Spiral galaxies take their name from the winding spiral shape they demonstrate. Most of the galaxies in the universe observed by scientists are spiral galaxies. These twisted collections of stars and gas often have beautiful shapes and are made up of hot young stars.
Credit: NASA/ESA/Hubble Heritage Team
Characteristics and classifications
Most spiral galaxies contain a central bulge surrounded by a flat rotating disk of stars. Made up of older, dimmer stars, the bulge in the center is thought to contain a supermassive black hole, though observing it can be a challenge. The dim light from the older stars can make the bulge difficult to pinpoint, and there are some spirals that lack this characteristic. The supermassive black hole thought to exist at the center is often blocked by dust and gas surrounding it.
Orbiting the bulge, a rotating disk gives the galaxy its distinctive classification. The disk separates itself into arms that circle the galaxy. These spiral arms contain young stars that shine brightly before their quick demise, as well as a wealth of gas and dust. The brilliant stars are the reason the arms are so well defined.
The exact mechanism for the formation of the spiral arms continues to puzzle scientists. If they were permanent features of the galaxy, they would soon wind up tightly and disappear in less than a billion years. Scientists think they could be a result of density waves traveling through the outer disk. Encounters between galaxies could be a cause of these waves. As two galaxies come near collision, the mass of the one about to be consumed could affect the structure of larger. Another potential cause deals with how the galaxy was initially formed.
Approximately two-thirds of spiral galaxies contain a barred structure through their center, making unbarred spiral galaxies a minority. The Milky Way is one of these, though its bar is a challenge to see and was not suspected until the 1990s. Its barred structure was confirmed in 2005.
In 1926, Edwin Hubble devised a system to classify galaxies. Known as the Hubble sequence, or the "Hubble tuning-fork", it organizes galaxies based on their shape. Spiral galaxies are classified by how tightly wound their arms are, as well as by the presence or absence of a bar.
Spiral galaxies make up roughly 77 percent of the galaxies that scientists have observed. However, they are not thought to be the dominant galaxy type. That honor goes to elliptical galaxies, which spirals are thought to ultimately degrade into. Because elliptical galaxies are made up of older, dimmer stars, they are more challenging to spot. In large, in-depth surveys of patches of the sky, elliptical galaxies have dominated, leading scientists to conclude that they are prevalent throughout the rest of the universe.
Credit: NASA's Goddard Space Flight Center/ESO/JPL-Caltech/DSS
History and formation
Spiral galaxies are filled with gas and dust, which results in a wealth of star formation. They are considered to be younger than elliptical galaxies, which contain less dust and form fewer stars.
Spiral galaxies come in a wide variety of shapes. Roughly 60 percent of spiral galaxies contain multiple arms, while another 10 percent have only two. Approximately 30 percent of spiral galaxies lack well-defined arms, as their features have faded over time.
Credit: ESO/P. Barthel Acknowledgments: Mark Neeser (Kapteyn Institute, Groningen) and Richard Hook (ST/ECF, Garching, Germany).
These twisted galaxies range from a billion to a trillion times as massive as the sun. The visible disk can be anywhere from 10 to 300 thousand light-years across. The largest known spiral galaxy is NGC 6872, which is 522,000 light-years across from the tips of its outstretched spiral arms, about 5 times the size of the Milky Way.
In the early universe, galaxies frequently collided and interacted with one another, so the spiral shape of these ancient giants would have been quickly disrupted. The oldest observed spiral galaxy, BX442, is approximately 10.7 billion years old. Because of the correlation between the distance and the amount of time it takes light to travel, scientists are able to see the galaxy only 3 billion years after the Big Bang formed the universe. [Big Bang to Now in 10 Easy Steps]
According to current scientific thought, as spirals burn through their gas and dust and star formation slows, they will lose their spiral shape and move on to the next stage of galactic evolution, elliptical galaxies.
