Spendthrift Spirals

Spirals are the most spectacular of galaxies. Like a giant vortex in space, a spiral galaxy spins through the cosmos, its mighty arms squeezing gas and dust into newborn stars. Blue supergiant stars and pink clouds of ionised hydrogen gas sparkle in the spiral arms, where the occasional flash of a supernova explosion casts newly minted chemical elements into the galaxy.

These magnificent galaxies lend grace and beauty to the night. Moreover, they work hard to create new light. Five out of every six stars born each year are born in spiral galaxies, some in normal spirals like the Whirlpool, others in barred spirals like the Milky Way.

Yet these same exquisite galaxies face an immense problem. As spiral galaxies spawn new suns, they exhaust the very gas and dust that enable them to be spirals in the first place.

The spiraliferous period

The modern universe abounds with so many spiral galaxies that it’s easy to imagine they will endure forever. About 70% of the bright galaxies within a billion light-years are spirals. Most galaxies in Charles Messier’s great catalogue are spirals. And the Local Group’s three most luminous galaxies — Andromeda, the Milky Way, and M33 — are all spirals. Just as geologists call the time when much of our planet’s coal formed the Carboniferous Period, so we might call the present time the Spiraliferous Period.

This spiral-spangled period took a long time to start, however, in part because the universe’s first galaxies were too small to spin fast. “Spirals tap rotational energy,” says astronomer Bruce Elmegreen (IBM Research Division). “Low-mass galaxies can’t form spirals because they rotate too slowly.” Even after galaxies grew massive enough to spin fast, spirals remained rare for billions of years after the universe’s birth. “You don’t get spirals then, because everything’s very turbulent,” he says. “The galaxy is accreting gas like