Of the 100 billion or more galaxies visible through existing telescopes, most are either spiral or elliptical in shape and about one-fourth are irregular in shape. In addition to their shape, star systems can be further classified using other characteristics, such as the type of material they contain or the angle from which they're viewed. This interactive activity from NOVA Online invites users to familiarize themselves with spiral galaxies, such as our own Milky Way. The annotated visible light photographs and composites offer opportunities to learn about a galaxy's scale, composition, and rotational pattern.
Scientists look for order and patterns by classifying or systematically grouping organisms and objects according to their characteristics. From the time Edwin Hubble first established that other galaxies exist outside our own Milky Way, and created the galaxy classification system, astronomers have classified galaxies based on their general appearance.
A spiral galaxy, viewed from the side, may appear as a long, thin strip, interrupted only by a bright central bulge midway across. Viewed face-on, spiral galaxies have two easily discernible components. A spherical component contains the bright central (or nuclear) bulge and a halo that contains clusters of old stars, but is largely devoid of gas and dust. A disk component contains most of the galaxy's gas, dust, and stars. The stars move around the bulge in a nearly circular orbit within spiral arms that emerge directly from the central bulge or from a distinct bar that runs through the bulge. These spiral arms contain many fully formed stars but are also the nurseries in which new stars are formed. New star formation results from the compression of gases and dust and yields the hottest and brightest stars. Hot stars emit most of their light at short wavelengths -- in the blue and ultraviolet part of the spectrum -- which explains the bluish tint of the arms.
The Milky Way is a spiral galaxy. Our Sun is positioned about two-thirds of the way from the center to the edge. Gas and dust prevent observations inward or outward along the plane of the disk using visible light, so it is difficult to know its precise shape, whether it has two or more spiral arms, or what lies at its center. Radio and infrared telescopes provide some understanding because they can detect gases, objects, and heat using wavelengths outside the visible part of the spectrum.
More common than spirals-- though less easily detected -- are the elliptical galaxies. These galaxies contain no visible gas and dust and lack the hot, bright stars that indicate new star formation. The presence of older, giant stars may give these galaxies a reddish tint. They are classified according to numerical index range of E0 (round) to E7 (highly elliptical). Stars in ellipticals lack the well-defined rotation axis of spirals, although their orbital paths are not entirely random.
Irregular galaxies, as their name suggests, have no specific form and come in two types. Type I irregulars contain many young stars and, much like spirals, contain bright nebulae (clouds of gas and dust). Type II irregulars are known as interacting or disrupting galaxies. They contain a large amount of gas and dust (i.e., star-forming potential) and result from the merger, collision, or near-miss of two or more galaxies.
Academic standards correlations on Teachers' Domain use the Achievement Standards Network (ASN) database of state and national standards, provided to NSDL projects courtesy of JES & Co.
We assign reference terms to each statement within a standards document and to each media resource, and correlations are based upon matches of these terms for a given grade band. If a particular standards document of interest to you is not displayed yet, it most likely has not yet been processed by ASN or by Teachers' Domain. We will be adding social studies and arts correlations over the coming year, and also will be increasing the specificity of alignment.