Introductory discussions of galactic collisions sometimes seem to imply that the stars in each galaxy pass right through the other galaxy as if it weren't even there, because the stars are so infinitely small in comparison to the vast distances between them that collisions or even near-collisions of stars are rare or nonexistent. But even though stars might not collide with each other, each galaxy's gravity so strongly affects the motion of stars in the other galaxy that the colliding galaxies may tear each other to pieces, scattering stars in all directions in intergalactic space, only to fall back together, under the combined influence of their mutual gravity, to form a new galaxy.
As dramatic as such effects might be, they are minor compared to the most noticeable effect of galactic collisions. For galaxies contain not only stars, but also huge amounts of gas and dust, and as galaxies near or pass through each other, the gases can be strongly or even violently compressed, causing extraordinary episodes of star formation known as starbursts. The heat and radiation emitted by the vast numbers of stars formed in such bursts energize the gas and dust left over from their formation, blowing them violently outward in a "superwind" of stupendous energy and magnitude.
The images immediately below show a relatively minor example of the effects caused by the recent near passage of two nearby galaxies, M81 and M82. As you examine the images and read their captions, you will see that the violent events recorded by these images may be the latest, but are by no means the most important episode of stellar interactions between these two galaxies, and that their past and future contain cataclysmic events that dwarf their present activity.
M81 (left) and M82 (right), two nearby (only 12 million light years from us) galaxies in Ursa Major. Each is visible as a faint smudge in a small telescope, the larger and brighter M81 of course a bit more noticeably. The two galaxies are orbiting each other about once every hundred million years, probably as a result of a near-collision about six hundred million years in the past; and each time they pass by each other, they undergo episodes of star formation caused by their gravitational interaction. M81's bright spiral arms are the site of star formation, partially caused by density waves raised by the most recent passage of M82, and the central portion of M82 is undergoing a starburst -- an explosively rapid surge of star formation and death -- which is violently blowing huge amounts of gas and dust out of the galaxy, causing the fuzzy blurring near its center, and emitting huge amounts of radiation, all the way from the infrared, to the X-ray region. Each hundred-million year passage will probably bring the two galaxies closer together. At first, this will probably produce more and more extreme waves of star formation; but within a few billion years, the two galaxies should tear each other to pieces, and after a period of considerable chaos, merge into a new, brighter and larger galaxy.
As an aside, it is interesting to compare this image, in which H-alpha images which emphasize the emissions from hydrogen gas were combined with visible-light images, with a very similar (apod000209) image, which did not enhance the hydrogen emissions. As discussed in False Colors in Astronomical Photographs, the more interesting an image looks, the more work had to be done to make it look that way, and the less likely it is that it looks like "the real thing". (Johannes Schedler (Panther Observatory), apod060415)
A closer look at M81 highlights the numerous hot, bright blue young stars which are lighting up the glowing clouds of hydrogen gas in its spiral arms. (Image Credit: ESA and the Hubble Heritage Team (STScI/AURA), NASA)
A closer view of M82 (a photomosaic of Hubble Space Telescope images released to celebrate the sixteenth anniversary of the space observatory) highlights the red light strongly emitted by superheated hydrogen gas, driven out of the core of the galaxy by a "superwind" created by large numbers of young, massive stars formed by the galaxy's most recent interaction with M81. In this image, filaments of glowing gas extend for more than ten thousand light years. The hot gas emits so much infrared radiation that at long wavelengths, M82 is the brightest galaxy in the sky. (NASA, ESA, The Hubble Heritage Team, (STScI / AURA), M. Mountain (STScI), P. Puxley (NSF), J. Gallagher (U. Wisconsin), apod060425)
Yet another image of M82, taken with the Hubble Space Telescope, concentrates more on the visible light of stars, and the dust obscuring that light, than on the infrared radiation of the heated gases sweeping away from the galaxy. Numerous globular clusters are revealed, which have a surprising property. Usually, globular clusters, such as those in our own galaxy, are ten to twelve billion years old, and date to the earliest days of galaxy formation; but studies of the cluster diagrams (Hertzsprung-Russell Diagrams of star clusters) for the globulars in M82 reveal that they are only 600 million years old (practically newborns, by stellar standards), as their hottest, brightest stars -- stars that are burning their fuel so quickly that they are nearly at the end of their Main Sequence lives -- are much brighter than the hottest, brightest stars in other globular clusters, and can't be much more than half a billion years old. The starburst that is ejecting huge amounts of heated gas and dust into intergalactic space is believed to have occurred as a result of the most recent passage of M81 by M82, sometime within the last hundred million years; but a far more more spectacular stage of star formation must have accompanied the formation of M82's young globulars, which may well have been caused by a near-collision of the two galaxies a little over half a billion years ago; and even more spectacular events await the two galaxies when they tear each other apart and merge into a new galaxy, a few billion years from now. (Richard de Grijs (Cambridge IoA) et al., ESA, NASA, apod010312)
Another view of M82, taken by the Spitzer Space Telescope, emphasizes the infrared radiation of the escaping gas and dust, and underplays the radiation from the stars within the body of the galaxy, which primarily emit visible light. Analysis of the wavelengths most strongly emitted by the gas shows that it contains large amounts of polycyclic aromatic hydrocarbons -- complex organic compounds frequently found in star-forming regions in our own galaxy -- which are almost literally cosmic smoke particles, as they are common products of combustion on Earth. (Considering that, on account of its visible-light shape, M82 is often referred to as the Cigar Galaxy, the presence of such noxious particles seems particularly appropriate.) The Spitzer image not only reveals the nature of the dust, but also shows that gas, smoke and dust extend even further than previously suspected -- nearly twenty thousand light-years above and below the plane of the galaxy. (C. Engelbracht (Steward Obs.), et al. JPL, Caltech, NASA, apod060414)
Yet another view of M82, combining various space telescope images in a single multi-false-color image. X-ray data from the Chandra space telescope are shown in blue, the bluest visible light radiation recorded by the HST are shown in yellow-green, Hubble's observations of (primarily red) hydrogen emission are shown in orange, and infrared radiation observed by the Spitzer space telescope are shown in red. (NASA, JPL-CalTech, STScI, SCS, UofA, ESA, AURA, JHU)