Galaxies are more than just collections of stars. The material between the stars affects the shape of a galaxy, how it behaves, and how its stars are formed. Interstellar matter consists mainly of hydrogen and helium left over from the immediate aftermath of the Big Bang, with some heavier chemical elements created in early generations of stars. The disks of spiral galaxies have an abundance of interstellar matter: in some places, it is thin and hot; in others, dense and cold. When the interstellar matter is hot and dense, we can see glowing nebulae, called H-II (hydrogen-II) regions. Hydrogen, the simplest atomic element, consists of one proton in the nucleus and one electron. H-II is ionized hydrogen — hydrogen that has lost its electron and acquired a positive electrical charge.
KeywordsStellar Wind Spiral Galaxy Young Star Bright Star Elephant Trunk
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