Their stars orbit the galactic center, like those in the disks of spiral galaxies, but they do so in more random directions.
Few new stars are known to form in elliptical galaxies. They are common in galaxy clusters. Lenticular galaxies , such as the iconic Sombrero Galaxy , sit between elliptical and spiral galaxies. Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form more often in densely populated regions of space.
Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Irregular galaxies—such as the Large and Small Magellanic Clouds that flank our Milky Way—appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by. They are full of gas and dust, which makes them great nurseries for forming new stars. Some galaxies occur alone or in pairs, but they are more often parts of larger associations known as groups, clusters, and superclusters.
Our Milky Way, for instance, is in the Local Group , a galaxy group about 10 million light-years across that also includes the Andromeda galaxy and its satellites. The Local Group and its neighbor galaxy cluster, the Virgo Cluster , both lie within the larger Virgo Supercluster , a concentration of galaxies that stretches about million light-years across.
The Virgo Supercluster, in turn, is a limb of Laniakea, an even bigger supercluster of , galaxies that astronomers defined in Galaxies in clusters often interact and even merge together in a dynamic cosmic dance of interacting gravity. When two galaxies collide and intermingle, gases can flow towards the galactic center, which can trigger phenomena like rapid star formation. Our own Milky Way will merge with the Andromeda galaxy in about 4.
Because elliptical galaxies contain older stars and less gas than spiral galaxies, it seems that the galaxy types represent part of a natural evolution: As spiral galaxies age, interact, and merge, they lose their familiar shapes and become elliptical galaxies. But astronomers are still working out the specifics, such as why elliptical galaxies follow certain patterns in brightness, size, and chemical composition. The universe's first stars ignited some million years after the big bang, the explosive moment Gravity had sculpted the first galaxies into shape by the time the universe turned million years old , or less than 3 percent of its current age.
The other galaxies are even further away from us and can only be seen through telescopes. The smallest galaxies may contain only a few hundred thousand stars and be several thousand light years across, while the largest galaxies have trillions of stars and may be hundreds of thousands of light years across. Galaxies can be found by themselves, in small groups and in large clusters. It is very rare to find stars in the space in between galaxies. Galaxies sometimes collide with each other, with interesting results.
These collisions can trigger bursts of star-formation in addition to changing the shapes of the galaxies that collide. However, when galaxy collisions occur, individual stars do not collide, due to the vast distances between them.
Interacting galaxies imaged by the Hubble Space Telescope. Ford JHU , G. Other galaxies may contain quasars — the most energetic bodies in the universe — at their core.
Galaxies are classified by their shape. Each type has different characteristics and a different history of evolution. Some, like the Milky Way, have arms spiraling outward around their center. Known as spiral galaxies , these groups make up most of the galaxies that astronomers can see. The gas and dust in a spiral galaxy circles the center at speeds of hundreds of miles per second, creating their pinwheel shape.
Some, known as "barred spirals," have a bar structure in their center, formed by dust and gas funneled into the center. The dust and gas in spiral galaxies are consistently fueling the formation of new stars. Elliptical galaxies lack the spiral arms of their more flamboyant cousins. Their appearance ranges from circular to very stretched out. Elliptical galaxies have less dust than their spiral counterparts, and so the star-making process has all but ended.
Most of their stars are older. Although they make up a smaller portion of the visible galaxies, astronomers think that over half the galaxies in the universe are elliptical. The remaining 3 percent of the galaxies in the universe are known as irregular galaxies. They are neither round nor boast spiral arms, and their shapes lack specific definition. While Hubble is unable to see the very first galaxies, it can track the development of galaxies over much of cosmic time.
The series of Hubble Deep Field images and other deep surveys have uncovered galaxies at many different distances out in the universe, and therefore at many different times in their development. Farther galaxies, seen earlier in time, have less developed structure.
Nearer galaxies, seen later in time, grow to resemble the familiar galaxy shapes we see in the local universe. The Building Blocks of the Universe. On This Page. Types of Galaxies. What Kinds of Galaxies Are There? This graphic compares illustrations of the three main types of galaxies top with actual photos of galaxies bottom that fit the categories.
Credit: A. Feild STScI. Dark Matter. What Is Dark Matter? This detailed view of our galactic next-door neighbor, the Andromeda galaxy, contains over million resolved stars and thousands of star clusterss. The panorama sweeps from the galaxy's central bulge across lanes of stars and dust to the sparser outer disk.
Dalcanton, B. Williams, and L. The Bullet Cluster was formed after the collision of two large clusters of galaxies. Hot gas detected by Chandra in X-rays is seen as two pink clumps in the image and contains most of the "normal," or baryonic, matter in the two clusters.
The bullet-shaped clump on the right is the hot gas from one cluster, which passed through the hot gas from the other larger cluster during the collision. An optical image from the Hubble and Magellan telescopes shows the galaxies in orange and white.
The blue areas in this image depict where astronomers find most of the mass in the clusters. Most of the matter in the clusters blue is clearly separate from the normal matter pink , giving direct evidence that nearly all of the matter in the clusters is dark. Markevitch et al. Clowe et al.
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