Originally published 17 October 1983
On the night of an August meteor shower, my son and I slept under the open sky. It was a night of exceptional clarity, far from the lights and haze of Boston. Meteors flashed against a background of stars so numerous the heavens seemed more light than dark.
Above our heads the Milky Way arched from Cassiopeia in the north to Sagittarius in the south. It was a dark-shoaled river of light, a luminous drapery, diamond dust on black velvet. As the hours passed and the stars tilted toward the west, we could almost feel ourselves whirled in the spiral arms of the Galaxy.
Among the ancients, who viewed the heavens with clearer skies, the Milky Way — the Via Galactica — was a bridge, a road on a river. The astronomer Robert Burnham has suggested that these images are closely related to the idea of human life as a journey or voyage between two worlds, and to the endless journey of the Universe itself toward an unknown destination. It is a powerful, evocative symbolism that goes a long way toward making us feel at home among the stars.
The modern view of the Milky Way is no less evocative. Our sun is one star in a disk-shaped swarm of several hundred billion stars. The stars in the disk are clustered in spiral arms that turn about the center like a pinwheel. The sun shares that motion, making the circular journey about the axis every 250 million years.
On the scale of the Galaxy, our Solar System is a pinprick, a mote of dust riding a whirlwind. All of the stars you see tonight, the thousands of stars you see on the best of nights, are but our near neighbors in one spiraling arm of the circular tide.
The eye of the hurricane
The brilliance of the summer Milky Way in the direction of the constellation Sagittarius hints at the great density of stars that cluster near the center of the spiral. Dust and gas in the central plane of the galactic disk obscures our view of the nucleus, but radio and x‑ray telescopes have penetrated the obscuring matter and revealed a powerful source of energy. The nucleus of the Milky Way Galaxy is apparently a site for cosmic violence on the grand scale, perhaps a place where countless suns are swallowed up by a massive gravitational “black hole.”
Above and below the plane of the galactic disk, like bees around the hive, are several hundred globular clusters. These ball-shaped associations of thousands of stars are small satellites of the Milky Way.
But this is the “traditional” picture of the Milky Way Galaxy. Recent studies have added surprising new features.
Radio studies of the emissions of atomic hydrogen show that the disk of the Galaxy is about twice as extensive as previously thought. Clouds of hydrogen extend well beyond the region of active star formation, rimming the spiral with suburbs of gas, reaching with four great arms into the void of space.
Another unexpected extension of the galaxy followed from the simple business of counting stars. Stars are now counted on photographic plates with computer-controlled scanning light beams. These new techniques have enabled astronomers to catalogue stars a hundred times fainter than in previous reliable determinations. They have discovered stars of low intrinsic brightness distributed above and below the galactic disk, enclosing the brilliant Milky Way spiral in a kind of egg-shaped aura of faint stars.
A more startling development has followed from studies of the rotational dynamics of the Milky Way galaxy. Recent observations of the motions of objects in the outer reaches of the galaxy — bright stars, clusters of stars in clouds of molecular gas, and globular clusters — have enabled astronomers to refine their calculations of the galactic mass. It seems that the Milky Way galaxy may be as much as ten times more massive than thought. How could so much matter have gone undetected until now?
Dark, exotic matter
The answer is that the newly discovered mass must consist of some exotic form of non-luminous matter. Not stars, which we should certainly see ablaze in the heavens. Nor interstellar gas or dust, which we would observe by its own radiation of by the absorption of light from distant sources.
If the newly discovered non-luminous matter of the Milky Way galaxy is typical of other galaxies (and there is reason to believe that it is) we will have to drastically increase our estimation of the mass of the Universe. What is this hidden component of the Universe, this dark matter that constitutes the greater part of all that exists?
Perhaps the dark halo of the galaxy consists of Jupiter-sized objects, too small to have “turned on” as stars. Perhaps is consists of some yet unknown type of celestial body, larger than dust but smaller than the smallest stars. Or perhaps the dark mass consists of black holes, stars that have collapsed into themselves and become do dense that light cannot escape the pull of their gravity.
The hidden matter of the Milky Way and other galaxies may decide the ultimate fate of the Universe. According to the theory of general relativity, it is the average mass density of the Universe that will determine whether the Universe expands forever in the cold void, or falls back upon itself to recreate the blazing fireball of its creation.
All of this we knew and felt that August night, as we watched the annual display of “shooting stars,” adrift on our backs in the stream of the Milky Way. We rode a spinning Earth about a sun that spun in a whirlpool of a trillion stars, sealed in an envelope of mysterious dark matter, wheels within wheels within wheels, sharing the journey of the Universe toward an uncertain destination. It was a ride that left us reeling.
The existence of so-called “dark matter” has been generally accepted by astrophysicists in the decades since this essay was first written. As a result, there is consensus among cosmologists that the Universe will expand forever, becoming more and more diffuse. ‑Ed.
