The Shark and the Jellyfish: More Stories in Natural History

A Dangerous Place

198 A Dangerous Place These days, we are passing through an exceptional diorama. Just now, our ride on the Earth—along the track the sun follows through space—is taking us across one of the spiral arms of our galaxy. Those arms are the namesake features of grand spiral galaxies, such as our own Milky Way. Seen from a perspective a hundred thousand light years above, set against the void of deep space, our galaxy is dominated by those great curved spokes. Lit with clusters of blue points of starlight and woven with red fluorescent clouds, they arch away across the massed brilliance of the billions of stars in the galactic disk. From that high vantage point, our sun is an inconsequential mote, invisible without a telescope. The vast, curving arms suggest that spiral galaxies are great pinwheels whose huge momentum holds their rotation down to just below the speed of perceptible movement. The illusion is reinforced where the ends of the spirals are bent back, as if by greater centrifugal forces at their faster-moving outer edges. In fact, any analogy with the familiar motion of a solid disk is misleading . Stars flow through galaxies not in unison but at different speeds— slower the farther they are from the center. Our sun takes hundreds of millions of years to complete one trip around the great circle. And the spiral arms themselves advance even more slowly. They are standing waves in a sea of light, continually falling behind the individual stars, which then pass right on through them. Now that we are in one of the arms, we have a sky full of brighter-thanusual lights to gaze upon at night—since the density of stars within the arms is greater than it is between them. But we are too close to the arm itself to see it. Only from a distant remove would its bright young stars and glowing clouds of gas shrink together and coalesce into a river of light streaming away from the galactic center. If we could spend a while gazing at a night sky in which a thousand years pass each second, we would be captivated by the stellar motion. The shimmering river of the Milky Way (made up of the galaxy’s more distant arms) would remain steadfast overhead, but we would see the nearby stars wan- 199 Day star. A supernova explosion bright enough to see by day lights the dark side of the moon at the end of the Permian period. 200 der from their constellations to continuously rearrange themselves into new figures for us to name, dissolving from one grouping into another. Stars tracking toward us would brighten—the larger of them coming to dominate the scene. Should we move into a star cluster—a stellar nursery full of new, young stars—we would be treated to a sky so crowded that our atmosphere, seen between the stars, would lighten at night from black to deep sapphire blue. As our imaginary night lengthened, most of the stars would dim and diffuse away. The night sky would lose its wonder as Earth’s orbit through the galaxy carried us into the safer, emptier quarters between the spiral arms. When the orbit of our sun through the galaxy leaves us between arms, the night sky presents a darker, lonelier vista. But the arc of sparkling stars that now dominates our evenings has a downside. Spiral arms can be dangerous places to visit. We are not as safe while we are within a spiral arm, because there—where the density of stars is greater—the odds of a close encounter with a star are greater. Within an arm, the chances increase that a blue supergiant star like Rigel or Deneb might come as close to the sun as Sirius is now. Such a passing giant would shine with the brightness of the moon concentrated into a single star point—250 times brighter than Venus. Its actinic blue spark would cast a shadow at night and be visible against the sky all day long. But even that close, the powerful ultraviolet radiation of such a star—or its fierce stellar wind of charged particles—would pose no threat to Earth. We would be insulated by the intervening gulf of light years. Isolation through distance protects us from every threat such a star would offer, with one exception. The Earth would come to great...