogue of Bright Nebulae.
- 9.
This signifies it is the 979th object in the Lynd’s Catalogue of Bright Nebulae.
- 10.
The name “planetary nebula” was first applied to these objects by Herschel who thought that the nebula looked like Jupiter when seen in a telescope.
- 11.
For the full details about this process, you are referred to the many astrophysics texts mentioned in the appendices.
- 12.
This description is VERY basic. More details of the processes involved can be found in the author’s book Astrophysics Is Easy!
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Inglis, M.D. (2012). Nebulae. In: A Field Guide to Deep-Sky Objects. Patrick Moore's Practical Astronomy Series. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-1266-3_4
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DOI: https://doi.org/10.1007/978-1-4614-1266-3_4
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