"An enormous variety of galaxies fill the abyss of space: small and large, near and far, in rich clusters and in near-total isolation. The Milky Way itself represents just one of at least two trillion such galaxies (and probably several times as many) within the observable Universe. Galaxies are collections of both dark matter and normal matter, where the latter includes plasmas, gas, dust"
"Ring galaxies make up only 1-in-10,000 of all the galaxies out there, where the first one known to humanity, Hoag's object, was only discovered in 1950. It's now more than 70 years since that serendipitous discovery, and we've finally figured out how the Universe makes them. Visually, when you look at a ring galaxy, there are numerous features that stick out as highly unusual among galaxies individually."
Galaxies appear as spirals, ellipticals, irregulars, and the rare ring type. Ring galaxies occur about one in ten thousand and show a compact, gas-poor central core of older stars, a low-density gap, and a bright, expanding ring of active star formation. Nearly head-on encounters between a smaller intruder and a larger disk launch radial density waves that sweep through the disk, compressing gas and igniting star formation in a ring while evacuating the inner region. Ring structures evolve over hundreds of millions of years as the wave expands and star formation migrates outward.
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