The Origin and Age of the Milky Way

The Milky Way Galaxy is approximately 13.2 billion years old, and formed as the universe cooled following the Big Bang. The Milky Way coalesced from a rotating cloud of dust that collapsed in the same fashion as the solar system, as described in Chapter 6. At its center lies a black hole—an extraordinary object whose gravitational tug is so powerful not even light can escape it, hence the name. The immense gravitational pull from the black hole at the Milky Way’s center, or galactic core, provides the anchor for all of its stars, holding them in place as they slowly orbit around it.

A black hole develops during the terminal stage in the life cycle of the highest mass stars; those with masses many times that of our sun. As these larger stars age and eventually deplete their hydrogen fuel supply, a series of violent explosions and physical reactions occur that result in the progressive breakdown of the very structure of their atoms. The final result is the formation of a black hole, which sits at an infinitesimally tiny point in space, yet with much of the mass of the original star!

The Milky Way’s black hole was first discovered in the late 1990s by Andrea Ghetz, who observed unexpectedly high orbital speeds of stars near its center. Their speeds were so high that only an object with the mass of the black hole could be responsible, yet the size of the then-unknown object was far too small to conform to any established form of matter. Although we have yet to directly image the region immediately adjacent to the Milky Way’s black hole, current research programs are underway to do so with unprecedented resolution.

Other studies have confirmed that most, if not all, galaxies in the observable universe also contain black holes at their centers. Although the precise sequence of events that leads to this arrangement is not yet completely understood, recent studies suggest that the black holes formed first and subsequently led to the presence of the surrounding stars.

The Size and Shape of the Milky Way

The shape and appearance of the Milky Way is similar to that of an enormous, swirled lollipop—nearly 100,000 light years in diameter,¹ yet only 1,000 light years in thickness, Figure 8.2(a). Its hundreds of billions of stars are distributed within distinct bands, or “arms,” along with residual clouds of dust and hydrogen gas. Our sun and solar system lies within the Orion-Cygnus arm and between the Perseus and Sagittarius arms. The solar system sits approximately two-thirds of the way from the galactic core, Figure 2.3, and approximately 90 light years above the Milky Way’s midplane.

The density of stars is far greater near the galactic core, Figure 8.2, than at positions further away. Its higher star density is why the galactic core appears to be so bright, and why the regions further away from it appear progressively dimmer. In fact, the uniformly bright appearance of the entire midplane region of the Milky Way to the naked eye is deceiving; just like viewing a sandy beach from a distance and not being able to resolve the individual grains of sand, the galactic core of the Milky Way is so far away that its billions of stars are indistinguishable and simply merge into a uniform glow.

8.2 The shape and appearance of the Milky Way: (a) the structure of the Milky Way is similar to that of a large, swirled lollipop—nearly one hundred thousand light-years in diameter and one thousand light years in thickness. The center of the Milky Way Galaxy, or “galactic core” is indicated by the yellow dot. (b) Early morning view of the bright, galactic core region of the Milky Way over Ellingson Island in Lake Superior, Minnesota, in the spring. The approximate location of the galactic core is shown.

Source: (a) NASA/CXC/SAO

Its flat, disc-like shape is why the midplane regions of the Milky Way often appear in the form of a bright stripe across the sky, Figures 8.1 and 8.2(b). In fact, its name is also the result of this shape; early Roman viewers likened its appearance to a river of spilled milk across the sky, and gave it the name, “via lactea.” Yet, as we saw in Chapter 2, the vast majority of all the objects we see at night reside within the Milky Way. Interpreted literally, each photograph of a starry night sky is thus a photograph of a region of the Milky Way Galaxy!

As landscape astrophotographers, we are often interested in the central band of the Milky Way as a photographic subject, and especially the bright, galactic core region. Therefore, to avoid any confusion, when I refer to “the Milky Way Galaxy” in this book, I am generally referring to its central band that, depending on the context, may or may not include the galactic core region. Finally, we will adopt the following viewing perspectives in our subsequent discussions: a “top view” refers to its appearance when viewed from a position above the plane of the Milky Way (of course, no human has ever been able to do so!); a “side view” simply refers to its appearance viewed edge-on.

Relative Orientations of the Earth, Solar System, and Milky Way

Astronomers have determined that the plane of the solar system, including the orbital plane of the earth, is inclined at an angle of approximately 63° to the midplane of the Milky Way, Figure 8.3(a) (overleaf). Coupled with the 23½° tilt of the earth’s axis relative to the normal to its orbital plane, the earth’s rotational axis is, therefore, aligned within a few degrees of the plane of the Milky Way. This fact explains, then, why the orientation of the Milky Way’s central band appears in positions ranging from roughly encircling, or parallel to the earth’s horizon, Figure 8.3(b), to arcing directly overhead in a general north to south alignment, Figure 8.3(c), at different times of night; and, as described below, at different times of year.

View of the Milky Way Core through the Year

So why are we only able to see the galactic core of the Milky Way during certain months of the year? The answer is straightforward; our vantage point is constantly changing from month to month, as illustrated schematically in Figure 8.4(a) (overleaf), just as we saw in Chapter 2, for example, Figure 2.10. The months of June, July, and August are by far the best time of the year to view the galactic core since we have a direct view of it at night. In contrast, the night sky view in November, December, and January is facing the opposite direction. During the daytime hours of these months, the sun sits between the earth and the galactic core and blocks our view. It is simply not possible to view the core at this time of year, barring a total solar eclipse. Finally, during the spring and fall, portions of the galactic core may be visible peeking over the horizon for a short time before sunrise and after sunset, respectively. Information for when the galactic core of the Milky Way is visible in the Northern Hemisphere throughout the year is summarized in Figure 8.4(b).

8.3 The relative orientation and inclinations of the earth, the solar system, and the plane of the Milky Way is shown here in (a). The 63° inclination of the earth’s orbit relative to the plane of the Milky Way, and the 23.5° tilt of its rotational axis relative to the normal to its orbital plane, combine to result in the earth rotating around an axis that is nearly parallel to the plane of the Milky Way. This phenomenon has the effect of causing the Milky Way’s central band to change position from roughly encircling the horizon to arcing vertically overhead in a general north to south alignment at different times of night and at different times of year, for observers at mid to central latitudes. (b, c) Simulated view of the Milky Way, facing southwest, on a date and time when the plane of the Milky Way is (b) parallel to the observer’s horizon; and (c) approximately 6 hours later, when the earth has rotated 90°, causing the Milky Way to now appear perpendicular to the observer’s horizon. The yellow dots in the inset schematic diagrams indicate the position of the local observer on Earth on this date and for these times. The Milky Way undergoes this change in orientation daily, although we may not be awake or in a position to observe it!

Source: Distant Suns

8.4 (a) Schematic illustrating how the changing position of the earth during the year affects our view of the Milky Way’s galactic core. The corresponding zodiacal constellations that are visible at night are also shown in relative position to the sun and earth. During the summer, the earth’s night sky has a direct view of the galactic core. In winter, however, the earth’s night sky view is in the opposite direction, and our view of the core is completely blocked by the sun. Thus, the best months for viewing the galactic core are May through September; in December and January it is never above the horizon at night. (b) This diagram illustrates the times when the galactic core of the Milky Way is above the horizon at mid northern latitudes (here, 47° N). The width of the white bar shows the duration of full darkness; the shaded region within each bar, when present, indicates the times when the Milky Way’s core region is best seen. The core is visible during the evening hours in the summer, the early morning hours in the spring, and in the early evening hours in the fall. During the winter, the core is blocked by the sun and not visible at night.

The two sweeping, panoramic images of the primary band of the Milky Way shown in Figure 8.5 clearly demonstrate several dramatic phenomena that are the direct result of the changes that occur in our viewing perspective throughout the year. The two images were made approximately five months apart, both in the Northern Hemisphere. The panorama in Figure 8.5(a) was made during the late summer, while the panorama in Figure 8.5(b) was made in the early winter. First, observe how the azimuth of our view of the Milky Way’s primary band changes: southeast during the late summer; northeast in the early winter. Next, note how we observe different portions of the Milky Way’s primary band; the bright galactic core region in the summer, and the fainter, outer fringe during the winter. Finally, and perhaps most revealing, observe how our viewing perspective has completely flipped; note the mirror-image relationship between the orientations and positions of the Andromeda Galaxy, the star Mirach, and the two circled dust clouds between the two images. The changes in the position of the earth during the intervening five months have turned our viewing position completely upside down!

So how do we use all this information? While a detailed description of landscape astrophotography planning is covered is Section IV of this book, we can set the stage with reference to Figure 8.6 (overleaf), where the appearance of the galactic core region of the Milky Way in the southern night sky in the Northern Hemisphere is shown for different times during the night, as well as for different months. For example, in the month of June, the appearance of the primary band of the Milky Way first appears angled just above the eastern horizon in the evening, arcs high across the sky as the night progresses and ends up nearly vertical in the western sky as dawn approaches. In contrast, it first appears oriented vertically above the southwestern horizon in the evening in September and then disappears below the western horizon within a few hours. A field example showing this movement during a single night in July is shown in Figure 8.7 (overleaf).

This movement has significant implications in landscape astrophotography images in which the Milky Way is an important feature. For example, you will want to understand it so that you may position yourself correctly relative to prominent foreground objects in order to view the Milky Way in a specific position and orientation. Since its position changes during the night and through the year, it is crucial to know how to account for these variations, as will be seen in Chapter 16.

Effect of Latitude on Milky Way Appearance

From terrestrial vantage points, the earth’s horizon always blocks part of our view of the Milky Way. Certain regions such as the Coal Sack rarely, if ever, rise above the horizon in the Northern Hemisphere; conversely, the Milky Way regions within Cassiopeia are never visible in portions of the Southern Hemisphere.

Horizon-to-horizon panoramas of the Milky Way at four different latitudes: 47° N, 34° N, 35° S and 45° S are shown in Figure 8.8(a)–(d) (overleaf) to illustrate the different appearance of the Milky Way from different latitudes. The Milky Way in each of these four panoramas, (a)–(d), has been aligned with respect to each other to assist in their direct comparison. As our view moves from north to south across the globe, slightly different regions of the Milky Way becomes visible above the horizon. At 47° N, the galactic core region barely clears the horizon, whereas at 45° S, it is directly overhead. Finally, to obtain a sense of how the Milky Way Galaxy might appear from space, these images from different latitudes and hemispheres are combined into a virtual panorama, Figure 8.8(e).

8.5 Panoramic images of the Milky Way made at two different times of year. The panorama in (a) was made during the late summer, while the panorama in (b) was made in the early winter. Observe how the azimuth of our view of the Milky Way’s primary band changes between these different times of year: (a) southeast; (b) northeast. Also, note how we observe different portions of the Milky Way’s primary band; the bright galactic core region in the summer compared to the fainter, outer fringe during the winter. Finally, observe how our viewing perspective has completely flipped by comparing the mirror-image relationship between the orientations and positions of the Andromeda Galaxy, the star Mirach, and the two circled dust clouds between the two images. The changes in the position of the earth during the intervening five months have turned our viewing position completely upside down!

8.6 Simulated appearance of the primary band of the Milky Way for different times of night and for different times of year, for latitude 47° N in the Northern Hemisphere.

Source: www.stellarium.org

8.7 Actual field photographs of the Milky Way at one-half hour intervals during a single night, and made from a fixed tripod position generally facing south. Note how the point of intersection between the primary band of the Milky Way and the horizon (arrow) steadily moves from east to west, or from left to right above, as the night progresses.

8.8 The appearance of the central band of the Milky Way viewed from different latitudes: from (a) Duluth, Minnesota (47° N), (b) Big Pine, California (34° N), (c) Brisbane, Australia (35° S); and (d) Sydney, Australia (45° S). Note that the Milky Way in each of these four panoramas, (a)–(d), has been aligned with respect to each other. (e) Panoramic view of the core region of the Milky Way obtained by combining these images into a single composite.

Stars, Gas, and Dust of the Milky Way

There are several features of the Milky Way worth noting for their photogenic and astronomical interest. The first are the regions of the galactic core containing gas and dust clouds that emit light. These emission or reflection nebula are either (a) physically hot or (b) located near stars whose light they absorb and re-emit. Several emission nebulae are located within the galactic core region of the Milky Way, including the Eagle Nebula, Swan Nebula and Lagoon Nebula. The colorful reddish light emanating from their rich stores of hydrogen gas often provides a distinctive highlight to Milky Way nightscapes.

In contrast, there are many large, dark areas clearly visible along the central band, Figure 8.9(a) (overleaf). These dark patches are enormous clouds of physically cold interstellar gas and dust that are part of the normal structure of the spiral arms of the Milky Way. They are remnants from the Big Bang and do not emit light; rather, they absorb all the light emitted from stars on the opposite side of them from earth and hence appear dark, as illustrated schematically in Figure 8.9(b). Especially prominent dust clouds or dark nebulae are the Great Rift in the Northern Hemisphere, Figure 8.10 (a) (overleaf) and the Coal Sack and Emu in the Sky in the Southern Hemisphere, Figure 8.10(b). Just like clouds blot out the sun, interstellar dust clouds blot out starlight.

Careful study of the dust clouds, however, will reveal a few stars that appear right in their midst, Fig. 8.9(a). These stars are positioned in between the dust clouds and earth, so we still are able to see them, as illustrated schematically in Figure 8.9(b). Especially on dark, moonless nights far away from city lights, this tendency of dust clouds to block light from faraway stars while providing a backdrop to intervening stars imparts a wonderful sense of the three-dimensionality of the Milky Way.

Colors of the Milky Way

The glowing, central band of the Milky Way generally appears to be a pale white in color to the naked eye. While some regions are brighter than others, their light intensity is too weak to activate the color-sensing parts of the human vision system, as described in Chapter 10. Camera sensors, however, are far more sensitive and easily capture and distinguish between the colors of the different regions of the Milky Way.

The ability of modern cameras to capture the colors of the Milky Way can lead to some unanticipated difficulties. As described in Chapter 21, e.g. Figure 21.5, it is important to decide how you wish to depict these colors. It is generally accepted that the core band of the Milky Way is a pleasant, whitish cream color, not the vivid blues, greens, and even purples that sometimes appear in nightscapes. Ultimately, of course, it is entirely the choice of the photographer how to proceed.

Notable Constellations of the Milky Way

There are several key constellations/asterisms associated with the Milky Way that should be noted: Sagittarius, Scorpius, Cassiopeia, Orion, Cygnus, and Altair in the Northern Hemisphere; and the Southern Cross in the Southern Hemisphere, as indicated in Figure 8.10. These constellations are often an extremely useful tool in identifying where the Milky Way “ought to be” under less than ideal viewing conditions, and can aid in rapidly orientating oneself to the night sky. In addition, by knowing where the Milky Way lies relative to these constellations, it is occasionally possible to

8.9 Dust and gas clouds of the Milky Way: (a) Photograph of the galactic core region of the Milky Way, where several prominent dust and gas clouds are visible; (b) side view schematic illustrating how the spatial arrangement of stars and a single dust cloud can lead to the types of view within (a).

make an image with your camera that reveals enough structure of the Milky Way to identify it, even though it may only be just barely discernible with your naked eye. Finally, many of the stars in these constellations are quite bright and hence the first to appear during astronomical twilight. They can be invaluable in gauging the precise location of the Milky Way as astronomical twilight deepens into complete darkness.

8.10 (a) The Great Rift and prominent summer asterisms, including the Teapot (Sagittarius), and the Fishhook (Scorpius), as viewed from the Northern Hemisphere. (b) Dust clouds of the Milky Way as viewed from 35° S, including the Emu in the Sky and the Coal Sack, along with the Southern Cross.

Beyond the Milky Way

There are only a few objects lying beyond the Milky Way visible to the naked eye. However, they not only make worthy targets by themselves, they impart a true element of mystery to any nightscape, as seen in Figure 8.11. Notable examples include the Large and Small Magellanic Clouds, Figure 8.11(a), and the Andromeda Galaxy, Figure 8.11(b).

8.11 There are only a few objects lying beyond the Milky Way visible to the naked eye: (a) Magellanic clouds; (b) The Andromeda Galaxy, M31.

Source: (a) Caren Zhao/The World At Night