The Milky Way is our home galaxy in the vast universe, but the structure and composition of the Milky Way remain mysterious. The vast interstellar space between the billions of stars is not empty, but filled with tenuous interstellar medium. The diffuse hydrogen gas radiates a spectral line with a frequency of around 1420 MHz. Some dense hydrogen atoms gather to form clouds of molecular hydrogen, and newly born stars are formed in the dense cores; young and bright stars can ionize the surrounding gas.
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Source: Phys.org