They Come in All Shapes and Sizes
You grew up in this galaxy. It’s what astronomers call a “barred spiral” and if you were an alien living on a planet a galaxy with a “top down” view of the Milky Way, this is what the view would be. The central area, called the “bulge” is filled with stars and, of course, our own supermassive black hole. The bar of light extending out from the center is a sort of “transport” mechanism for gas and other materials toward the core. The spiral arms are where a lot of the latest star-forming (and star death) action is taking place. Our planet is about 2/3 of the way out from the center, between a pair of spiral arms. We don’t live in the center of this stellar city, but more like in the outskirts where the action is a bit quieter. That’s good for us, since being too close to the center might not be good for our solar system’s health.
Astronomers are well aware that not all galaxies look like this. In fact, the Milky Way didn’t always look as it does now. It has evolved, just as all other galaxies have throughout the cosmos. To study the changes that galaxies go through, astronomers have categorized them by shape (their “morphology”) and their sizes, as well as other characteristics like the ages of their stars and the metal content they have.
NASA’s WISE mission (the Wide-field Infrared Survey Explorer), is studying distant galaxies and today has released images of an assort mix of colorful and shapely galaxies. Just as people come in all sizes and variations on the two-arms, two-legs, height and weight arrangements of our bodies, galaxies also exhibit a wide array of variations on the standard theme.
The new collection of nine galaxies shows off this diversity, with members of different sizes, colors and shapes. Infrared light from the galaxies, which we can’t see with our eyes, has been translated into visible-light colors that we can see. Blue colors show older populations of stars, while yellow indicates dusty areas where stars are forming.
This collage of WISE images shows everything from “grand design spirals,” with their elegant swirling arms, to so-called “flocculent” galaxies, which look more patchy and nebulous. All these galaxies are close enough to us that WISE can see details of their structures. Some show sinuous arms and central bulges filled with packed-together stellar populations and possibly even central supermassive black holes.
Some of the galaxies are oriented toward us nearly face-on, such as Messier 83, and others are partly angled away from us, for example Messier 81. One galaxy, NGC 5907, is oriented completely edge-on, so that all we can see is its profile. The edge of its main galaxy disk appears pencil-thin, and its halo of surrounding stars is barely visible as a green glow above and below the disk.
The arms of the galaxies come in different shapes too. Messier 51 has arms that look like a spiral lollipop, while the arms of the flocculent galaxy NGC 2403 look choppy, perhaps more like layered frosting. Astronomers think that gravitational interactions with companion galaxies may lead to more well-defined spiral arms. One such companion can be seen near Messier 51 in blue. Some of the galaxies also have spokes, or spurs, that join the arms together, such as those in IC 342.
As astronomers scan the universe, they’ll be able to dig more deeply into the different galaxy shapes they see. Just as images of different people at different ages tell us about how humans are born, age, and die, images such as these give important clues about a galaxy’s evolutionary history and the stars it contains. Not only will this work help us understand the life stories of all galaxies, it contributes to a greater appreciation of our own Milky Way and the changes it went through that led to the creation of our own Sun and planets.