Well-Mannered Galaxies

Well-Mannered Galaxies

Even a galaxy so seemingly well-mannered as the Milky Way has its stirrings and its dances. Radio observations show two enormous clouds of hydrogen gas, enough to make millions of suns, plummeting out from the galactic core, as if a mild explosion happened there every now and then.

A high-energy astronomical observatory in Earth orbit has found the galactic core to be a strong source of a particular gamma ray spectral line, consistent with the idea that a massive black hole is hidden there. Galaxies like the Milky Way may represent the staid middle age in a continuous evolutionary sequence, which encompasses, in their violent adolescence, quasars and exploding galaxies: because the quasars are so distant, we see them in their youth, as they were billions of years ago.

The stars of Milky Way move with systematic grace. Globular clusters plunge through the galactic plane and out the other side, where they slow, reverse and hurtle back again. If we could follow the motion of individual stars bobbing about the galactic plane, they would resemble a froth of popcorn. We have never seen a galaxy change its form significantly only because it takes so long to move.

The stars of Milky Way move with systematic grace, but the spiral pattern persists. Image : A Blue and Nuances Swirl by © Elena

The Milky Was rotates once every quarter billion years. If we were to speed the rotation, we would see that the Galaxy is a dynamic, almost organic entity, in some ways resembling a multy-cellular organism. Any astronomical photograph of a galaxy is merely a shapshot of one stage in its ponderous motion and evolution (but this is not quite true. The near side of a galaxy is tens of thousands of light-years closer to us than the far side; thus we see the front as it was tens of thousands of years before the back. But typical events in galactic dynamics occupy tens of millions of years, so the error in thinking of an image of a galaxy as frozen in one moment of time is small).

The inner region of a galaxy rotates as a solid body. But beyond that, like the planets around the Sun following Kepler’s third law, the outer provinces rotate progressively more slowly.

The arms have a tendency to wind up around the core in an ever-thightening spiral, and gas and dust accumulate in spiral patters of greater density, which are in turn the locales for the formation of young, hot, bright stars, the stars that outline the spiral arms. These stars shine for ten millions years or so, a period corresponding to only 5 percent of a galactic rotation. But as the stars that outline a spiral arm burn out, new stars and their associated nebulae are formed just behind them, and the spiral pattern persists. The stars that outline the arms do not survive even a single galactic rotation; only the spiral pattern remains.