A spiral galaxy centers around a hub, which is roughly spherical & bulges outward from the disk. The hub is the gravitational middle of the galaxy, & usually contains a supermassive black hole. Surrounding the hub are spiral arms, which are bunched-up waves of stars & gas orbiting the middle. The spiral arms usually have more gas & dust, & they contain most of the newly formed stars, which make them appear blue in color imagery. A spiral galaxy may have a horizontal bar passing through its hub; our own galaxy, the Milky Way, is believed to contain a bar.
A galaxy is a cluster of stars, nebulae, dark matter, & other astronomical objects. Most galaxies are tens of thousands of light years in diameter, contain billions of stars. Galaxies come in primary shapes; spiral galaxies are narrow disks, with spiral arms surrounding a central hub; elliptical galaxies are uniform, oval-shaped agglomerations; irregular galaxies have tiny or no positive structure.
Elliptical galaxies have a massive, central hub, but they lack the arms of a spiral galaxy; they usually appear more yellowish in color imagery from elderly, reddish stars. Irregular galaxies often have no discernible structure, but often they can be recognized as having a distorted spiral or elliptical shape. Elliptical and irregular galaxies often result from collisions, which cause massive bursts of star formation and distort the galaxies' structure due to the complex gravitational interactions. Finally, the galaxies usually combine to form massive galaxy, which has cast off most of its gas and dust clouds.
There's roughly a hundred billion galaxies in the visible universe; most of them occur in massive agglomerations called galaxy superclusters. Between these superclusters are voids with few or no galaxies, often for a hundred million light years or more. By studying the rotation of spiral galaxies, astronomers found that most of the matter in the universe is not stars and gas, but invisible �dark matter� which cannot be seen but still exerts gravity. Dark matter is thought to be responsible for these massive clusters, as the gravitational attraction of the dark matter pulled galaxies together.
A galaxy is a cluster of stars, nebulae, dark matter, & other astronomical objects. Most galaxies are tens of thousands of light years in diameter, contain billions of stars. Galaxies come in primary shapes; spiral galaxies are narrow disks, with spiral arms surrounding a central hub; elliptical galaxies are uniform, oval-shaped agglomerations; irregular galaxies have tiny or no positive structure.
Elliptical galaxies have a massive, central hub, but they lack the arms of a spiral galaxy; they usually appear more yellowish in color imagery from elderly, reddish stars. Irregular galaxies often have no discernible structure, but often they can be recognized as having a distorted spiral or elliptical shape. Elliptical and irregular galaxies often result from collisions, which cause massive bursts of star formation and distort the galaxies' structure due to the complex gravitational interactions. Finally, the galaxies usually combine to form massive galaxy, which has cast off most of its gas and dust clouds.
There's roughly a hundred billion galaxies in the visible universe; most of them occur in massive agglomerations called galaxy superclusters. Between these superclusters are voids with few or no galaxies, often for a hundred million light years or more. By studying the rotation of spiral galaxies, astronomers found that most of the matter in the universe is not stars and gas, but invisible �dark matter� which cannot be seen but still exerts gravity. Dark matter is thought to be responsible for these massive clusters, as the gravitational attraction of the dark matter pulled galaxies together.