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Galaxies Gobble Galaxies

Two spiral galaxies, NGC 6050 and IC 1179, collide.

Two spiral colliding galaxies, NGC 6050 and IC 1179, 450 million light-years away. Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)/ESA/HUBBLE Collaboration, and A. Evans (NRAO)

IfA astronomer David Sanders gave the Frontiers of Astronomy Community Lecture, "Galactic Cannibalism: The Ultimate Fate of Our Milky Way," on Thursday, September 23, at the UH Manoa Art Building Auditorium.

Sanders explained that detailed surveys of the sky reveal that galaxy formation is often a messy process. As big galaxies grow by gobbling up smaller ones, their shapes change from picturesque spirals with their bluish young stars into more rotund, reddish elliptical galaxies with their mostly "red and dead" old stars.

Our home galaxy, the Milky Way, is a relative latecomer to this process of galactic cannibalism. First it will merge with small nearby dwarf galaxies such as the Small and Large Magellanic Clouds. Eventually, it will undergo a spectacular merger with our nearest big neighbor, the Andromeda Galaxy (also called M31). When this happens, in four to five billion years, the pattern of bright stars in the nighttime sky will take on a dramatic new appearance, and much of the gas and dust that would have formed new stars in the Milky Way's spiral disk will instead be swept into the center of the new merged system, where it will build a billion-solar-mass black hole that will signal the birth of a quasar. 

Sanders tried to give the audience a sense of the intergalactic distance scale. He said that if the island of O`ahu were the Milky Way Galaxy, our solar system would have the diameter of a pinhead and the thickness of a fingernail. If the solar system were located in the Art Auditorium, then nearby star-forming regions would be at Bachman Hall, and the Galactic Center containing a black hole of 6 million solar masses would be in Mililani.

He also explained the history of our understanding of galaxies. In the 1770s, the French astronomer Charles Messier published an astronomical catalog of deep sky objects that included nebulae and star clusters to help comet hunters distinguish between permanent and transient objects in the sky. We now know that some of these nebulae, including the one called "Messier 31" (M31), are galaxies. It was the American astronomer Edwin Hubble who figured out in the 1920s that some of these nebulae were galaxies outside the Milky Way. In the 1950s, Halton Arp looked at some of the weirdly shaped galaxies and theorized they were falling apart. We now know that these unusually shaped galaxies are result of mergers.

Many colorful pictures and movies of galaxies in the process of merging are on the Web: