| Description: | Neutral hydrogen (HI) is the fuel for star formation. It is routinely observed with radio telescopes at a wavelength of 21cm. However, before the HI can clump together and form stars, it needs to cool down. During this process the hydrogen atoms stick together and form hydrogen molecules. This molecular hydrogen is difficult to observe, and astronomers usually observe the CO molecule, which is formed under similar conditions as the hydrogen molecule. The image shows two nearby galaxies, NGC 5194 (also known as M51) and NGC 2903, as seen in HI (gray) and CO (red). The HI has been observed with the Very Large Array in the US as part of the THINGS survey of nearby galaxies. The CO was observed at the IRAM 30m telescope as part of the HERACLES survey for CO in nearby galaxies. We see that the molecular gas is mainly formed in the inner parts of galaxies, where stars are seen to form. The molecular gas also traces the spiral arms, and in the case of NGC 2903, is also found in the central bar. These kind of observations of nearby galaxies, in combination with maps of star formation, help us pin down which laws govern star formation and the evolution of galaxies.
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