Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask an astronaut and Space Radio, and author of How to Die in Space.
Galaxies there are shining cities, massive metropolises full of stars, dust, gas, black holes, magnetic fields, cosmic rays, dark matter and more. Separated by millions of light-years from basically nothing, galaxies are incredibly isolated, each island.
Astronomers have identified three types of galaxies: spiral, elliptical and irregular, and the differences between such galaxies reveal their complicated histories.
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If you were to leave a galaxy largely left alone, subjected only to the occasional minor fusion (or more precisely, consumption) of a dwarf galaxy, it would naturally develop a set of beautiful spiral arms.
These spiral arms are created by waves of density rippling through the disk of the galaxy, rotating and overlapping on themselves. These waves come from sources such as nearby passing galaxies, occasional galactic food and even chains of supernova explosions.
Despite the bold appearance of spiral arms, these structures are not much denser than a galactic average, only about 10%. But this slightly higher density causes stellar traffic congestion, igniting the formation of new sets of stars. Essentially spiral arms are the sites of active star formation within spiral galaxies.
All kinds of stars can form in star-shaped regions. Big, bright, blue. Medium white. Small, dark, red. But from millions of light-years away, our eyes and our telescopes most easily choose the bright ones, making the arms visually stand out much more than just their density would result.
The key to beautiful spiral arms in a galaxy is some sort of occasional gravitational encounter or disturbance – an emphasis on the word “occasional”. If a galaxy suffers too many collisions or only one large one, the turbulence can permanently disfigure a galaxy, forever robbing it of the opportunity for beauty.
The problem is the merging event itself. When galaxies collide, the stars themselves don’t actually hit each other – there’s too much empty space for that to happen. But all of these extra gravitational interactions, plus a healthy dose of gas clouds too close to each other, trigger a circle of high-intensity star formation. In the midst of a merger event that can last hundreds of millions of years, star formation can explode to tens or even hundreds of times more than normal.
For a while everything is spectacular. Flaming with newly created stars, the galaxy shines brighter than ever before. Alas, it will not last. All of these new stars come at a price: using so many star-making materials so quickly, the post-fusion galaxy will stop producing stars sooner. In just a few hundred million years, all the massive stars formed during the fusion event die and disappear, with no new stars replacing them.
The result is a giant cluster of dark, red stars. The once beautiful spiral arms are ragged and the galaxy deformed. With enough time, the now giant galaxy settles in an elliptical shape, but despite its huge size remains relatively obscure, clinging to a sad and broken existence for billions of years.
While most galaxies in our universe are either beautiful, grandiose spirals or giant, boring ellipses, there are many strange things. Some galaxies seem to be tense and distorted as we look at them in a funny mirror. Some galaxies appear to have holes exploded directly through their centers. Some are just tangled, mistreated wrecks, barely visible as any structure.
The culprit behind these weirdos is gravity. The gravitational influence of the galaxies on each other twists and distorts the pair, pulling huge tails of gas, dust and stars out of each galaxy as a spiral empties. For several hundred million years, these two galaxies will look very irregular.
Smaller collisions can also create some interesting galaxies. For example, some galaxies look like they were decapitated after a neighboring galaxy plowed through its center, clearing a path through the stars.
Irregular galaxies usually arrange continuous star formation, precisely because whatever influence distorted the galaxy created dense tubercles and nodes where enough material can condense to form stars.
This makes irregular some of the most interesting galaxies out there – no two look alike, but all are annoyingly beautiful.
Learn more by listening to the episode “AAS! 144: ASTRO101 PART VI – WHAT ARE GALAXIES?” on the Ask A Spaceman podcast, available on iTunes and online at http://www.askaspaceman.com. Thanks to Mitchell L. for the questions that led to this piece! Ask your own question on Twitter via #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on Twitter @Spacedotcom and on Facebook.