The most common form of gas in the disk of the Milky Way galaxy is what?

The most common form of gas in the disk of the Milky Way galaxy is atomic hydrogen gas. In the interstellar medium, which constitutes a significant portion of the galaxy's disk, hydrogen is the most abundant element. While hydrogen can exist in different forms, such as molecular hydrogen (where two hydrogen atoms bond together) or atomic hydrogen (individual hydrogen atoms), atomic hydrogen is especially prevalent in the warmer regions of the galaxy where conditions allow for its existence without forming molecules.

Atomic hydrogen is crucial as it serves as the primary building block for star formation. In colder regions of the galaxy, atomic hydrogen can combine to form molecular hydrogen, which is essential for the birth of new stars. However, in the overall context of the Milky Way’s disk, atomic hydrogen represents a larger fraction of the gas content compared to other gases. Helium, while the second most abundant element, is less prevalent than atomic hydrogen in the gas that makes up the galactic disk. Carbon dioxide is found in much smaller quantities and is mainly associated with specific regions or processes, making it considerably less common than the dominant atomic hydrogen.