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The Black Hole at the Center of the Milky Way

How would the world look if you saw only gamma rays? Instead of the normal tapestry of colors, everything would appear rather uniform and mundane. Lifting your gaze upward would reveal an uneven glow with a bright band corresponding to the plane of the Milky Way Galaxy. This largely unchanging picture—unaffected by day or night—of the gamma-ray sky also shows an artifact of a more violent time in our galaxy—a period hostile to life as we currently know it. 

Scattered throughout the irregular background, the occasional bright spot indicates the presence of a specific source of gamma rays. Most of these sources reside far beyond the confines of the Milky Way. Galaxies in their own right, these sources indicate the presence of a massive black hole (some with masses larger than the entirety of our galaxy) voraciously feeding on gas, dust, and stars that fall prey to powerful gravitational forces. The feeding frenzy spews out huge jets, millions of light-years long and containing matter traveling at nearly the speed of light. The frenetic conditions associated with the jets and hungry black hole generate enormous amounts of x-ray and gamma-ray radiation that bathes the host galaxy. Other processes may actually form the jets, but all of them result in the deadly radiation.

Fortunately, the Milky Way currently hosts a far more life-friendly environment with minimal amounts of radiation. However, bulging out from the center of the Milky Way,two large lobes of diffuse, low-level gamma-ray emission tell the story of a past feeding frenzy occurring in our own backyard.1  

Scientists continue to investigate the origin and nature of the large gamma-ray lobes. Maybe they are signs of an active period when our galaxy produced large jets beaming gamma rays out into the universe. Maybe the black hole generated a wind that formed the bubbles or perhaps a period of particularly violent star formation. Any reasonable scenario for producing the gamma-ray lobes also brings an increase in radiation to Earth. Yet today, humanity enjoys a radiation environment that permits the use of satellites and all the related technology that it brings. It looks like we live in a special time period. Even gamma-ray eyes see evidence of design.

For more on the ways in which humanity’s habitat is design to support our existence and our technology, check out these previous articles.

  1. Anna Franckowiak and Stefan Funk, “Giant Gamma-ray Bubbles in the Milky Way,” Physics Today 67 (July 2014): 60–61.