Spiral galaxies get their name from the shape of their disks. In a spiral galaxy, the stars, gas and dust are gathered in spiral arms that spread outward from the galaxy's center.
Properties of Spiral Galaxies
The range of masses for spiral galaxies is ~ 109 - 1012 solar masses, with the typical mass being ~ 1011 solar masses. The typical range of luminosities corresponds to absolute blue magnitude -16 to -23, and the typical diameter of the visible disk is 5-100 kpc.
Properties of Spiral Galaxies
The range of masses for spiral galaxies is ~ 109 - 1012 solar masses, with the typical mass being ~ 1011 solar masses. The typical range of luminosities corresponds to absolute blue magnitude -16 to -23, and the typical diameter of the visible disk is 5-100 kpc.
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
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
Types of Spiral Galaxy
Spiral galaxies are grouped not only into ordinary spirals and barred spirals but it is also divided into three main types depending on how tightly wound their spiral arms are by means of a lettering system: a, b and c. The Hubble classification scheme, lists spiral galaxies as type S, and barred spiral galaxies have the symbol SB.
Sb's or SBb’s are between the two extremes, having moderately wound arms around an average sized nucleus.
Spiral galaxies are grouped not only into ordinary spirals and barred spirals but it is also divided into three main types depending on how tightly wound their spiral arms are by means of a lettering system: a, b and c. The Hubble classification scheme, lists spiral galaxies as type S, and barred spiral galaxies have the symbol SB.
- Sa or SBa galaxies have very tightly wound arms around a larger central nucleus.
- Sc or SBc galaxies have very loosely wound arms around a smaller nucleus.
Sb's or SBb’s are between the two extremes, having moderately wound arms around an average sized nucleus.
Barred spiral galaxies (like our Milky Way) contain a long bar in the middle with spirals arms coming off the ends. Around two-thirds of spiral galaxies contain a barred structure in their center. 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.