Irregular galaxy

As you can imagine, not all of these galaxies have the same characteristics, and they definitely don’t look the same. Astronomers have recognized several types of galaxies according to their visual appearance. This galaxy morphological classification system, known as the Hubble sequence, or Hubble Tuning Fork, was invented by American astronomer Edwin Hubble in 1926, and it’s a significant part of the study of galaxy evolution . Hubble sequence expanded by Gérard de Vaucouleurs. The scheme divides galaxies into categories based on their shape. It is roughlydivided into elliptical galaxies and spiral galaxies. Hubble gave the elliptical galaxies numbers from zero to seven, with E0 galaxies having an almost round shape and E7 very stretched out and elliptical. The spiral galaxies were given letters from "a" to "c," with "Sa" galaxies appearing more tightly wound and "Sc" galaxies more loosely wound. The spiral galaxies were furthersub-divided into normal spirals and barred spirals (which have a B in their designation), with barred spirals containing a bar of stars running through the central bulge. Lenticular galaxies, designated S0, represent a transition between ellipticals and spirals. Hubble also found that some galaxies did not fit into this classification system - they had odd shapes, were very small or very large, etc. These are termed irregular galaxies. The Hubble system was later extended byGérard de Vaucouleurs, whoargued that rings and lenses are also important s...

\( \newcommand\) • • • • • • • Learning Objectives By the end of this section, you will be able to: • Describe the properties and features of elliptical, spiral, and irregular galaxies • Explain what may cause a galaxy’s appearance to change over time Having established the existence of other galaxies, Hubble and others began to observe them more closely—noting their shapes, their contents, and as many other properties as they could measure. This was a daunting task in the 1920s when obtaining a single photograph or spectrum of a galaxy could take a full night of tireless observing. Today, larger telescopes and electronic detectors have made this task less difficult, although observing the most distant galaxies (those that show us the universe in its earliest phases) still requires enormous effort. The first step in trying to understand a new type of object is often simply to describe it. Remember, the first step in understanding stellar spectra was simply to sort them according to appearance (see Analyzing Starlight in Chapter 10). As it turns out, the biggest and most luminous galaxies come in one of two basic shapes: either they are flatter and have spiral arms, like our own Galaxy, or they appear to be elliptical (blimp- or cigar-shaped). Many smaller galaxies, in contrast, have an irregular shape. Spiral Galaxies Our own Galaxy and the Andromeda galaxy are typical, large spiral galaxies (see Figure \(26.1.1\) in Section 26.1). They consist of a central bulge, a halo,...

A galaxy that does not have the clearly defined shape and structure of typical elliptical, lenticular, or spiral galaxies. Irregular galaxies typically contain large amounts of gas and dust, and their stars are often young. They account for only a small percentage of known galaxies. Some irregular galaxies are the result of gravitational interactions or collisions between formerly regular galaxies. Many irregular galaxies orbit larger regular ones; the Magellanic Cloud galaxies orbiting the Milky Way are examples. Compare elliptical galaxy lenticular galaxy spiral galaxy. See more at Hubble classification system.

These systems exhibit certain characteristic properties. They have complete rotational symmetry; i.e., they are figures of revolution with two equal principal axes. They have a third smaller axis that is the presumed axis of rotation. The surface brightness of ellipticals at optical wavelengths decreases monotonically outward from a maximum value at the centre, following a common mathematical law of the form: I = I 0( r/ a +1 ) −2, where I is the intensity of the light, I 0 is the central intensity, r is the radius, and a is a scale factor. The isophotal b/ a = 0.3, with b and a the minor and major axes of the elliptical image, respectively. Ellipticals contain neither Subclasses of elliptical galaxies are defined by their apparent shape, which is of course not necessarily their three-dimensional shape. The n, where n is an integer defined by n = 10( a− b)/ a. A perfectly circular image will be an E0 galaxy, while a flatter object might be an E7 galaxy. (As explained above, elliptical galaxies are never flatter than this, so there are no E8, E9, or E10 galaxies.) Spirals are characterized by circular symmetry, a bright nucleus surrounded by a thin outer disk, and a superimposed spiral structure. They are divided into two parallel classes: normal spirals and barred spirals. The normal spirals have arms that emanate from the nucleus, while barred spirals have a bright linear feature called a bar that straddles the nucleus, with the arms unwinding from the ends of the bar. Th...