How many times have you bumped into someone and left a hole in their body where you hit? That's what happened when one galaxy collided with another, the Cartwheel Galaxy. When two spiral galaxies collide, stars, gas, and dust are drawn away into space and form long tails. The impact sometimes creates a black hole in the center with a cloud of dust and gas around it.
The Cartwheel Galaxy is located 500 million light years away in the constellation Sculptor. Before the collision, the galaxy was probably looked like our Milky Way, a normal spiral. Now, new stars are being created, and the galaxy is trying to regain its former shape.
Photo. The Cartwheel Galaxy after collision, photographed in January 1995. When the two galaxies slammed into each other, the so-called "intruder" galaxy threw off a ring of energy that collected the gas and dust that form the brilliant ring. The bright blue areas in the ring, which NASA refers to as "knots," are clusters of newly created stars and of exploded stars, called supernovae. According to the Space Telescope Science Institute, this is one of the most popular Hubble images for download and it made their HST Greatest Hits 1990-1995 Gallery. Courtesy K. Bourne/Space Telescope Science Institute, NASA.
A quasar was formed by the collision. Quasars shine a hundred times brighter than a galaxy, yet occupy the same volume of space as our solar system. This is like comparing the size of the North American continent to a coffee cup.
Astronomers have been confounded as to how an area this small could produce that much energy. They believe quasars are usually formed by two merging galaxies with similar masses, like the one in the Cartwheel Galaxy.
Most astronomers think that a black hole already exists, or must be created, in the center of merging galaxies to form such a bright quasar. The energy of the black hole ignites the quasar, allowing it to shine brightly.
Photo. A brightly shining quasar. Courtesy Space Telescope Science Institute, J. Bahcall/Princton University, NASA.
The Cartwheel Galaxy is so large -- 150,000 light years across -- that our Milky Way would fit inside it. Hubble's view of this galaxy can't allow us to determine who ran into whom. One galaxy exhibits new star formation which suggests it caused the wreck. However, the other galaxy has no gas which is consistent with the idea that it was stripped of gas while passing through the Cartwheel Galaxy.
Maybe from now on these galaxies will use their turn signals!