Interstellar medium

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Interstellar medium

Though the space between the stars appears to be empty it is actually filled with gas and dust, the interstellar medium (ISM). The ISM is composed of gas, that is mainly hydrogen and helium, and dust, which accounts for about 1 % of the mass of the gas. The dust, which is not the same as dust on Earth, consisting of elements other than hydrogen, such as carbon, silicon, etc., as well as their compounds, CO, HCN, etc. It can be thought of as being more like tiny material grains, akin to sand though much smaller.

The material comprising the ISM is not spread evenly through space. There are dense regions and other regions of lesser density. Also, there are some areas of the ISM that are hot, while there are others that are cooler. There are therefore 2 parameters that are most important concerning the ISM, the temperature and the quantity known as the number density, n. The latter is the number of particles per unit volume, per cubic metre, and it can be individual atoms, neutral, ionised, combined in molecules, or a combination of all 4. As there is far more hydrogen in the ISM than anything else, we can say to a good approximation that the particle density n is the number of hydrogen atoms per cubic metre and this is the n_H.

The enormous range of temperatures and number densities that occur in the ISM is the important point that it is important to realise. The number of particles per cubic metre can be as low as 100, (n = 100 m^-3) to about (n = 10¹⁷/m^-3). The temperature can similarly be as low as 10 K and as high as a few million K.

Most of the ISM is accounted for by the intercloud medium, whether hot or warm. All other regions of the ISM are located in the intercloud medium. The regions are:

Emission Nebulae – Hot intercloud medium – is widespread, and though it is hot, has an extremely low density that consists mostly of ionised hydrogen. This does not obscure the view of space as it is transparent. The warm intercloud medium is also transparent for similar reasons.

All the other regions of the ISM present a much more visual aspect and are therefore important to observers. They can be divided into 2 groups: those regions of the ISM that are concerned with the formation of stars, the diffuse and dense clouds and the HII regions, and those that deal with the death of stars – planetary nebulae, supernovae remnants, and circumstellar shells.

There are methods that allow the observations of these clouds – radio astronomy measures the hydrogen 21 cm line, microwave telescopes measure the CO molecule, and infrared telescopes measure the far infrared emission of the dust.

The ISM is composed mostly of gas, mainly hydrogen and dust, so it is invisible without the high tech telescopes used by astronomers; though there are parts of the galaxy, nebulae, where certain conditions tend to aggregate the material, and these can be seen by smaller low tech telescopes.

Nebulae

Though many nebulae are rather similar in appearance they are actually disparate in nature. They are associated with areas in which stars are forming, cover several aspects of the stars life, and end with the process of star death.

Emission nebulae

These gas clouds are associated with very hot O-and B-type stars, which produce immense amounts of ultraviolet radiation. Typically, they have masses of about 100-1,000 solar masses. However, this huge mass is spread across a correspondingly large area, possibly reaching up to a few light years across, so density of the clouds is extremely low, maybe only a few thousand hydrogen atoms per cubic centimetre. These very luminous stars actually form within and from the material of the clouds, with the result that many emission nebulae are “star nurseries.” Radiation emitted from these stars causes the gas that is usually hydrogen, to undergo a process called fluorescence and it is this that is responsible for the glow that is observed from the gas clouds.

The hydrogen in the cloud is ionised by the energy of the ultraviolet radiation from the young and hot stars. I.e. energy, in this case in the form of ultraviolet radiation, is absorbed by the atom and transferred to an electron in the energy level or orbital shell. Electrons with a large amount of energy are in the outer orbits, but electrons with less energy are in orbits closer to the nucleus. Quantum mechanics doesn’t allow all orbits. Electrons need a very specific amount of energy to move up to higher energy levels; if they have too much energy or too little energy they remain in the same energy level. An electron that gains extra energy can move into a higher energy level, or orbit further from the nucleus, and in some instances they can gain enough energy to allow them to break free of the nuclear attraction and from the atom. The remaining atomic nucleus has been ionised.

The hydrogen cloud will contain some hydrogen atoms that don’t have electrons, if electrons escape from their parent atoms – ionised hydrogen, ionised hydrogen, also known as protons, as well as a corresponding number of free electrons. The time spent is very short before recombining – millionths of seconds – though also depends on the amount of radiation present and the density of the gas cloud. Eventually, the electrons recombine with the atoms, though the electron can’t just settle down back to their original state before they absorbed the extra energy, and it needs to lose this extra energy received from UV before it can return to its original energy level. As the electron emits energy it moves down the atomic energy levels until reaching its original level. In the case of hydrogen, the most common gas in the nebula, an electron that moves down from the 3^rd energy level to the 2^nd energy level emits a photon of light at 656.3 nm. This is the origin of the “hydrogen alpha line,” usually written as the H-alpha. This light that is emitted is a red-pink colour and is the cause of all the red and pink glowing gas clouds that is seen in photos of emission nebula. This glow is usually too weak to be seen when observing through the eye piece of a telescope, only becoming apparent when photographed.

When electrons jump down for other energy levels of the atom, other specific wavelengths of light is emitted. This is the case when an electron jumps from the 2^nd energy level to the 1^st, as it emits a photon in the UV part of the spectrum. This emitted wavelength is the Lyman alpha line of the hydrogen, in the UV part of the spectrum.

Nearly all the light seen in emission nebulae is produced by electrons cascading down the energy levels of an atom, as a result of the process of the atoms absorbing radiation to ionise a gas, then subsequently, the extra energy is re-emitted at specific wavelengths. In clouds that are particularly dense, the oxygen gas in the cloud may also be ionised, and the recombination of the electron with the atom produces lines that are doubly ionised, at wavelengths of 495.9 nm and 500.7 nm. These lines can be seen as a rich blue-green colour in the Orion Nebula, M42, under good viewing conditions and if the optics of the telescope are clean.

Emission nebulae are sometimes called HII regions. The astrophysical term referring to hydrogen that has lost 1 electron by ionisation. Hydrogen atoms that have not absorbed any radiation are neutral hydrogen, HI. OIII is the doubly ionised oxygen line (“oh three”); the “doubly” means that 2 of the outermost electrons have been lost from the atom by ionisation. In astrophysical contexts as occurs in the centre of quasars, the conditions are such that Fe23 can be produced. The radiation density is so phenomenal that the iron (Fe) atom has been ionised to such an extent that it has lost 22 electrons.

The shape of an emission nebula depends on several factors: the amount of radiation available, the gas cloud density, and the amount of gas available for ionisation. When a significant amount of radiation is coupled with a small, low-density cloud, all of the cloud is likely to be ionised, and therefore the HII region that resulted will have an irregular shape, just the shape of the cloud itself. These nebulae are therefore termed matter bounded.

However, if the gas cloud is large and dense, the radiation can penetrate only a certain distance before all of it is absorbed by the hydrogen atoms, i.e., there is only a fixed amount of radiation that is available for ionisation. In this case the HII region the shape will be a sphere, that is often referred to as the Stromgren sphere, often surrounded by the remainder of the gas cloud, which does not fluoresce. These nebulae are therefore referred to as radiation bounded.

Many of the irregularly shaped emission regions include M42 (the Orion Nebula), M8 the Lagoon Nebula), and M17 in Sagittarius. There are 2 which exhibit a circular shape, so are circular nebulae, M20 (the Trifid Nebula) and NGC 2237 (the Rosette Nebula).

After a period of usually several million years, the group of O and B-type stars that are located at the centre of the nebulae will produce so much radiation that they in effect sweep away the residual gas and dust clouds that surround them. This results of there being a “bubble” of clear space around the star cluster. This situation is exhibited by several emission regions. Examples are NGC 6276 and M78 that show the star cluster in the centre of a circular clear area within the larger emission nebula.

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