A serious problem with ellipticals, however, is determination of their real shape: A flat elliptical may look round if seen from above or below or face‐on in the same manner that a dinner plate can look very different depending upon the position of the viewer. An E0 is a round galaxy, whereas an E6 is a rather flattened system (but not a disk in the sense of a flat spiral galaxy) (See Figure ). If the long axis is measured to have a dimension of a and the perpendicular short axis is measured as b, then an ellipticity can be defined as ϵ = 10 (1 – b/ a) rounded to the nearest unit, ϵ is used as a subtype to distinguish between ellipticals (E) with different shapes. Many ellipticals are round, but others are noticeably elongated or flattened. In other words, the brightness roughly falls off as the inverse square of the distance from the center of the galaxy. The decrease in surface brightness with distance may be expressed in different ways, but one reasonable approximation is I(r) = I ⊙/(a + r) 2 where I ⊙ is a central brightness, r is the distance from the center, and a is a distance at which the brightness is one‐quarter of that in the center. In general, they show no obvious structural features other than a smooth concentration of light to the center. Although originally based solely on optical appearance, appearance is also closely correlated with other physical properties of galaxies.Įllipticals (also sometimes called early‐type galaxies) were so named because they look like elliptical blobs of light. The Hubble classification has proven to be immensely valuable to the study of galaxies. A classification based on their optical appearance or morphology is limited to four fundamental types of galaxies-ellipticals, spirals, irregulars, and S0s. Hubble quickly realized that the vast majority of galaxies have only a small number of shapes. It is estimated that about 100 billion galaxies in the universe are observable with existing telescopes. Dwarf galaxies are by far the most numerous. More thorough studies show that galaxies exhibit a wide range of properties, from giant galaxies of 10 13 solar masses and sizes over 150,000 pc in diameter to dwarf galaxies of 10 6 solar masses and about 1,000 pc in size. Application of the Period‐Luminosity Relation for these variables in the Magellanic Clouds established that these were objects outside the confines of the Milky Way and of sizes comparable to it. The identification of other galaxies, or independent stellar systems, goes back to 1924, when Edwin Hubble found Cepheid variables in the nearby galaxies Messier 33 and Messier 31.
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