Abstract
When we look up into the night sky we see stars, and not much else. So we get the impression that between the stars, space is empty. There doesn’t seem to be any sort of material that lies between one star and another. At the same time, we know intuitively that this cannot be true, for if space were empty, from what did stars form? This then leads us to the conclusion that perhaps space is not quite so empty, but filled with some sort of material that, to our eyes, is all but invisible yet is responsible for providing the source material for stars.
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Notes
- 1.
An important distinction is that n is not the same as the number of hydrogen atoms per cubic meter, nH. If the hydrogen is in a molecular form, H2, then the number of separate particles is nH/2.
- 2.
As mentioned earlier in this book, HII is pronounced “aitch 2.”
- 3.
Recall that the ISM is made up of about 74% hydrogen (by mass), 25% helium and the rest metals.
- 4.
And in some cases, star death, namely supernovae remnants, covered later.
- 5.
Our simple model of an atom has a central nucleus with electrons orbiting around it, somewhat like planets orbiting a sun. Electrons with a lot of energy are in the outer orbits, while electrons with less energy are closer to the nucleus. Not all orbits are allowed by quantum mechanics. To move up to higher energy levels, electrons need a very specific amount of energy; too much or too little, and an electron will not move.
- 6.
The time spent before recombining is very short—millionths of seconds—but also depends on the amount of radiation present and the density of the gas cloud.
- 7.
Unfortunately, the red glow is usually too weak to be seen at the eyepiece.
- 8.
These lines are a rich blue-green color and, under good seeing conditions and with clean optics can be glimpsed in the Orion Nebula, M42.
- 9.
In some astrophysical contexts, such as in the center of quasars, conditions exist that can give rise to terms such as Fe23. The amount of radiation is so phenomenal that the atom of iron (Fe) has been ionized to such an extent, it has lost 22 of its electrons!
- 10.
This is often called the Stromgren sphere, named after the astronomer Bengt Stromgren, who did pioneering work on HII regions.
- 11.
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 fifth magnitude star.
- 12.
Note that scattering of water molecules, and not dust, is responsible for the blue sky on Earth.
- 13.
This signifies it is the 741st object in the Lynd’s Catalogue of Bright Nebulae.
- 14.
This signifies it is the 979th object in the Lynd’s Catalogue of Bright Nebulae.
- 15.
Other molecules such as ammonia (NH3) and alcohol (CH3OH!!!) have also been detected.
- 16.
In areas where the average density exceeds, say, a million solar masses, clouds referred to as giant molecular clouds can form.
- 17.
Molecular clouds can be found outside of spiral arms, but current ideas suggest that the spiral arms are regions where matter is concentrated, due to gravitational forces. The molecular clouds pass through the arms and are ‘squeezed.’ This dense region then gives rise to star-forming regions.
- 18.
See the section on dark nebulae for observable examples of Barnard objects.
- 19.
James Jeans (1877–1946) was a British astronomer, and was the first person to mathematically describe the necessary conditions for star collapse.
- 20.
At this point, the increase in density is thought to be a more important condition than a commensurate increase in temperature.
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Inglis, M. (2023). The Interstellar Medium and Protostars. In: Astrophysics Is Easy!. The Patrick Moore Practical Astronomy Series. Springer, Cham. https://doi.org/10.1007/978-3-031-16805-5_5
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