Galactic collisions are among the most ferocious and stunning events in our universe. These cosmic pile-ups occur whenever galaxies become gravitationally attracted to one another. Multiple galaxies may spiral around each other for billions of years, creating odd distortions and beautiful trails of stars as they pass. Eventually, the objects crash together in a forceful embrace. Since the first galaxies coalesced several hundred million years after the Big Bang, their collisions have been influential in shaping the history of our universe. “As small galaxies merge, they make larger galaxies, and those will then merge to make still larger galaxies, and so on, up to and including the present-day galaxies,” said astronomer Kirk Borne of George Mason University. Because of the vast distances between them, there's a low probability that stars within galaxies will actually hit head-on. But gravitational forces can wrest stars from their previous orbits, scrambling the shape of the galaxies involved. Friction between diffuse gas and dust inside each galaxy raises temperatures, and interstellar material often combines into huge molecular clouds. All this mass in one place triggers prodigious star formation, with stellar birth rates increasing by a hundredfold. The increased light from this extra star formation allows astronomers to see galaxy mergers in the distant universe, helping them learn about some of the earliest periods of cosmic history. Understanding how larger galaxies coalesce from smaller galaxies provides important information on our own cosmic origin story: the formation of the Milky Way galaxy. The Milky Way formed through the long-ago merging of progenitor galaxies and is on course to hit our neighbor, the Andromeda galaxy, in about 4 billion years. Simulations indicate that the Earth and solar system won’t be destroyed in this collision, though the sun may be tossed into a brand new galactic region. Along with other astronomers, Borne is a member of the Galaxy Zoo project, an online collaboration between scientists and interested citizens to sift through astronomical telescope datasets and classify galaxies and their behavior. His specialty is the Galaxy Zoo: Mergers project, which studies galactic collisions. Their website has an online Java applet that lets anyone simulate galaxies smashing together. Just input parameters like the galaxies’ relative masses, collision speeds and angles, and then watch the results. The applet's purpose is to find the initial conditions for different real-world galaxy collisions. Volunteers on the project have run more than 3 million simulations and found models describing 54 real-world galaxies, said astronomer John Wallin, the Galaxy Zoo: Mergers project lead. The project's initial data was recently presented at a cosmology workshop in Santa Fe, New Mexico, and they're still looking for more help. Volunteers can participate in Merger Wars, where different simulated outcomes are pitted against one another to see which best describes actual galaxy collisions. On the following pages, Wired takes a look at some of the most amazing images ever taken of these cosmic pile-ups. Above: Antennae Galaxies One of the most spectacular galactic mergers is known as the Antennae galaxies. Located about 62 million light-years away in the constellation Corvus, the object contains a jewelry case of stars that shine in the visible, infrared, and X-ray wavelengths in this image. The colliding galaxies have nearly engulfed one another since they first began to unite around 100 million years ago, triggering an intense period of star formation. Image: X-ray: NASA/CXC/SAO/J.DePasquale; IR: NASA/JPL-Caltech; Optical: NASA/STScI

Mayall’s Object Looking like one galaxy chomping another to bits, Mayall’s Object is a galactic collision located about 500 million light-years away in the constellation Ursa Major. The strange ring-and-tail shape likely happened when the two parent galaxies first collided, drawing matter into the object’s center. A shockwave eventually propagated outward, creating the enormous ring. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), K. Noll (STScI), and J. Westphal (Caltech)

Mice Collision The Mice galaxies, also known as NGC 4676, are two sweeping objects undergoing the earliest stages of merging. Already, one galaxy has swept past the other, creating a long tail of stars that sticks out to the right. The pair can bee seen in the constellation Coma Berenices and are approximately 300 million light-years away. Image: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin (STSci), G. Hartig (STSci), and the ACS Science Team

Bizarre Shape Zw II 96, as this object is called, has an odd shape, probably resulting from the angles that the galaxies forming it came from. It is located 500 million light-years away in the constellation Delphinus. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Feather Merger The gorgeous collision looks like a delicate sail pushing a small boat through the cosmic ocean. The object, known as ESO 593-8, is located 650 million light-years away in the constellation Sagittarius. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Double Merger A beautiful pair of galaxies has each distorted the other as the two gravitationally interact. Known as Arp 87, the object is located some 300 million light-years away in the constellation Leo. Stars, gas, and dust, can be seen flowing from one galaxy to the other in the image. Image: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

Dusty Merger Dust from the foreground galaxy in this image obscures much of the action. The merger, AM1316-241, is found 400 million light-years away in the constellation Hydra, and is created from two galaxies colliding. The background light appears red as it travels through the dust. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and W. Keel (University of Alabama, Tuscaloosa)

Early Merger A very wide 130,000 light-years separates the two galaxies in the object known as 2MASXJ09133888-1019196. But they are caught in one another’s gravitational spell and have begun a slow dance that will one day result in a merger. The galaxies are about 700 million light-years away in the constellation Hydra. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Cosmic Owl The odd object, known as ESO 148-2, looks like some weird water bug seen head-on. Located about 600 million light-years from us, it is the result of two galaxies merging, with their cores located in the central “body” and a great deal of matter sweeping out into two curved “wings.” Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Linked Arms The two galaxies seen here, NGC 6050 and IC 1179, are located about 450 million light-years away in the constellation Hercules. The spiral bodies are seen crashing together, with an enormous eddy of stars seeming to form between their conjoined arms. This object is part of the Hercules Galaxy Cluster, itself located within the Great Wall of superclusters, the largest known structure in the universe. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and K. Noll (STScI)

Middle Merger The galaxies seen here are caught mid-merger. The bodies of the two parent galaxies have more or less fused but two independent central nuclei can still be seen in the image. The collision is thought to have begun approximately 300 million years ago. This object, known as NGC 520, is one of the brightest galaxy pairs on the sky, and is located about 100 million light-years away toward the constellation Pisces. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and B. Whitmore (STScI)

Mirror Image Two galaxies appear as mirror reflections of one another as they enter the first stages of their collision. The object is known as NGC 5331 and is located 450 million light-years from Earth in the constellation Virgo. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Galaxy Quartet An impressive arc of four galaxies can be seen smashing together in this image. The object is known as ESO 255-7 and is located 550 million light-years away in the constellation Puppis. While three galaxies are clearly visible, the top object is made up of two distinct galaxies that have partially merged already. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Skater Galaxies Connected via a long stellar bridge, the two galaxies in UGC 8335 are pulling one another gravitationally with all their might. The collision is happening 400 million light-years away in the constellation Ursa Major. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Strongly Interacting The galaxies seen here — NGC 6621, on the right, and NGC 6622, to the left – are seen about 100 million years into their merger. Gravitational forces have wrapped a long tail of stars around both galaxies as well as triggering an extensive burst of stellar formation. The pair is located in the constellation Draco, approximately 300 million light-years from Earth. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and W. Keel (University of Alabama, Tuscaloosa)