What Is That? Hoag-Type Galaxies
It hasn’t been too long ago that we humans thought that our galaxy was the entire universe. Edwin Hubble discovered that this is far from the case only in 1925. Since then we’ve come to realize that there are almost an inconceivable number of galaxies floating around in the the universe, and they won’t stop telling us stories.
Take their shapes, for example. Galaxy shapes tend to fall into two main categories: elliptical and spiral. Those two categories are fairly descriptive; elliptical galaxies are shaped like an ellipse and spiral galaxies – like the Milky Way – look like a cosmic whirlpool. Galaxy shapes are so distinct that Hubble created a method for classifying them based solely on their morphology called the Hubble Tuning Fork.
The idea is that galaxies evolve from left to right, from elliptical to some type of spiral galaxy. However, elliptical and spiral galaxies are just common galaxies. That doesn’t mean there aren’t more types out there. There are lenticular galaxies, represented in the image above as an S0 type galaxy. It’s kind of the link between elliptical and spiral galaxies. There are irregular galaxies whose morphology doesn’t really fit in the Hubble classification. (The Large and Small Magellanic Clouds fall into this category.)
Then there are “peculiar” galaxies, which are…peculiar. One such type of peculiar galaxy is called a ring galaxy. Again, the name is fairly descriptive. A ring galaxy looks like a ring. They are rare and mysterious objects. No one is sure quite how they form, but it’s thought that it’s the result of a galactic collision. A small galaxy punching through the center of a larger one. There galaxies are strange enough, but there is a sub-type of ring galaxy that is even more strange and more rare: the Hoag-type galaxy.
Hoag-type galaxies were discovered by Arther Hoag in 1950. Here is an image of the original Hoag’s Object taken in 2001 by the Hubble Space Telescope:
It’s gorgeous, isn’t it? It consists of a yellow elliptical-ish core surrounded by a ring of blue stars. That the center is yellow and the ring is blue tells us that these two features are not the same age. The outer ring is younger than the core. This is a particularly mysterious object because, while normal ring galaxies are thought to be result of a galaxy collision, there’s no sign of a collision in Hoag’s Object.
Super freaky. But we’re not done. Recently, a Hoag-type galaxy was discovered that has not one but two rings. What could cause a galaxy to evolve in such an odd way? We need to study more to find out.
Featured image: Wikipedia