If one of these days you find yourself under a dark night sky, have a look at the constellation Andromeda. With bare eyes you should just be able to spot a faint smudge in this constellation. You need sharp eyes that are well-adapted to the dark. It definitely helps if you happen to carry with you a pair of binoculars. And the dark should be real dark. That means a spot far away from city lights. Also the moon, with its overwhelming brightness, needs to be out of sight.
Once you have spotted it, look more closely at that faint smudge. It is the furthest object you can see with bare eyes. You are looking at a galaxy comparable to but somewhat larger than our Milky Way galaxy. It is the enormous distance you are away from Andromeda that reduces it to a faint fuzzy in the night sky. The light from this galaxy has been traveling an amazing 2.5 million years to reach you. In comparison, no human has ever reached a spot from which light would need to travel more than 1.3 seconds to reach Earth. The distance traveled by light in two-and-a-half million years is a distance way beyond human comprehension. Yet you are more strongly bonded to Andromeda than to Earth.
You read that correctly. You are gravitationally more strongly bonded to Andromeda than you are to earth.
Let me make that more precise. Gravitation makes you stick to earth. And this gravitational binding to earth is pretty strong. To escape earth’s gravitational pull from your present position, you would need to jump up at a speed of about 11 km per second (7 mi/s). No small task. And that is ignoring any drag due to Earth’s atmosphere. However, to escape that faint smudge in the sky, you need to jump much more fiercely. In fact, you need to jump such that you achieve a speed of 88 km/s (55 mi/s) relative to the same smudge. And no, I am not cheating, it’s a like-for-like comparison. It is you again jumping from your same present position, and that is again ignoring atmospheric drag.
Few people realize the amazing reach of gravity. Gravity adds up. Andromeda with its trillion stars is incredibly more heavy than earth, and an overwhelming gravitational attraction comes with it that easily compensates for the enormous distance. The fact that you are gravitationally bound to Andromeda, makes everything around you – earth, the solar system and the whole Milky Way – bound to Andromeda. It should therefore not surprise you that the Milky Way is on a head-on collision course with Andromeda. Both galaxies are falling into each other. Don’t be worried, this is a long fall, and you and I won’t witness the final stage of it, and neither will your children, your grand-children, your grand-grand-children, … , and so on including your grand-to-the-power-100,000,000-children. And when the galaxy merger finally takes place, it will perhaps be a most welcome event as around that time we – if indeed we still exists – will need some forceful intervention to pull us away from sun, which soon thereafter will blow up and turn into a red giant.