MAPPING THE MILKY WAY

A 1939 biography of Albert Einstein offers a poignant example of the perspective that helped shape the famous scientist’s relativistic view of Earth in space: “The world is moving along rapidly in space: your office in the morning will not be where it was when you left it at the close of business. It will never be in the same place in space again!”1 Indeed, the Sun plunges daily some 12.5 million miles through the empty wastes of space, never to return to its former location. The Earth orbits the Sun at roughly 67,100 miles per hour even as the Sun roars around the Milky Way galaxy at about 490,000 miles per hour. Meanwhile our galaxy is reeling to­ward the Virgo Cluster, the largest and nearest cluster of galaxies, roughly 54 million light-years distant, at a speed of about 864,000 miles per hour. But in relation to the background radiation of the universe, the Milky Way is racing through space at approximately 1.3 million miles per hour.2 As astronomers grapple to understand our place in the universe, every second, our planet, Sun, and Solar System, as well as the Milky Way galaxy, whirl blindly into the depths of outer space.

It’s some consolation, however, that our neighboring stars and galaxies are barreling into the unknown abyss along with us. The sameness of the night sky has been recorded for millennia and is of great value to humankind. Long before written records, the positions of stars were used as directional aids in traversing the unmapped and largely unpopulated expanse of Earth’s surface,

its deserts and wastelands, and in navigating uncharted seas. The same stars traced the seasons of Earth’s passage on its perdurable orbit around the Sun. When humans as a species were first form­ing words, the rising, setting, and annual return of the stars and constellations in the night sky must have offered a sense of perma­nence in a savage world. Deeply embedded in our primal imagina­tion were the stars as guides in navigating novel landscapes, lo­cating seasonal fruits and vegetables, following migratory animals that provided food and pelts, and preparing for oncoming seasons.

Much older than our Sun, the Milky Way galaxy began to form approximately 13 billion years ago and is a barred-spiral compris­ing some 200-400 billion stars. Its spiral arms, strewn with mas­sive clouds of gas and dust coalescing into newborn stars, sweep in magnificent arcs around the millions of stars comprising the gal­axy’s central bulge. Because of the scale of this whirlpool of dust and planetary systems, from our perspective neighboring stars ap­pear to form a fixed pattern of constellations and they only change their relative positions over millennia (plate 13).

Imagine if our planet circled a star that in turn was orbiting within a globular cluster (among millions of stars clustered to form a soft-edged spherical structure), all the nearby star patterns would change over a lifetime, possibly never to recur! The night sky would be disorienting rather than a familiar point of refer­ence. The seeming sameness of the night sky is a result of nearby stars hurtling along with our Sun as it circumnavigates the Milky Way every 226 million years, their high speeds and small relative motions reminiscent of racing cars on a circular track, but on a galactic scale. Of these stars, even the closest are so unimaginably far away, their actual motion through interstellar space is all but imperceptible. Hipparcos mission lead scientist Michael Perryman explains:

The bright stars forming Ursa Major, for example, one of the larg­est and most prominent of the northern constellations, known vari­ously as the Big Dipper or the Plough, look the same now as they did hundreds of years ago—Ptolemy listed it, Shakespeare and Ten­nyson wrote about it, and Van Gogh painted it. And they will look just the same to our children, and to theirs. But to earliest humanity, a hundred thousand years ago, and to those equally far in the future, the constellation would be unrecognisable, grossly distorted from its present shape.3

In the prehistoric past, the stars were beyond human investiga­tion and perhaps even comprehension. Their extreme remoteness compared to our neighboring planets prevented us from initially realizing their true nature. Over time we recognized that stars are fundamentally like our Sun, replete with worlds we are only now surveying. As will become clear, the apparent sameness of the night sky over long periods of time has been an invaluable natural phe­nomenon for humankind coincident with our very survival.