Nebulae

Tuesday 20th November, 2018

Our speaker was Owen Brazell who came to talk to us about "Nebulae". He is Director of the Galaxies Section of the Webb Deep Sky Society. He began by saying that he is primarily a visual observer and produces drawings of what he sees through the eyepiece. He commented that before the 1920s photographic plates were so "slow" (i.e. took such a long time to capture an image) that many observers drew what they saw.

He then said that the term nebula was originally used to describe any hazy astronomical object. Strictly speaking the word "nebula" should be used only for gas and dust clouds and not for an object made up of a collection of stars. So, most classification systems divide them up into a number of types: emission, reflection, dark, planetary and supernova remnants.

An emission nebula consists of clouds of high temperature ionized gas, which have been heated by the ultraviolet light from a nearby star. They are usually red in colour due to the abundance of hydrogen gas re-radiating at red wavelengths of light. One well-known example is the Orion Nebula in the constellation of Orion (Messier 42) where stars are forming from the collapse of huge clouds of gas. Seen best in the winter sky it is bright enough in dark skies to be visible to the naked eye.

The next type, a reflection nebula, is made up of clouds of dust that simply scatter the blue light of nearby stars. In the case of a reflection nebula the starlight is not intense enough to ionize the surrounding gas, so it is just scattered. A bright example of a reflection nebula is the blue luminosity around the Pleiades star cluster (Messier 45). This is visible in the winter sky and is also known as The Seven Sisters.

Dark nebulae simply block the light from whatever is shining behind them. These dark regions are mostly seen as irregular patches against a backdrop of stars but sometimes form a distinct shape such as the Horsehead Nebula in the constellation of Orion just south of the star Alnitak.

Planetary nebula consist of shells of gas ejected by some stars as the nuclear reactions in their core comes to an end. This results in the star becoming unstable and throwing off its outer atmosphere into the surrounding space. Our own Sun will produce one in another 5 billion years. They come in a variety of shapes, with some resembling rings, others long elongated patterns. The Ring Nebula (Messier 57) is found in the constellation of Lyra, which is highest in the summer months but is best seen in binoculars or a telescope.

A supernova remnant is the debris from a star that has imploded and depending on the type of supernova the central star may or may not survive. Some remnants appear quite symmetric, taking on shell-like shapes but others can be quite irregular depending on the density of surrounding material. In 1054, in the constellation of Taurus, a supernova explosion was observed by Chinese astronomers leaving a remnant known as the Crab Nebula (Messier 1). It can be seen in binoculars or telescopes given dark skies but its filamentary structure will not be apparent.