Star forming regions. Coloured infrared image showing the chain of star forming regions collectively known as W51. These reddish areas are made of compact and extended HII regions and molecular clouds. Stars in the centres of these regions are typically young and massive, in spectral class O and B, and radiation from them is responsible for ionising hydrogen surrounding them. W51 is in the Sagittarius arm of the galaxy at a distance of about 24,000 light years from Earth. This image was made as part of the Two- Micron All-Sky Survey (2MASS) project.