The Milky Way Appears to Us as a Patchy and Irregular Band Across the Sky. What Accounts for This?
Learning Objectives
By the stop of this section, you lot will be able to:
- Describe the properties and features of elliptical, spiral, and irregular galaxies
- Explain what may cause a milky way'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 backdrop as they could measure. This was a daunting chore in the 1920s when obtaining a unmarried photograph or spectrum of a galaxy could have a full night of tireless observing. Today, larger telescopes and electronic detectors have fabricated this task less hard, although observing the near distant galaxies (those that bear witness u.s. the universe in its primeval phases) still requires enormous effort.
The first step in trying to empathize a new type of object is frequently simply to describe it. Remember, the first step in understanding stellar spectra was simply to sort them according to appearance (see Analyzing Starlight). Equally it turns out, the biggest and most luminous galaxies come in i of two basic shapes: either they are flatter and have spiral arms, similar our ain Galaxy, or they appear to be elliptical (blimp- or cigar-shaped). Many smaller galaxies, in contrast, have an irregular shape.
Screw Galaxies
Our own Milky way and the Andromeda galaxy are typical, large screw galaxies. They consist of a central burl, a halo, a deejay, and spiral arms. Interstellar material is usually spread throughout the disks of spiral galaxies. Bright emission nebulae and hot, immature stars are present, particularly 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 most edge on (Figure).
Figure i: Screw Galaxies. (a) The spiral arms of M100, shown hither, are bluer than the rest of the galaxy, indicating young, high-mass stars and star-forming regions. (b) We view this spiral milky way, NGC 4565, nearly exactly edge on, and from this angle, we tin 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 run into face on, the bright stars and emission nebulae brand the arms of spirals stand out like those of a pinwheel on the fourth of July. Open up star clusters can be seen in the arms of nearer spirals, and globular clusters are often visible in their halos. Screw galaxies incorporate a mixture of immature and one-time stars, just as the Galaxy 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.
Near 2-thirds of the nearby spiral galaxies have boxy or peanut-shaped bars of stars running through their centers (Figure 2). Showing great originality, astronomers call these galaxies barred spirals.
Effigy ii: Barred Spiral Galaxy. NGC 1300, shown here, is a barred spiral galaxy. Notation that the spiral artillery brainstorm at the ends of the bar. (credit: NASA, ESA, and the Hubble Heritage Team(STScI/Aura))
As we noted in The Galaxy Galaxy chapter, our Galaxy has a modest bar as well. The screw arms usually begin from the ends of the bar. The fact that bars are so common suggests that they are long lived; information technology may be that well-nigh spiral galaxies form a bar at some betoken during their evolution.
In both barred and unbarred spiral galaxies, we discover a range of different shapes. At one extreme, the central burl is large 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 extreme may have no articulate screw arm construction, resulting in a lens-like 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 farthermost, the key bulge is small and the arms are loosely wound. In these Sc galaxies, luminous stars and emission nebulae are very prominent. Our Galaxy and the Andromeda galaxy are both intermediate between the two extremes. Photographs of screw galaxies, illustrating the different types, are shown in Figure 3, along with elliptical galaxies for comparison.
Figure 3: Hubble Classification of Galaxies. This figure shows Edwin Hubble'due south original classification of galaxies. Elliptical galaxies are on the left. On the right, y'all can come across the bones spiral shapes illustrated, alongside images of actual barred and unbarred spirals. (credit: modification of piece of work by NASA, ESA)
The luminous parts of spiral galaxies appear to range in diameter from about 20,000 to more than 100,000 calorie-free-years. Recent studies have found that there is probably a large amount of galactic material that extends well beyond the credible edge of galaxies. This material appears to be thin, cold gas that is difficult to detect in most observations.
From the observational data available, the masses of the visible portions of spiral galaxies are estimated to range from 1 billion to i trillion Suns (ten9 to 1012 Thou Sun). The total luminosities of most spirals fall in the range of 100 one thousand thousand to 100 billion times the luminosity of our Sun (108 to 10eleven 50 Sun). Our Milky way and M31 are relatively large and massive, as spirals go. There is as well considerable nighttime thing in and effectually the galaxies, but as there is in the Galaxy; nosotros 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 one-time stars and accept shapes that are spheres or ellipsoids (somewhat squashed spheres) (Figure iv). They contain no trace of spiral artillery. Their calorie-free is dominated past older cherry-red stars (the population 2 stars discussed in The Milky Way Milky way). In the larger nearby ellipticals, many globular clusters can be identified. Grit and emission nebulae are not conspicuous in elliptical galaxies, merely many do contain a small amount of interstellar matter.
Figure 4: Elliptical Galaxies. (a) ESO 325-G004 is a behemothic elliptical galaxy. Other elliptical galaxies can be seen effectually the edges of this image. (b) This elliptical galaxy probably originated from the collision of two spiral galaxies. (credit a: modification of piece of work by NASA, ESA, and The Hubble Heritage Team (STScI/AURA); credit b: modification of work by ESA/Hubble, NASA)
Elliptical galaxies show various degrees of flattening, ranging from systems that are approximately spherical to those that approach the flatness of spirals. The rare giant ellipticals (for instance, ESO 325-G004 in Figure 4) reach luminosities of 1011 50 Sun. The mass in a giant elliptical can exist every bit big as tenthirteen M Sunday. The diameters of these big galaxies extend over several hundred grand light-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, as occurs in spirals. Therefore, ellipticals don't appear to rotate in a systematic way, making it difficult to estimate how much dark affair they comprise.
We notice that elliptical galaxies range all the way from the giants, only described, to dwarfs, which may be the most common kind of milky way. Dwarf ellipticals (sometimes called dwarf spheroidals) escaped our discover for a long time because they are very faint and difficult to see. An case of a dwarf elliptical is the Leo I Dwarf Spheroidal milky way shown in Figure 5. The luminosity of this typical dwarf is about equal to that of the brightest globular clusters.
Intermediate betwixt 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 every bit Leo I.
Figure v: Dwarf Elliptical Galaxy. M32, a dwarf elliptical galaxy and one of the companions to the giant Andromeda galaxy M31. M32 is a dwarf by galactic standards, as it is but 2400 calorie-free-years across. (credit: NOAO/AURA/NSF)
Irregular Galaxies
Hubble classified galaxies that do non take the regular shapes associated with the categories we just described into the catchall bin of an irregular galaxy, and we proceed to use his term. Typically, irregular galaxies have lower masses and luminosities than screw galaxies. Irregular galaxies often announced disorganized, and many are undergoing relatively intense star formation activity. They contain both young population I stars and quondam population Two stars.
The ii best-known irregular galaxies are the Big Magellanic Cloud and Modest Magellanic Cloud (Effigy 6), which are at a altitude of a fiddling 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 circular-the-world journey, were the first European travelers to notice them. Although not visible from the United states and Europe, these two systems are prominent from the Southern Hemisphere, where they look similar wispy clouds in the night sky. Since they are simply well-nigh 1-tenth as distant every bit the Andromeda milky way, they nowadays an excellent opportunity for astronomers to study nebulae, star clusters, variable stars, and other key objects in the setting of another galaxy. For instance, the Big Magellanic Cloud contains the thirty Doradus complex (besides known as the Tarantula Nebula), one of the largest and most luminous groups of supergiant stars known in any galaxy.
Figure 6: 4-Meter Telescope at Cerro Tololo Inter-American Observatory Silhouetted against the Southern Sky. The Milky way is seen to the right of the dome, and the Large and Small Magellanic Clouds are seen to the left. (credit: Roger Smith/NOAO/AURA/NSF)
The Modest Magellanic Cloud is considerably less massive than the Large Magellanic Deject, and it is six times longer than it is broad. This narrow wisp of material points directly toward our Milky way like an pointer. The Modest Magellanic Cloud was about probable contorted into its current shape through gravitational interactions with the Galaxy. A large trail of debris from this interaction between the Galaxy and the Small Magellanic Deject has been strewn across the heaven and is seen every bit a series of gas clouds moving at abnormally high velocity, known equally the Magellanic Stream. Nosotros will see that this kind of interaction between galaxies volition help explain the irregular shapes of this whole category of pocket-size galaxies,
View this beautiful album showcasing the unlike types of galaxies that have been photographed by the Hubble Space Telescope.
Galaxy Evolution
Encouraged by the success of the H-R diagram for stars (see Analyzing Starlight), astronomers studying galaxies hoped to notice some sort of comparable scheme, where differences in appearance could be tied to different evolutionary stages in the life of galaxies. Wouldn't it exist nice if every elliptical galaxy evolved into a screw, for instance, just every bit every main-sequence star evolves into a red behemothic? Several elementary ideas of this kind were tried, some past Hubble himself, only none stood the test of time (and ascertainment).
Considering no simple scheme for evolving 1 type of galaxy into another could exist found, astronomers and then tended to the contrary point of view. For a while, most astronomers thought 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 thing was converted speedily into stars. Spirals were galaxies in which star germination occurred slowly over the entire lifetime of the galaxy. This idea turned out to be too unproblematic as well.
Today, nosotros understand that at least some galaxies have changed types over the billions of years since the universe began. As we shall encounter in later capacity, collisions and mergers between galaxies may dramatically change spiral galaxies into elliptical galaxies. Fifty-fifty isolated spirals (with no neighbor galaxies in sight) can modify their appearance over time. As they consume their gas, the charge per unit of star formation will slow downwardly, and the screw artillery will gradually get less conspicuous. Over long periods, spirals therefore begin to look more like the galaxies at the middle of Effigy 3 (which astronomers refer to as S0 types).
Over the past several decades, the study of how galaxies evolve over the lifetime of the universe has become i of the virtually active fields of astronomical research. We will talk over the evolution of galaxies in more particular in The Evolution and Distribution of Galaxies, but permit's offset see in a picayune more particular but what unlike galaxies are like.
Fundamental concepts and summary
The majority of bright galaxies are either spirals or ellipticals. Spiral galaxies contain both sometime and immature stars, too every bit interstellar thing, and accept typical masses in the range of 109 to 1012 K Sun. Our own Galaxy is a large spiral. Ellipticals are spheroidal or slightly elongated systems that consist almost entirely of old stars, with very niggling interstellar thing. Elliptical galaxies range in size from giants, more than massive than any spiral, down to dwarfs, with masses of but about 10vi 1000 Sun. Dwarf ellipticals are probably the most mutual blazon of milky way in the nearby universe. A modest percentage of galaxies with more disorganized shapes are classified as irregulars. Galaxies may alter their appearance over time due to collisions with other galaxies or past a modify in the rate of star germination.
Glossary
elliptical milky way: a galaxy whose shape is an ellipse and that contains no conspicuous interstellar material
irregular galaxy: a milky way without any clear symmetry or design; neither a spiral nor an elliptical milky way
spiral milky way: a flattened, rotating milky way with pinwheel-like artillery of interstellar material and young stars, winding out from its central bulge
pasleymonexte1974.blogspot.com
Source: https://courses.lumenlearning.com/astronomy/chapter/types-of-galaxies/