Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form more often in densely populated regions of space. They're called “lenticular” because they resemble lenses: Like spiral galaxies, they have a thin, rotating disk of stars and a central bulge, but they don't have spiral arms. Lenticular galaxies, such as the iconic Sombrero Galaxy, sit between elliptical and spiral galaxies. Few new stars are known to form in elliptical galaxies. Their stars orbit the galactic center, like those in the disks of spiral galaxies, but they do so in more random directions. Elliptical galaxies may also be small, in which case they are called dwarf elliptical galaxies.Įlliptical galaxies contain many older stars, but little dust and other interstellar matter. The universe's largest-known galaxies-giant elliptical galaxies-can contain up to a trillion stars and span two million light-years across. Our Milky Way, like other spiral galaxies, has a linear, starry bar at its center.Įlliptical galaxies are shaped as their name suggests: They are generally round but can stretch longer along one axis than along the other, so much so that some take on a cigar-like appearance. That spinning motion, at speeds of hundreds of kilometers a second, may cause matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel. A spiral galaxy has a flat, spinning disk with a central bulge surrounded by spiral arms. More than two-thirds of all observed galaxies are spiral galaxies. In 1936, Hubble debuted a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies. Despite the immense distance, Andromeda is the closest large galaxy to our Milky Way, and it's bright enough in the night sky that it's visible to the naked eye in the Northern Hemisphere. Since it is so far from us, it takes light from Andromeda more than 2.5 million years to bridge the gap. But in the 1920s, astronomer Edwin Hubble showed that the Andromeda “nebula” was a galaxy in its own right. Types of galaxiesīefore the 20th century, we didn't know that galaxies other than the Milky Way existed earlier astronomers had classified them as as “nebulae,” since they looked like fuzzy clouds. Some of those distant systems are similar to our own Milky Way galaxy, while others are quite different. One 2016 study estimated that the observable universe contains two trillion-or two million million-galaxies. The deeper we look into the cosmos, the more galaxies we see. In our own galaxy, the Milky Way, the sun is just one of about 100 to 400 billion stars that spin around Sagittarius A*, a supermassive black hole that contains as much mass as four million suns. Nearly all large galaxies are thought to also contain supermassive black holes at their centers. Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity.
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