why might you expect to find giant elliptical galaxies at the centers of old, dense clusters?
Learning Objectives
By the end of this section, you will exist able to:
- Describe the properties and features of elliptical, spiral, and irregular galaxies
- Explain what may cause a galaxy'southward appearance to change over fourth dimension
Having established the being of other galaxies, Hubble and others began to observe them more closely—noting their shapes, their contents, and equally many other properties as they could mensurate. This was a daunting job in the 1920s when obtaining a single photo or spectrum of a milky way could accept a full dark of tireless observing. Today, larger telescopes and electronic detectors have fabricated this task less hard, although observing the most distant galaxies (those that show us the universe in its earliest phases) still requires enormous effort.
The starting time step in trying to understand a new type of object is oftentimes simply to describe it. Think, the first step in understanding stellar spectra was only to sort them co-ordinate to advent (see Analyzing Starlight). As it turns out, the biggest and nearly luminous galaxies come in 1 of two basic shapes: either they are flatter and accept spiral arms, like our ain Galaxy, or they announced to be elliptical (blimp- or cigar-shaped). Many smaller galaxies, in contrast, have an irregular shape.
Spiral Galaxies
Our ain Milky way and the Andromeda galaxy are typical, large screw galaxies. They consist of a key bulge, a halo, a disk, and screw arms. Interstellar textile is normally spread throughout the disks of spiral galaxies. Bright emission nebulae and hot, young stars are nowadays, especially in the spiral arms, showing that new star formation is still occurring. The disks are often dusty, which is especially noticeable in those systems that we view nearly edge on (Figure).
Effigy 1: Spiral Galaxies. (a) The spiral artillery of M100, shown here, are bluer than the residuum of the galaxy, indicating young, high-mass stars and star-forming regions. (b) We view this spiral galaxy, NGC 4565, almost exactly edge on, and from this bending, we can see the dust in the plane of the milky way; it appears dark because it absorbs the light from the stars in the galaxy. (credit a: modification of work by Hubble Legacy Archive, NASA, ESA, and Judy Schmidt; credit b: modification of work by "Jschulman555″/ Wikimedia)
In galaxies that we see face on, the bright stars and emission nebulae make the arms of spirals stand up out similar those of a pinwheel on the fourth of July. Open star clusters can exist seen in the arms of nearer spirals, and globular clusters are often visible in their halos. Screw galaxies contain a mixture of young and old stars, just equally the Milky way does. All spirals rotate, and the direction of their spin is such that the arms appear to trail much like the wake of a boat.
Almost two-thirds of the nearby screw galaxies accept boxy or peanut-shaped bars of stars running through their centers (Figure 2). Showing great originality, astronomers call these galaxies barred spirals.
Effigy 2: Barred Screw Milky way. NGC 1300, shown here, is a barred spiral galaxy. Notation that the spiral arms begin at the ends of the bar. (credit: NASA, ESA, and the Hubble Heritage Team(STScI/Aura))
As nosotros noted in The Milky Way Galaxy chapter, our Milky way has a small-scale bar besides. The spiral artillery unremarkably begin from the ends of the bar. The fact that bars are and so common suggests that they are long lived; it may exist that most spiral galaxies form a bar at some bespeak during their evolution.
In both barred and unbarred screw galaxies, we find a range of different shapes. At i extreme, the central bulge is big and luminous, the arms are faint and tightly coiled, and bright emission nebulae and supergiant stars are inconspicuous. Hubble, who developed a system of classifying galaxies by shape, gave these galaxies the designation Sa. Galaxies at this farthermost may have no clear screw arm construction, resulting in a lens-similar appearance (they are sometimes referred to as lenticular galaxies). These galaxies seem to share as many properties with elliptical galaxies as they do with spiral galaxies
At the other extreme, the key burl is minor and the artillery are loosely wound. In these Sc galaxies, luminous stars and emission nebulae are very prominent. Our Galaxy and the Andromeda milky way are both intermediate betwixt the ii extremes. Photographs of spiral galaxies, illustrating the different types, are shown in Effigy 3, forth with elliptical galaxies for comparison.
Figure iii: Hubble Classification of Galaxies. This effigy shows Edwin Hubble'southward original classification of galaxies. Elliptical galaxies are on the left. On the correct, y'all tin see the basic spiral shapes illustrated, alongside images of bodily barred and unbarred spirals. (credit: modification of work past NASA, ESA)
The luminous parts of spiral galaxies appear to range in bore from well-nigh twenty,000 to more than 100,000 light-years. Contempo studies have found that in that location is probably a large amount of galactic material that extends well beyond the credible edge of galaxies. This material appears to be sparse, cold gas that is difficult to observe in nigh observations.
From the observational data available, the masses of the visible portions of spiral galaxies are estimated to range from 1 billion to ane trillion Suns (tenix to x12 Chiliad Sun). The total luminosities of almost spirals autumn in the range of 100 meg to 100 billion times the luminosity of our Sun (108 to tenxi L Sun). Our Milky way and M31 are relatively large and massive, as spirals go. In that location is likewise considerable dark thing in and effectually the galaxies, merely every bit there is in the Milky Manner; we deduce its presence from how fast stars in the outer parts of the Galaxy are moving in their orbits.
Elliptical Galaxies
Elliptical galaxies consist almost entirely of old stars and have shapes that are spheres or ellipsoids (somewhat squashed spheres) (Effigy iv). They contain no trace of spiral arms. Their low-cal is dominated past older reddish stars (the population II stars discussed in The Milky way Galaxy). In the larger nearby ellipticals, many globular clusters tin be identified. Dust and emission nebulae are not conspicuous in elliptical galaxies, but many do comprise a small-scale amount of interstellar matter.
Figure 4: Elliptical Galaxies. (a) ESO 325-G004 is a behemothic elliptical galaxy. Other elliptical galaxies can be seen around the edges of this prototype. (b) This elliptical galaxy probably originated from the collision of two screw galaxies. (credit a: modification of work by NASA, ESA, and The Hubble Heritage Squad (STScI/AURA); credit b: modification of work past ESA/Hubble, NASA)
Elliptical galaxies evidence diverse degrees of flattening, ranging from systems that are approximately spherical to those that arroyo the flatness of spirals. The rare giant ellipticals (for case, ESO 325-G004 in Figure 4) reach luminosities of xxi L Sun. The mass in a giant elliptical can exist equally large as tenxiii Thousand Sun. The diameters of these large galaxies extend over several hundred thousand calorie-free-years and are considerably larger than the largest spirals. Although individual stars orbit the center of an elliptical milky way, the orbits are not all in the same direction, every bit occurs in spirals. Therefore, ellipticals don't announced to rotate in a systematic manner, making it hard to guess how much night affair they contain.
Nosotros observe that elliptical galaxies range all the mode from the giants, but described, to dwarfs, which may exist the most common kind of galaxy. Dwarf ellipticals (sometimes called dwarf spheroidals) escaped our notice for a long fourth dimension because they are very faint and difficult to meet. An example of a dwarf elliptical is the Leo I Dwarf Spheroidal galaxy shown in Figure v. The luminosity of this typical dwarf is about equal to that of the brightest globular clusters.
Intermediate between the giant and dwarf elliptical galaxies are systems such as M32 and M110, the two companions of the Andromeda galaxy. While they are often referred to every bit dwarf ellipticals, these galaxies are significantly larger than galaxies such equally Leo I.
Figure 5: Dwarf Elliptical Galaxy. M32, a dwarf elliptical milky way and i of the companions to the giant Andromeda galaxy M31. M32 is a dwarf by galactic standards, every bit it is simply 2400 light-years beyond. (credit: NOAO/AURA/NSF)
Irregular Galaxies
Hubble classified galaxies that do not have the regular shapes associated with the categories we simply described into the catchall bin of an irregular milky way, and we continue to use his term. Typically, irregular galaxies have lower masses and luminosities than screw galaxies. Irregular galaxies often appear disorganized, and many are undergoing relatively intense star formation activity. They contain both young population I stars and former population Ii stars.
The 2 best-known irregular galaxies are the Large Magellanic Cloud and Small Magellanic Cloud (Effigy half-dozen), which are at a distance of a little more than 160,000 lite-years away and are among our nearest extragalactic neighbors. Their names reflect the fact that Ferdinand Magellan and his crew, making their round-the-globe journey, were the get-go European travelers to discover them. Although non visible from the Us and Europe, these two systems are prominent from the Southern Hemisphere, where they await like wispy clouds in the night sky. Since they are only about one-tenth as afar every bit the Andromeda galaxy, they present an excellent opportunity for astronomers to written report nebulae, star clusters, variable stars, and other fundamental objects in the setting of another milky way. For example, the Big Magellanic Cloud contains the 30 Doradus complex (likewise known as the Tarantula Nebula), one of the largest and near luminous groups of supergiant stars known in any galaxy.
Figure 6: 4-Meter Telescope at Cerro Tololo Inter-American Observatory Silhouetted against the Southern Heaven. The Milky Way is seen to the correct of the dome, and the Large and Small Magellanic Clouds are seen to the left. (credit: Roger Smith/NOAO/Aura/NSF)
The Small Magellanic Cloud is considerably less massive than the Large Magellanic Cloud, and it is six times longer than it is wide. This narrow wisp of material points direct toward our Galaxy similar an arrow. The Minor Magellanic Cloud was near likely contorted into its current shape through gravitational interactions with the Milky Way. A big trail of debris from this interaction betwixt the Milky way and the Small Magellanic Cloud has been strewn beyond the heaven and is seen equally a series of gas clouds moving at abnormally high velocity, known equally the Magellanic Stream. We will run across that this kind of interaction betwixt galaxies will aid explicate the irregular shapes of this whole category of small galaxies,
View this beautiful album showcasing the different types of galaxies that have been photographed past the Hubble Space Telescope.
Galaxy Evolution
Encouraged past the success of the H-R diagram for stars (see Analyzing Starlight), astronomers studying galaxies hoped to discover some sort of comparable scheme, where differences in appearance could be tied to dissimilar evolutionary stages in the life of galaxies. Wouldn't it exist overnice if every elliptical galaxy evolved into a screw, for instance, only equally every principal-sequence star evolves into a red giant? Several elementary ideas of this kind were tried, some by Hubble himself, but none stood the test of time (and ascertainment).
Because no simple scheme for evolving one type of galaxy into some other could be found, astronomers and then tended to the opposite betoken of view. For a while, most astronomers idea that all galaxies formed very early in the history of the universe and that the differences between them had to do with the rate of star formation. Ellipticals were those galaxies in which all the interstellar matter was converted rapidly into stars. Spirals were galaxies in which star formation occurred slowly over the entire lifetime of the galaxy. This idea turned out to be too uncomplicated too.
Today, we understand that at to the lowest degree some galaxies have changed types over the billions of years since the universe began. As nosotros shall see in later chapters, collisions and mergers betwixt galaxies may dramatically modify spiral galaxies into elliptical galaxies. Even isolated spirals (with no neighbor galaxies in sight) can change their advent over time. As they consume their gas, the charge per unit of star formation will ho-hum downwardly, and the spiral artillery will gradually become less conspicuous. Over long periods, spirals therefore begin to look more like the galaxies at the heart of Figure three (which astronomers refer to every bit S0 types).
Over the past several decades, the study of how galaxies evolve over the lifetime of the universe has get i of the most active fields of astronomical research. Nosotros volition discuss the development of galaxies in more item in The Evolution and Distribution of Galaxies, but let'southward showtime meet in a little more detail just what dissimilar galaxies are similar.
Central concepts and summary
The majority of brilliant galaxies are either spirals or ellipticals. Spiral galaxies contain both one-time and young stars, also every bit interstellar affair, and have typical masses in the range of ten9 to x12 M Dominicus. Our own Galaxy is a large spiral. Ellipticals are spheroidal or slightly elongated systems that consist almost entirely of former stars, with very trivial interstellar matter. Elliptical galaxies range in size from giants, more than massive than any spiral, downward to dwarfs, with masses of only about 106 K Sun. Dwarf ellipticals are probably the most common type of milky way in the nearby universe. A modest percentage of galaxies with more disorganized shapes are classified as irregulars. Galaxies may modify their advent over time due to collisions with other galaxies or by a change in the rate of star formation.
Glossary
elliptical galaxy: a galaxy whose shape is an ellipse and that contains no conspicuous interstellar material
irregular galaxy: a galaxy without any clear symmetry or pattern; neither a spiral nor an elliptical galaxy
spiral galaxy: a flattened, rotating milky way with pinwheel-like artillery of interstellar textile and young stars, winding out from its cardinal bulge
Source: https://courses.lumenlearning.com/astronomy/chapter/types-of-galaxies/
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