Chapter 27 Active Galaxies, Quasars, and Supermassive Black Holes
27.0 Thinking Ahead
![The Distant Universe. The panel at left shows the deepest picture of the sky in visible light. The filed is uniformly scattered with galaxies of all types. The panel at right is the deepest picture of the sky taken in X-rays. The field is uniformly covered with tiny points of light, much smaller than the images of galaxies seen on the left.](https://pressbooks.bccampus.ca/practicalphysics1104/wp-content/uploads/sites/302/2017/08/OSC_Astro_27_00_HUDFQuasar-1.jpg)
During the first half of the twentieth century, astronomers viewed the universe of galaxies as a mostly peaceful place. They assumed that galaxies formed billions of years ago and then evolved slowly as the populations of stars within them formed, aged, and died. That placid picture completely changed in the last few decades of the twentieth century.
Today, astronomers can see that the universe is often shaped by violent events, including cataclysmic explosions of supernovae, collisions of whole galaxies, and the tremendous outpouring of energy as matter interacts in the environment surrounding very massive black holes. The key event that began to change our view of the universe was the discovery of a new class of objects: quasars.