字幕表 動画を再生する 英語字幕をプリント See that strip of stars splashed across the night sky? That's our home galaxy, the Milky Way. You can see it from Earth because we live right on the outskirts, far away from all the fun stuff happening in its center. The Milky Way is huge - about 100,000 light-years across. It has four large, spiral arms encircling its center. That makes it a barred spiral galaxy, just like most galaxies in the observable universe. If the Earth packed up its belongings and moved to the heart of the Milky Way, things would be looking pretty bleak for the survival of any kind of life on the planet. But not because of the Milky Way's supermassive black hole... The Earth is located in a quiet neighborhood in one of the smallest spurs of the Milky Way - the Orion Arm. We've got approximately 27,000 light-years between us and the galactic center. It's a comfortable spot to live in. The temperatures on Earth are just right to sustain life. And there aren't too many space hazards that could wipe out our existence. I didn't say it's completely safe, did I? Living anywhere near the center of the Milky Way would be... different. It would depend on what part of that center the Earth had the misfortune to occupy. The closer you get to the heart of the galaxy, the tighter the stars are packed together. Because of this high star density, the Earth would be blasted with a lot more radiation than what the Sun throws at you right now. Under these circumstances, the odds of life ever evolving would be pretty small. But if the Earth moved to the center together with all its inhabitants, you'd find yourself very unlucky. Earth's magnetosphere wouldn't be able to protect you from space radiation blasting from all directions. This could change the Earth's climate, and cause everything on the planet to either mutate or die. If you lived, your biggest concern would be a close encounter with a supernova. A supernova occurs when a massive star collapses releasing radio waves, X-rays, cosmic rays and gamma rays into space. For all we know, one massive stellar body could throw the Earth out of the Sun's orbit. Our planet would be surrounded by foreign stars until one of them exploded in a supernova and wiped us out for good. But it would look spectacular from Earth, if only for a brief moment. There's another big hazard that lies within the Milky Way's center - a supermassive black hole called Sagittarius A*. If getting sucked into a black hole isn't on your to-do list, consider yourself lucky. The Earth might stay far enough from it that we wouldn't get engulfed by this cosmic beast. But we could get close enough to get caught in the black hole's orbit. At a distance of 20 billion km (12 billion mi) from the black hole's event horizon, the Earth could develop a speed of 25 million km/h (15.5 million mph). That's over 230 times faster than the speed our planet is moving at right now. It sure wouldn't be good for you, or anything else on the Earth's surface. The Earth is perfectly designed to be right where it is. It can survive on the outskirts of the galaxy, but you wouldn't want it to be anywhere else. The most dangerous place for a planet to be located is anywhere near the center of a galaxy - any galaxy. If you want a nice view, your best bet might be to teleport the Earth to the middle of a nebula instead. But that's a story for another WHAT IF.