TYPES OF GALAXIES, PART 1:
What are galaxies? Scientists sometimes categorize galaxies based on their shapes and physical features. Other classifications organize galaxies by the activity in their central regions – powered by a supersized black hole – and the angle at which we view them.
What are active galaxies?
Around 10% of known galaxies are active, which means their centers appear more than 100 times brighter than the combined light of their stars. They can be spiral, elliptical, or irregular. The Milky Way is not currently an active galaxy, although it likely experienced a burst of activity in the past few million years. They think this happened due to nearby supermassive black holes that are hundreds of thousands to billions of times the mass of the sun.
Active galaxies can also be categorized by their brightness in radio wavelengths. Radio-loud galaxies typically emit from both the accretion disk and the jets. Radio-quiet galaxies tend to have little-to-no emission from jets. The observed luminosity is also thought to be another aspect of our viewing angle. Jets directed more toward our line of sight, viewed “down the barrel,” appear brighter and more variable than those viewed at wider angles.