In the early autumn sky of the southern hemisphere, a dazzling band of bright stars runs from the southeast to the west. Look directly at the western end of this band in the mid-evening hours to see the blue-white star Sirius, the brightest star in the night sky. Turn a little to the southwest to see the bright white star Canopus, the second-brightest star in the sky. Both stars are much higher during the summer months, and you will visit their home constellations of Canis Major and Carina during that season.
All the stars, nebulae, and star clusters you see at night are part of a single galaxy: our Milky Way. But the Milky Way is just one of hundreds of billions of galaxies in the universe. The Hubble Space Telescope and large earth-based telescopes have mapped and catalogued galaxies out to a distance of some 12-13 billion light years. The light from these galaxies left just a couple of billion years after the formation of the universe. Where stars are the constituents of galaxies, galaxies are the constituents of the universe.
As you have discovered in the last sections, stars are born in emission nebulae like the Great Orion Nebula, grow up in open star clusters like the Pleiades, then disperse into the galaxy where they glow for tens of millions to tens of billions of years with energy created from atoms fusing in their hot cores. [Read more…]
After the hot stars near the center of an emission nebula push away the remaining gas and dust, a group of a few dozen to a few hundred young stars remain clustered together. These groups are called open star clusters, and they are often found along the Milky Way. They are beautiful to observe, especially in dark sky where they look like dazzling jewels set against the black velvet of deep space.
On a dark night, as you gaze along the arc of the Milky Way, you will see small patches of diffuse light and other patches where there appear to be few stars. These are diffuse nebulae and dark nebulae, respectively. These patches are made of gas and dust, and the are places where new stars form.