Fact of the Day - Glorious Galaxies Galore

After last Tuesday’s lunar eclipse, let’s consider galaxies. Until just over a century ago, galaxies were not recognized as different from fuzzy stars. As telescopes got better, astronomers realized galaxies were distinct objects in the sky. Today there are an estimated 100,000,000,000 (100 billion) galaxies in the observable universe.  Our Milky Way, a typical galaxy, has around 100 billion stars. Do the math.

Besides many stars most galaxies have three distinct characteristics, two cannot be seen: dust, black holes and dark matter. Every galaxy has dust. When we look at those amazing photos of galaxies from the Hubble telescope the filmy reddish wisps are really stellar dust. It’s left over from the huge explosions when stars die. Most of it gets repackaged as new stars, planets and or your body. You are stardust.  

Black holes live unseen at the center of most galaxies. Black holes form when massive stars collapse at the end of their lives. Gravity is so powerful in black holes that even light cannot escape. Our Milky Way contains a supermassive black hole of about 4.3 million times as big as our Sun.

All the stars, oceans and the stuff you curse when you stub your toe on it, makes up only 5 % of all known matter.  Dark matter makes up the rest. Dark matter does not interact with light or anything else we know about except gravity.  Dark matter makes the galaxies spin. But it is even more invisible than black holes.