Donald, who is much more civic-minded than I, recently raised an interesting community health question: "Has anyone bothered to calculate just how long we have before the gigantic black hole at the center of the galaxy swallows up everything around it, including us?".
It does seem conclusive that something 15 times as wide as the sun and weighing at least four millions times more is sitting at the center of the Milky Way, darkly consuming everything it can draw in with its considerable gravity. True, it's located a comfortable 25,000 light-years from us, but gravity is a patient and inexorable force. The question is whether the rotational velocity of our particular arm of the galaxy is fast enough to offset the inward pull exerted by the object and the mass of stars between us and it, all of which are doing their part to draw us to doom.
There is some indication that centripetal force is winning out over centrifugal. One (admittedly sketchy) source gives the Earth's speed toward the galactic center as about 10 kilometers per second. That could put us smack in the gravity well of our supermassive black hole in only 6 billion years.
6 billion years may sound like a leisurely pace to doomsday, but consider that the Earth itself is only about 4 billion years old. That puts us almost halfway through our lifespan already, and we've only just managed to come up with tool-using inhabitants here. The Milky Way itself is probably no more than 15 billion years old. With only 6 billion left until the better part of its mass is consumed, it's well past its heyday.
But in researching these numbers, I came across a lot more information on the dynamics of the galaxy as well as the things in and around it. Space, galactic and intergalactic, is a hotbed of activity when you start looking at a timescale of billions of years. Far from the silent and still expanse of emptiness that we sense when we look up, it's more like a vast barrage of collisions and explosions that we're caught in the middle of. Indeed, it's remarkable that we've managed to survive this long.
For instance, there are two smaller companion galaxies to the Milky Way, the Lesser and Greater Magellanic Clouds. These may have been independent galaxies once, but they're now orbiting minions of us, their shape distorted by our galaxy's titanic gravity, vast shreds of their matter infalling upon ours. They're slowly spiraling in toward us, 180,000 light-years away and counting down.
Far vaster is the Andromeda Galaxy, a couple of million light-years away, but headed here fast under the mutual attraction of its gravity and ours. It's on a collision course, and when it arrives, our quiet corner of the galaxy will be destabilized by several million solar masses coming our way. If we're still around in a few billion years we could find ourselves spending all our time dodging stars.
In fact, there's so much to worry about, and wonder over, that I've made a timeline of — well, let's not call them "likely disasters", but just "dynamic life-altering events" that living in space will bring us.
| < 1 million years | Solar output variations bring on another Ice Age or global heatwave |
| < 1 billion years | Major asteroid collision with the Earth |
| 4 billion years | Andromeda galaxy collides with the Milky Way |
| 5 billion years | Sun expands to become a red giant, wiping out all life on the planet. |
| 6 billion years | Death spiral into the galactic center |
The great distance and stately pace of the stars beguiles us. By and large we have forgotten that they are whirling, speeding, colliding balls of colossal thermonuclear explosions that are basically out to get us, and which outnumber us by a few million to one. Hard to believe that we used to be afraid of invaders from space, when space itself now looks so perilous.
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