12.1 The Milky Way Galaxy

Milky Way Galaxy

The Milky Way. The central bulge of the Milky Way (to the right of center) shines brightly in the dark Utah skies. Silhouetted in the foreground is Square Tower in Hovenweep National Monument.
Figure 12.1. The Milky Way rises over Square Tower, an ancestral pueblo building at Hovenweep National Monument in Utah. Many stars and dark clouds of dust combine to make a spectacular celestial sight of our home Galaxy. The location has been designated an International Dark Sky Park by the International Dark Sky Association.
The Milky Way above Hovenweep Castle by NPS/Jacob W. Frank, Public Domain

Today, we know that our Sun is just one of the many billions of stars that make up the huge cosmic island we call the Milky Way Galaxy. How can we “weigh” such an enormous system of stars and measure its total mass?

One of the most striking features you can see in a truly dark sky—one without light pollution—is the band of faint white light called the Milky Way, which stretches from one horizon to the other. The name comes from an ancient Greek legend that compared its faint white splash of light to a stream of spilled milk. But folktales differ from culture to culture: one East African tribe thought of the hazy band as the smoke of ancient campfires, several Native American stories tell of a path across the sky traveled by sacred animals, and in Siberia, the diffuse arc was known as the seam of the tent of the sky.

In 1610, Galileo made the first telescopic survey of the Milky Way and discovered that it is composed of a multitude of individual stars. Today, we know that the Milky Way comprises our view inward of the huge cosmic pinwheel that we call the Milky Way Galaxy and that is our home. Moreover, our Galaxy is now recognized as just one galaxy among many billions of other galaxies in the cosmos.

The Milky Way Galaxy surrounds us, and you might think it is easy to study because it is so close. However, the very fact that we are embedded within it presents a difficult challenge. Suppose you were given the task of mapping New York City. You could do a much better job from a helicopter flying over the city than you could if you were standing in Times Square. Similarly, it would be easier to map our Galaxy if we could only get a little way outside it, but instead we are trapped inside and way out in its suburbs—far from the galactic equivalent of Times Square.

With modern instruments, astronomers can now penetrate the “smog” of the Milky Way by studying radio and infrared emissions from distant parts of the Galaxy. Measurements at these wavelengths (as well as observations of other galaxies like ours) have given us a good idea of what the Milky Way would look like if we could observe it from a distance.

Figure 12.2 sketches what we would see if we could view the Galaxy face-on and edge-on. The brightest part of the Galaxy consists of a thin, circular, rotating disk of stars distributed across a region about 100,000 light-years in diameter and about 1000 light-years thick. (Given how thin the disk is, perhaps a CD is a more appropriate analogy than a wheel.) In addition to stars, the dust and gas from which stars form are also found mostly in the thin disk of the Galaxy. The mass of the interstellar matter is about 15% of the mass of the stars in this disk.

Schematic Representation of the Galaxy

A schematic representation of the Milky Way Galaxy. On the left is the face-on view of the spiral disk, with the central bar in the center, the Cygnus spiral arm on the lower left, the Perseus arm labelled on the bottom, the smaller Orion spur labelled above that, and the Carina arm labelled on the right. On the right of the schematic is the edge-on view of the spiral disk, surrounded by serval globular clusters. The nuclear bulge is labelled in the center of both views, and the Sun is labelled on the Orion spur. The distance between the Sun and the nuclear bulge is labelled 26,000 light years.
Figure 12.2. The left image shows the face-on view of the spiral disk; the right image shows the view looking edge-on along the disk. The major spiral arms are labelled. The Sun is located on the inside edge of the short Orion spur.

As the diagram in Figure 12.2 shows, the stars, gas, and dust are not spread evenly throughout the disk but are concentrated into a central bar and a series of spiral arms. Recent infrared observations have confirmed that the central bar is composed mostly of old yellow-red stars. The two main spiral arms appear to connect with the ends of the bar. They are highlighted by the blue light from young hot stars. We know many other spiral galaxies that also have bar-shaped concentrations of stars in their central regions; for that reason they are called barred spirals. Figure 12.3 shows two other galaxies—one without a bar and one with a strong bar—to give you a basis for comparison to our own. We will describe our spiral structure in more detail shortly. The Sun is located about halfway between the centre of the Galaxy and the edge of the disk and only about 70 light-years above its central plane.

Unbarred and Barred Spiral Galaxies

Unbarred and Barred Spiral Galaxies. Panel (a), at left, shows the beautifully symmetric spiral form of M74. The blue spiral arms and dust lanes spiral neatly into the bright nucleus at center. Panel (b), at right, shows the barred spiral NGC 1365. A bar of yellow stars projects out from the nucleus at center, with a nearly straight blue arm at each end of the bar.
Figure 12.3. (a) This image shows the unbarred spiral galaxy M74. It contains a small central bulge of mostly old yellow-red stars, along with spiral arms that are highlighted with the blue light from young hot stars. (b) This image shows the strongly barred spiral galaxy NGC 1365. The bulge and the fainter bar both appear yellowish because the brightest stars in them are mostly old yellow and red giants. Two main spiral arms project from the ends of the bar. As in M74, these spiral arms are populated with blue stars and red patches of glowing gas—hallmarks of recent star formation. The Milky Way Galaxy is thought to have a barred spiral structure that is intermediate between these two examples.
Credit a: PESSTO snaps Supernova in Messier 74 by ESO/PESSTO/S. Smartt, CC-BY-4.0.
Credit b: Fine Details in a Barred Galaxy by ESO, CC BY-4.0.

Our thin disk of young stars, gas, and dust is embedded in a thicker but more diffuse disk of older stars; this thicker disk extends about 3000 light-years above and below the midplane of the thin disk and contains only about 5% as much mass as the thin disk.

Close in to the galactic centre (within about 10,000 light-years), the stars are no longer confined to the disk but form a central bulge (or nuclear bulge). When we observe with visible light, we can glimpse the stars in the bulge only in those rare directions where there happens to be relatively little interstellar dust. The first picture that actually succeeded in showing the bulge as a whole was taken at infrared wavelengths as shown in Figure 12.4.

Inner Part of the Milky Way Galaxy

Infrared Image of the Inner Part of the Milky Way Galaxy. This 2MASS image, centered on the central bulge, perfectly illustrates how thin and flat is the disk of our galaxy.
Figure 12.4. This beautiful infrared map, showing half a billion stars, was obtained as part of the Two Micron All Sky Survey (2MASS). Because interstellar dust does not absorb infrared as strongly as visible light, this view reveals the previously hidden bulge of old stars that surrounds the centre of our Galaxy, along with the Galaxy’s thin disk component.
Two Micron All Sky Survey by NASA/2MASS, NASA Media License.

The fact that much of the bulge is obscured by dust makes its shape difficult to determine. For a long time, astronomers assumed it was spherical. However, infrared images and other data indicate that the bulge is about two times longer than it is wide, and shaped rather like a peanut. The relationship between this elongated inner bulge and the larger bar of stars remains uncertain. At the very centre of the nuclear bulge is a tremendous concentration of matter, which we will discuss later in this chapter.

In our Galaxy, the thin and thick disks and the nuclear bulge are embedded in a spherical halo of very old, faint stars that extends to a distance of at least 150,000 light-years from the galactic centre. Most of the globular clusters are also found in this halo.

The mass in the Milky Way extends even farther out, well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the centre of the Galaxy. This invisible mass has been give the name dark matter because it emits no light and cannot be seen with any telescope. Its composition is unknown, and it can be detected only because of its gravitational effects on the motions of luminous matter that we can see. We know that this extensive dark matter halo exists because of its effects on the orbits of distant star clusters and other dwarf galaxies that are associated with the Galaxy.

Some vital statistics of the thin and thick disks and the stellar halo are given in Table 12.1, with an illustration in Figure 12.5. Note particularly how the ages of stars correlate with where they are found. As we shall see, this information holds important clues to how the Milky Way Galaxy formed.

Table 12.1. Characteristics of the Milky Way Galaxy
Property Thin Disk Thick Disk Stellar Halo (Excludes Dark Matter)
Stellar mass 4 × 1010MSun A few percent of the thin disk mass 1010MSun
Luminosity 3 × 1010LSun A few percent of the thin disk luminosity 8 × 108LSun
Typical age of stars 1 million to 10 billion years 11 billion years 13 billion years
Heavier-element abundance High Intermediate Very low
Rotation High Intermediate Very low

Major Parts of the Milky Way Galaxy

Schematic of the Milky Way. Our galaxy is seen edge-on in this illustration, with the major components labelled. At the center of the diagram is the “Galactic center” indicated with a white dot located in the middle of a white horizontal line labelled “Thin disk”. The Sun is about 2/3 of the way from the center to the left edge of the thin disk and indicated with a white dot. The “Thick disk” is shown in pink above and below the thin disk. The “Bulge” surrounds the galactic center and the “Halo”, drawn as a semi-transparent sphere, surrounds nearly the entire galaxy.
Figure 12.5. This schematic shows the major components of our Galaxy.

Establishing this overall picture of the Galaxy from our dust-shrouded viewpoint inside the thin disk has been one of the great achievements of modern astronomy (and one that took decades of effort by astronomers working with a wide range of telescopes). One thing that helped enormously was the discovery that our Galaxy is not unique in its characteristics. There are many other flat, spiral-shaped islands of stars, gas, and dust in the universe. For example, the Milky Way somewhat resembles the Andromeda galaxy, which, at a distance of about 2.3 million light-years, is our nearest neighbouring giant spiral galaxy. Just as you can get a much better picture of yourself if someone else takes the photo from a distance away, pictures and other diagnostic observations of nearby galaxies that resemble ours have been vital to our understanding of the properties of the Milky Way.

Our radio observations of the disk’s gaseous component indicate that the Galaxy has two major spiral arms that emerge from the bar and several fainter arms and shorter spurs. You can see a recently assembled map of our Galaxy’s arm structure—derived from studies in the infrared—in Figure 12.6.

Milky Way Bar and Arms

Map of the The Milky Way Galaxy. Over-plotted on this data-based illustration of the Milky Way is a coordinate system centered on the Sun, which is located about half way from the center and the bottom of the image. It is a polar coordinate system, with zero degrees straight up from the Sun, 90O to the left, 180O straight down and 270O to the right. Distances are shown as circles of increasing radius centered on the Sun. Distances from 15,000 ly to 75,000 ly are indicated in increments of 5,000 ly. Moving outward from the Sun along the zero degree line are the “Near 3kpc Arm”, “Far 3 kpc Arm” and the “Sagittarius Arm”. Moving outward from the Sun along the 330O line (to the right of zero) are the “Norma Arm” and the “Scutum-Centaurus Arm”. Moving outward from the Sun along the 90O line are are the: “Orion Spur”, “Perseus Arm” and the “Outer Arm”.
Figure 12.6. Here, we see the Milky Way Galaxy as it would look from above. This image, assembled from data from NASA’s WISE mission, shows that the Milky Way Galaxy has a modest bar in its central regions. Two spiral arms, Scutum-Centaurus and Perseus, emerge from the ends of the bar and wrap around the bulge. The Sagittarius and Outer arms have fewer stars than the other two arms.
The Milky Way Galaxy by NASA/JPL-Caltech/R. Hurt (SSC/Caltech), NASA Media License.

The Sun is near the inner edge of a short arm called the Orion Spur, which is about 10,000 light-years long and contains such conspicuous features as the Cygnus Rift (the great dark nebula in the summer Milky Way) and the bright Orion Nebula. Figure 12.7 shows a few other objects that share this small section of the Galaxy with us and are easy to see. Remember, the farther away we try to look from our own arm, the more the dust in the Galaxy builds up and makes it hard to see with visible light.

Orion Spur

The Sun and the Orion Spur. Portions of three spiral arms of the Milky Way are shown in this illustration. The “Cygnus arm” at top, the “Perseus arm” at center and the “Sagittarius arm” at bottom. The “Orion spur” is a stream of stars and gas runs from the Cygnus arm diagonally downward to the right through the Perseus arm and on to the Sagittarius arm. The Sun is located in the portion of the spur between the Perseus and Sagittarius arms. Objects of interest are indicated with arrows from above and below the figure. At top, from left to right are: “Sn2 289”, “Perseus transit”, “Rosetta nebula” and the “Orion nebula”. At bottom, from left to right are: “Turner S”, “Vela molecular ridge”, “Gum nebula”, “Sun”, “Cygnus X-1” and “W51”.
Figure 12.7. The Sun is located in the Orion Spur, which is a minor spiral arm located between two other arms. In this diagram, the white lines point to some other noteworthy objects that share this feature of the Milky Way Galaxy with the Sun.
Modification of The Milky Way by NASA/JPL-Caltech, NASA Media License.

Attribution

25.0 Thinking Ahead“, “25.1 The Architecture of the Galaxy“, and “25.2 Spiral Structure” from Douglas College Astronomy 1105 by Douglas College Department of Physics and Astronomy, is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Adapted from Astronomy 2e.

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Fanshawe College Astronomy Copyright © 2023 by Dr. Iftekhar Haque is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.