Great Orion Nebula. Optical image of the emission nebulae M42 (right, large) and M43 (centre left, smaller) in the constellation Orion, together making up what is known as the Great Orion Nebula. Emission nebulae are clouds of gas and dust that glow due to the ionisation of hydrogen by hot young stars within them. These nebulae are about 1500 light years away. NGC 1977 (blue, upper left) is a reflection nebula. This image includes filters for the light from hydrogen (H-alpha), oxygen (OIII) and sulphur (SII) ions.