Galaxies are the major building blocks of the universe. They are large masses of stars and gases (from several million to several billion stars) galaxies are found in large groups or "clusters" of galaxies,and all expand away from each other as the universe continues to expand. Galaxies are some of the most treasured objects for visual and photographic studies. They reveal the cronological evolution of our universe.
Galaxies formed very early in the Universe's life and continue to form and change today. Four general categories describe basic galaxy structure. They are elliptical, spiral, barred spiral and irregular. Elliptical galaxies are subcategorized as E0 through E7, E0 being nearly spherical without internal structure to E7 being lenticular without internal structure. Spiral galaxies are S0,Sa,Sb and Sc. S0 is a spiral galaxy with an outer ring of stars. Sa is a multiple spiral armed galaxy where the arms are tightly "wrapped" around the nucleus. Sb has fewer arms that are loosely "wrapped" compared to Sa galaxy arms. Our Milky Way galaxy is an Sb type. Sc galaxies are usually two-armed galaxies. Barred spiral galaxies are similar to Sa,Sb and Sc galaxies but exhibit a bar structure connecting the nucleus to the first arm(s) on either side of the galaxy. Irregular galaxies exhibit no uniform structure overall and have no discernable arms. Structure densities vary greatly. Irregular galaxies seem to form from "collisions" with other galaxies. External galaxies show the same types of stars, clusters and gaseous nebulae that our Milky Way exhibits (Only the abundances vary). Also many exhibit evidence of black hole formation in their nuclei. BlackHoles exist when matter density is great enough to cause the gravitational retraction of matter and radiation. Our galaxy appears to have a black hole in it's nucleus. Many earlier galaxies contain Quasars in their cores. Quasars are huge black hole accretion disks that are much brighter than their host galaxy. Scientists are continuing to study galaxy morphology. Current studies show early galaxies exhibit prolific star formation and heavy element production compared to later galaxies as a result of higher galaxy collision rates.
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