Australia: The Land Where Time Began

A biography of the Australian continent 

Dark Nebulae

Dark Nebulae, aka dense clouds, differ in nature from all others in one major respect, they don’t shine. This type of cloud is observed by their ability to block light that is emitted behind them when viewed from Earth. They are clouds of vast dimensions that are composed of molecules of gas such as H₂, HCN, OH, CO and CS, as well as dust grains. These grains don’t resemble dust on Earth, being of microscopic size, in the region of 20-30 nm. Ice, however, either water ice of ammonia ice, may condense on them to form a “mantle,” which then increases their size to 300 nm. The dust grains are in the shape of long spindles, some of which rotate. There is much debate concerning the actual composition of these dust grains, though it is believed they are composed of various unknown amounts of carbon in the form of graphite, as well as silicon carbides and magnesium silicates and aluminium. 

It is believed the dust grains form in the outer regions of stars – in particular the cool supergiants and the R Corona Borealis-type stars. Therefore it is believed the dense molecular clouds are another possible site of star formation. The grains are believed to have a temperature of 10-100 K, which is cool enough to allow molecules to form. There may be anywhere from 10⁴ to 10⁹ particles consisting of atoms, various molecules and dust grains in a typical dark nebula.

The nebulae appear dark due to their vast size and so are very effective at scattering all of the light, the result of which is that hardly any reaches the naked eye. The light scattering process is so effective, for instance, that the visible light emitted from the centre of the Milky Way galaxy, the home galaxy of Earth, is almost 100 % extinguished by the dust clouds between the centre of the galaxy and the Earth. This is the reason the central region of our galaxy in visible light is still unknown. Extinction is the term used for the scattering and absorption of light.

The dark nebulae are not dense dust clouds in which the grains are very close together. Most of the cloud material is molecular hydrogen, as well as carbon monoxide, which is responsible for their radio emission, and the density is therefore very low. According to Inglis there is also some evidence that suggests the properties of the dust in the clouds are different from those of the dust in the interstellar medium.

Many dark nebulae have been shown to be interacting with their environs, as can be seen in the spectacular images of M16 in Serpens taken with the Hubble Space Telescope. Dust clouds are shown by the images to contain dense regions, or globules, resisting the radiation pressure from close, young stars, with the result that many of the globules are trailing long trails of material. The area near the Horsehead Nebula in Orion is famous for its image that shows the radiation from the supergiant stars of Orion’s Belt impacting on the dark clouds on either side of the Horsehead, resulting in the ionisation of that material that is seen streaming from the surface of the cloud.

Most of the dark clouds listed in this book have shapes that are vastly different, which is due to several reasons:

·         The original shape of the cloud may have been spherical, therefore presenting a circular shape in an image, though this will have been disrupted by radiation pressure and stellar winds.

·         Shock fronts from nearby supernovae can also affect them.

·         These effects can be caused by gravitation from other clouds, stars or the Milky Way.

·         Some effects are thought to result from magnetic fields.

·         Many dark clouds are part of a much larger region of star formation; therefore the new stars will also influence and alter the shape of the cloud.

The “opacity” of a dark nebula is a measure of how opaque the cloud is to light and therefore of how dark it will appear. The rough classification system of dark nebulae; 1 indicates the cloud attenuates light from the background Milky Way very slightly; a value of 6 indicates the cloud is nearly black and it is allocated a special symbol.

Sources & Further reading

1. Inglis, Michael, 2015, Astrophysics is Easy, An Introduction for the Amateur Astronomer, 2^nd Edition, Springer International Publishing.