No, that's not an interstellar bull's-eye. This remarkable image, captured by Japan's Subaru Telescope on Mauna Kea in Hawaii, actually shows something much more special: an extremely rare triple-ring galaxy located about 800 million light-years from Earth, officials from the National Astronomical Observatory of Japan wrote in a statement. How it formed remains a cosmic mystery.
Under the standard Hubble sequence method of classification, galaxies typically fall into one of four categories: elliptical, lenticular, spiral and irregular. Elliptical galaxies appear fairly smooth and egg-shaped through a telescope, with an even distribution of stars. Lenticular galaxies look a bit like flattened ellipses with a bulge in the center — imagine viewing a fried egg from the side. Spiral galaxies, such as our Milky Way, have a similar central bulge, but instead of an outer disk, they have swirling stellar "arms." And irregular galaxies, as their name suggests, lack a predictable, organized shape.
But ring galaxies don't fit into these neat categories. This has led some researchers and citizen scientists to argue that the Hubble sequence — the most widely used method of categorizing galaxies — needs a serious update.
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While Hubble sequence galaxies tend to follow one of two predictable evolutionary paths, nobody knows for certain how ring galaxies form. One of the leading hypotheses is that they result from collisions between galaxies. When another galaxy hits the center of a spiral galaxy, it creates ripples of gas and dust that radiate like a shock wave, producing a ring, the theory suggests.
Another hypothesis is that galactic rings result from natural resonance phenomena. As the interstellar gas within a galaxy moves, it tends to congregate in certain regions because of gravitational effects. Under certain circumstances, this could produce a distinctive ring shape around the galaxy — or even more than one.
Galaxy Cruise, the citizen science project in charge of classifying Subaru images, currently regards ring galaxies as the result of collisions. Whether this triple-ring galaxy was formed through multiple collisions, a single collision or some other means is still unknown. But one thing is certain: Finding oddballs like this helps us refine our overall understanding of the universe and galactic formation.
