Nebulosity in Sagittarius

These nebulosities form part of a huge gas cloud in the constellation of Sagittarius, close to the centre of our Milky Way galaxy. This area of sky shows dense regions of stars, so close together that they cannot easily be separated, interspersed with blank regions indicating the presence of dark clouds of gas and dust which obscure the light from the more distant stars. In some areas of sky the clouds are revealed as blue "reflection nebulosity" or red "emission nebulosity". Two well-known nebulae, the "Trifid Nebula" and the "Lagoon Nebula" are very close by and also form part of the same gas cloud (but are not seen in this poster). New stars are formed in these clouds and the intense radiation from very hot, young, blue stars ionises the hydrogen gas (the main constituent of the gas cloud). The ionised hydrogen reveals itself by re-radiating at a wavelength of 656nm in the red part of the spectrum; this is why these clouds appear red. Such clouds are known as H II ("H two") regions to indicate that they are mainly composed of ionised hydrogen. The blue "reflection nebulae" are caused by dust grains in the clouds scattering the light from nearby hot stars; in this case the stars are not quite hot enough to ionise the hydrogen. The blue colour is because the grains scatter the shortest (blue) wavelengths of light most effectively. The sky appears blue for a similar reason, although the Sun appears yellow.

The nebulae in this poster are very faint and would only be seen as faint smudges even when viewed through a powerful telescope. Their colours were only revealed when photographs such as this became available. They do not therefore have popular names and are only known by their catalogue names. IC 1274-5 is the nebulosity to the top left (NE) and NGC 6559 is that at the south.

The part of the sky shown here is at declination -24 degrees and can therefore only be seen under very clear skies from Europe although never very easily from Britain.

North is up and East to the left. The area of sky in this photograph is about 1 degree across (the full moon is about half a degree across).

(This photograph was made from three black and white photographic plates, exposed through blue, green and red coloured filters, taken with the UK Schmidt Telescope in Australia. The colour picture is produced by sequentially printing the original photographs through the appropriate colour filter onto colour material. Colour film, as used for conventional photography, does not register the colour in objects as faint as these nebulae. The three-colour process as used here allows the beautiful colours of the nebulosity to be revealed.)