Exposure

Jacobus Kapteyn, a Dutch astronomer of the late nine-teenth and early twentieth centuries, took advantage of the new information gleaned from spectroscopy and updated Herschel’s mapping technique. His representation of our stellar system, which became known as the ‘‘Kapteyn Universe,’’ required a lifetime of patient effort to put together. The Kapteyn Universe was an important model of the distribution of stars until the 1910s, and some aspects of it survived beyond that period.


The American astronomers Harlow Shapley and Edwin Hubble, contemporaries who were notorious rivals, provide the denouement in this account of our discovery of our place in the cosmos. Shapley astounded the astronomical world in the 1910s with the news that the ‘‘Kapteyn Universe’’ was only a small part of a vast galaxy. Hubble, a successor of Shapley at the Mount Wilson Observatory in California, established that our galaxy was beyond doubt only one of many similar systems of stars, or galaxies, scattered throughout space.

Viewing the galaxy from within

The word ‘‘galaxy’’ is a familiar one. Today even elementary-school children know that we live in a galaxy—a system of billions or even trillions of stars, bound by gravity and orbiting a massive center—and that our Sun is one of the lesser lights in the Milky Way galaxy.

 One can even buy T-shirts showing stars in the classic whirlpool pattern, with the words ‘‘YOU ARE HERE’’ and an arrow pointing to the Sun’s location in one of the spiral arms.

How do we know we live in a galaxy? Many of my students seem to think we know because we have seen pictures of it. This is not an unreasonable assumption in light of the stunning photo-graphs collected by the Hubble Space Telescope and other ground-based and satellite-based telescopes.

 In the so-called ‘‘Hubble deep field’’ photograph, for example, space looks posi-tively crowded with galaxies. There are a few bright points in this image that represent foreground stars, but the rest, yellow, blue or reddish in color, are galaxies.

 Some are wide, flat spirals that we see nearly face-on, presenting a disk-like appearance. Some look spherical. Some appear as thin lines—these are the disk-shaped galaxies seen edge-on. 

The variety of colors stems from the different chemical compositions and ages of the stars making up the galaxies, and the presence of dust and gas clouds among the stars, which lend a reddish hue to the galaxy. 

Such photographs make it seem eminently reasonable that we live in one such galaxy, in our own group or cluster of galaxies. In fact, although we have a good idea of what our galaxy must look like from a distance, and we know quite a bit about neighboring galaxies in our group, no one has ever seen a photograph of the Milky Way galaxy in its entirety.

 We cannot get far enough away to put our stellar system in perspec-tive. Our most far-flung robotic eye, the Voyager 1 spacecraft, was launched in 1977. Traveling through space at hundreds of millions of kilometers (or hundreds of millions of miles) per year, Voyager 1 is scheduled to reach the outer edge of the solar system—not even as far away as the nearest star—in the first quarter of the twenty-first century. To pass through the disk, rise above the plane of the Galaxy, and look back with.

The ‘‘Hubble Deep Field’’—a view taken with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. As described in the text, most of the objects seen here are distant galaxies.

 A foreground star, within our own galaxy, has ‘‘rays’’ extending from it—an artifact of the imaging system. The view is actually a synthesis of separate images in red, green, and blue light. 

A cosmic bird’s-eye view across the entire span of its spiral arms would require billions of years more travel time.

What we know about the shape and size of our galaxy emerged from the efforts of many astronomers, beginning in the late eighteenth century and culminating in the early part of the twentieth century. 

Detective work of an astronomical sort was required to make sense of the available information. The problem of studying our galaxy from within it is like trying to learn about a crowd of people from a vantage point inside the throng. Consider, for example, that you are part of a graduation procession at a large school.

 Looking to your left and right, you might see only one or two neighbors, while the head and tail of the line may be out of sight. Clearly you are in a line of people, but your perspective gives you only limited information about the size and shape of the procession crowd. 

Similarly, from our vantage point in a spiral arm of the Galaxy, we have some information about the nearby disk, while some parts of the galaxy are obscured from view. And to complicate matters, astronomers have had to devise methods of estimating distances that allow them to gauge the extent of the starry congregations without leaving the surface of the Earth.

The most important clue to the distribution of stars is the phenomenon we call the Milky Way. The term ‘‘Milky Way’’ has two possible, related meanings: it refers to our home galaxy, and it also means the misty band of milky-white light we see arching across the sky (figure 1.2).

 Residents of countries in the northern hemisphere see the Milky Way band of light most prominently in the late summer, fall, and winter. Southern hemisphere observers see it best in spring and summer.

The Greeks gave us the term ‘‘Milky Way,’’ a translation of ‘‘kiklos Galaxias’’ or milky circle. The story behind this name is that the infant Heracles (Hercules in the Roman version) tried to suckle at the breast of the goddess Hera (Juno, to the Romans). 

In what nursing mothers everywhere recognize as a sign of a powerful let-down reflex, some of the milk sprayed out, missing Heracles’ mouth. By failing to latch on to this divine stream, Heracles missed out on his chance for immortality. The milk that spurted up into the sky formed the Milky Way.5

When Galileo first turned a telescope to the Milky Way in 1609, a tapestry of close-packed stars sprang into view.



He correctly inferred that the misty glow of the Milky Way is nothing other than the combined light of these stars, much more tightly condensed in this region than in other parts of the sky. For him, the question of the Milky Way was nicely settled by this tele-scopic view and left no more to wonder about. 

‘‘All the disputes which have vexed philosophers through so many ages have been resolved, and we are at last free from wordy debates about it,’’ Galileo wrote in his popular booklet, the Starry Messenger. ‘‘The Galaxy is in fact nothing but congeries of innumerable stars grouped together in clusters. 

Upon whatever part of it the telescope is directed, a vast crowd of stars is immediately presented to view, many of them rather large and quite bright, while the number of smaller ones is quite beyond calculation.’’6 Galileo also noted that several other ‘‘nebulous’’ or cloudy patches of light could be seen scattered about the night sky, and that the telescope revealed these, too, to be groups of stars.