What is a galaxy? What are the kinds of galaxies? Information about galaxy.
GALAXY; a large star system that contains millions or billions of stars. A total of approximately 500 million galaxies is estimated to be within reach of the largest reflecting telescopes, which can probe to a distance of at least 5 billion light-years. The faintly glimmering band of the Milky Way across the sky at night marks the central plane of our own galaxy, the Milky Way system. All of the individual stars that are visible to the naked eye or are within reach of binoculars also belong to our galaxy, which contains in all about 100 billion stars. Our sun, a star of average mass and brightness, is situated in the outskirts of the galaxy, close to the central galactic plane. The greatest diameter of our galaxy is about 100,000 light-years.
In the Northern Hemisphere, one of the galaxies beyond our own star system can be seen with the naked eye—the galaxy in the constellation Andromeda. It is often referred to as Messier 31 because it is the 31st entry in the catalog of nebulae prepared by the comet-hunting French astronomer Charles Messier in the late 18th century. Messier 31 is similar to our galaxy but is about twice as large in diameter. A good pair of binoculars or a small telescope reveals two smaller galaxies that accompany Messier 31. Observers in the Southern Hemisphere can view the two companions of our own galaxy—the Large Magellanic Cloud and the Small Magellanic Cloud. All of these nearby galaxies appear nebulous to the naked eye or through telescopes, but on close photographic inspection they can be resolved into their individual stars. Galaxies at distances more than twice that of Messier 31 are too far away for individual stars to be seen.
KINDS OF GALAXIES
There are four major classes of galaxies: spiral, barred spiral, elliptical, and irregular.
Spiral. Within the class of spiral galaxies (generally referred to by the letter S) there are several varieties. In the most common classification system, developed by the U. S. astronomer Edwin P. Hubble in the 1920’s, the normal spiral galaxies are subdivided into three subclasses: Sa, Sb, and Sc. The first of these, Sa, is the most tightly wound spiral, and the last, Sc, is the most open. Messier 31 is an Sb spiral, while our own galaxy is an Sb that approaches Sc in the openness of its spirals.
Barred Spiral. Hubble and other astronomers noted that many spiral galaxies, on photographs, show a barred structure. That is, their arms seem to emanate from both ends of a luminous bar that passes through the center of the galaxy. Barred spirals number about one fourth of the total number of galaxies, and there are very few among the brighter galaxies. Some astronomers classify the Magellanic Clouds as barred spirals. Hubble subdivided the barred spirals into SBa, SBb, and SBc varieties, a sequence that parallels the sequence of the normal spirals.
Elliptical. Whereas spiral galaxies are rich in very luminous stars, interstellar gas, and cosmic dust, the elliptical galaxies contain little gas and dust, and their stars are rather dim. They are classified as E0 to E7 according to the degree of flattening of their elliptical shape. E0 represents the nearly spherical systems and E7 the most highly flattened ones. The number after the E classification is equal to 10 X (a — b)/a, where a is the major axis of the ellipse and b is the minor axis. Small elliptical galaxies are called dwarf ellipticals, or dE. There is also a transition class, SO, between the most flattened elliptical galaxies and the most tightly wound spirals.
Irregular and Other. Irregular galaxies are small and show no obvious shape or organization. The number of irregulars in any catalog of galaxies is generally no more than 2% or 3% of the total number in the catalog.
The galaxies described are normal galaxies. Many other galaxies emit radiation in the radio region of the electromagnetic spectrum, however, as well as in the optical range. Some of these Tadio galaxies are intrinsically very luminous, especially the starlike quasars that outshine normal galaxies by a factor of 10,000 or more.