Stars and Nebulae

Great clouds of gas and dust, called nebulae, extend across space. Bight clouds are limited regions stimulated to shine by the light of nearby hot stars. An example is the faint patch of light just discernible in the sword of the constellation Orion. In reality this is a nebula about 15 light years across, the birthplace of thousands of new stars.

More extensive are the dark nebulae. The bulk of these is transparent gas, but the dust in them blocks the light from stars beyond. The dark “lanes” that we seen in the Milky Way are actually dust clouds blocking our view of the piled up banks of stars lying beyond and preventing us from seeing to the center of the galaxy, 30,000 light years away.

 Gas in the galaxy is pretty much the same as the primordial matter that emerged from the Big Bang, and from which the galaxy formed perhaps 12 billion years ago. Three quarters of its mass consists of hydrogen, nearly all the rest of helium, but some of the gas and all of the dust consists of new elements formed since then in stars. The hydrogen, though dark, can be mapped by ultra high frequency radio emissions.