Australia: The Land Where Time Began

A biography of the Australian continent 

Reflection Nebulae

Reflection nebulae shine by the light that is reflected from stars within the nebulae, or from nearby stars. As with emission nebulae, these vast clouds consist of both gas and dust, though in this case, the dust is much less concentrated than dust in emission nebulae. The property of scattering selectively light of a particular wavelength is one of the characteristics of particles, or grains, which are so small in proportion to the wavelength of light. When a beam of white light shines on a cloud that contains the grains the blue light scatters in all directions, which is a phenomenon similar to what occurs in the atmosphere of the Earth making the sky appear blue (12). This is one of the reasons for reflection nebulae appearing to be blue on images; it is caused by the usually hot blue stars that are nearby.

Such nebulae should actually be called scattering nebulae, to be scientifically accurate. A property of the scattered light is that polarisation is caused by the scattering process, which is useful in studies of the composition of the grains and the structure.

Also, if a star located behind a dust cloud is observed, some of the blue light is removed from light that has been emitted by the star by the process discussed above, an effect known as interstellar reddening, which causes the light from the star to appear redder than it actually is. A result of this is that a further phenomenon that is associated with dust grains, interstellar extinction, which affects all observations. I had been noticed by astronomers that the light from star clusters was fainter than expected, which is due to the dust between the cluster and the observer. It is this dust that causes all objects to appear fainter than they are which leads to underestimation of the luminosity and an overestimation of their distance from the Earth. Therefore, when making measurements the interstellar extinction must be taken into account. The cause of the blue sky phenomenon of the Earth is water molecules, not dust.

Several reflection nebulae have been found within the same gas clouds as emission nebular, a perfect example being the Trifid Nebula. The inner regions of the nebula glow with a pink colour, which indicates the ionisation process that is responsible for the emission, whereas the edge material further from the centre is definitely blue, a telltale sign of the scattering nature of the nebula.

Reflection nebulae are visually very faint objects that have low surface brightness, which makes them difficult targets. Most require telescopes with large apertures and moderate magnification to be seen, though there a few that can be seen with binoculars and small telescopes. To view these objects also requires excellent viewing conditions and very dark skies.

Some reflection nebulae

Caldwell 4 – NGC 7023

This is a small reflection nebula. There is a 7^th magnitude star at its centre. This reflection nebula is surrounded by a larger area of dark nebulosity, which Inglis suggests is probably part of the same nebula complex. The contrast between the background stars, the dark nebula and the reflection nebula makes this a very interesting region.

NGC 1333 – LBN 741  

This reflection nebula is an elongated hazy patch. Towards the north and south of it are 2 fainter dark nebulae, Barnard 1 and 2.

Merope Nebula – NGC 1435

This faint patch of reflection nebular is within the Pleiades, the most well-known star cluster. The nebula surrounds the star Merope, which is one of the brighter members of the cluster. It is a cloud shaped like a comet, and in 1859 W. Tempel described it as “a breath on a mirror.”

Caldwell 31 – IC 405 aka the Flaming Star Nebula

This is comprised of several nebulae, which includes IC 405, 410 and 417, as well as the variable star AE Aurigae.

NGC 1999 - LBN 979

This reflection nebula is small, bright, and resembles a planetary nebula, and has a star of 10.5 magnitude, V380 Orionis, in its central region. It is about 1^o south of M42.

Messier 78 – NGC 2068

This is a bright, small reflection nebula. Its shape has been likened to a fan or a comet. There are 2 10^th magnitude stars within the nebula that can give the false impression of 2 cometary nuclei. It is part of a group of reflection nebulae NGC 2064 about 7′ to the southwest and NGC 2067 6′ to the west-northwest.

 Sources & Further reading

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