"What Is Galaxy Gas? Understanding the Building Blocks of the Universe

  Galaxies are huge systems made up of stars, gas, dust and dark matter, all held together by gravity. They vary greatly in size and shape, but generally fall into three categories: spiral, elliptical and irregular galaxies. While stars are the most visible components of galaxies, a significant portion of their mass is made up of galaxy gas, dust and dark matter, which are often invisible to the naked eye. Here is a description of these components:                                                          Stars: Stars are the most recognizable elements in a galaxy. These glowing balls of gas (mainly hydrogen and helium) produce light and energy through nuclear fusion. Galaxies can contain anywhere from millions to trillions of stars, depending on their size.                        ...