In turn, the stirring action of the rotating central bar may give rise to spiral arms. Bars are seen in the majority of spiral galaxies and are thought to form from gravitational instabilities in a galaxy’s dense disk. NGC 1300 has a bright, linear structure in its center, which astronomers call a bar, and two bluish spiral arms that start at the ends of the bar and wend slowly outward as they encircle it. The nearby spirals NGC 1300 and Messier 101 (M101) provide good examples of how the Milky Way might look from afar. Spirals, along with rounded ellipticals, are common types of galaxies. This revelation upended the notion that the Milky Way might encompass the entire universe.Īstronomers figured out that we live in a spiral galaxy by measuring the motions of gas throughout the disk-the large, pancake-shaped region that makes up the main body of the Milky Way. Debate on these points continued into the early 1900s, until Edwin Hubble, using a technique developed by Henrietta Leavitt to measure the distance to bright stars, showed that the Whirlpool and similar spirals were far outside the Milky Way. Without knowledge about how far away it was or about the scale of the Milky Way, however, it was unclear whether the Whirlpool was a small structure inside our galaxy or a large nebula similar to it. He observed and drew what we now call the Whirlpool Galaxy, which clearly had a spiral pattern. In the early 1800s William Parsons, the third Earl of Rosse, built a 72-inch telescope-huge for its time. In addition to gaining a better understanding of what the Milky Way looks like, we are starting to clarify why galaxies such as ours exhibit spiral structure and how our astronomical home fits into the universe as a whole. Our initial results offer a new and improved view of the Milky Way. For this effort, we were granted an unprecedented amount of observing time-5,000 hours-on the Very Long Baseline Array, a system operated by the National Radio Astronomy Observatory and funded by the National Science Foundation. This emerging vista is the result of several large projects involving advanced radio and optical telescopes, including our program, the Bar and Spiral Structure Legacy (BeSSeL) Survey. Recent efforts, however, have begun to map the Milky Way from the inside out, allowing us to assemble an accurate snapshot of its structure for the first time. We are left with many open questions about our cosmic home, such as how many spiral arms the galaxy has, whether the large structure closest to the sun counts as an arm and where in the galaxy our solar system lies. Imagine sending a spacecraft on a multimillion-year journey to go beyond our galaxy, look back and snap a picture: clearly impractical. The reason is obvious: we cannot get outside it to take a peek. Yet as well as we have come to know our astronomical backyard, our image of the larger neighborhood-our Milky Way galaxy-is blurry. Hundreds of years ago explorers sailed across oceans and traversed uncharted continents to map Earth, and in the past half a century space probes have photographed most of our solar system.
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