The Century that Defined Galaxy

Explanation: As the twentieth century dawned a debate raged over whether the Milky Way was the entire universe. The 1920s brought observations of spiral-nebulae that housed familiar but faint variable stars , and hence placed these nebulae much farther than anything else ever found. The Milky Way, apparently, is itself a spiral nebula seen from the inside. Soon interstellar dust was identified as the opaque material that blocks our view of distant spiral arms , confirming that our Sun was well away from the center. As the twentieth century closes , galaxies are seen forming across the universe , and our Milky Way is referred to as a spiral galaxy quite similar to NGC 1288 , as pictured above .