Galaxies are clusters of astronomical objects, with three primary forms: spiral, elliptical, and irregular. Spiral galaxies have a central hub with spiral arms, while elliptical galaxies lack arms and irregular galaxies have no structure. Most galaxies occur in superclusters with voids in between, and dark matter is responsible for their gravitational pull.
A galaxy is a cluster of stars, nebulae, dark matter and other astronomical objects. Most galaxies are tens of thousands of light-years in diameter and contain billions of stars. Galaxies come in three primary forms; spiral galaxies are thin disks, with spiral arms surrounding a central hub; elliptical galaxies are uniform agglomerations of oval shape; and irregular galaxies have little or no definite structure.
A spiral galaxy is centered around a hub, which is roughly spherical and protrudes outward from the disk. The hub is the gravitational center of the galaxy and usually contains a supermassive black hole. Surrounding the hub are spiral arms, which are piled-up waves of stars and gas that orbit the center. The spiral arms usually have more gas and dust and contain most of the newly formed stars, making them appear blue in color images. A spiral galaxy may have a horizontal bar passing through its hub; our galaxy, the Milky Way, is believed to contain a bar.
Elliptical galaxies have a large central hub, but lack the arms of a spiral galaxy; they usually appear more yellowish in color images than older reddish stars. Irregular galaxies often have no recognizable structure, but can often be recognized as having a distorted spiral or elliptical shape. Elliptical and irregular galaxies often result from collisions, which cause large bursts of star formation and distort the structure of galaxies due to complex gravitational interactions. Eventually, the two galaxies usually merge to form a large galaxy, which has expelled most of its clouds of gas and dust.
There are about one hundred billion galaxies in the visible universe; most of them occur in large clusters called superclusters of galaxies. Between these superclusters are voids with few or no galaxies, often spanning a hundred million light-years or more. By studying the rotation of spiral galaxies, astronomers have discovered that most of the matter in the universe is not stars and gas, but invisible “dark matter” that can’t be seen but still exerts gravity. Dark matter is thought to be responsible for these large clusters, as the gravitational pull of the dark matter has pulled the galaxies together.
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