How big is our Milky Way Galaxy?
If we were to just try and make sense of the distance across our galaxy the Milky Way, we would already surpass all meaningful numbers we can comprehend. But let's try anyway. Imagine that our entire Solar System were the size of a quarter.
On this scale, the edges of the outer perimeter of our Milky Way Galaxy would touch both the Western and Eastern borders of a map of the United States! In terms of actual distance, it is roughly 100,000 light years across. How far is that in understandable terms? It would be the equivalent of traveling at light speed (186,000 miles per second) for 100,000 years. That's a long way.
How far away is the nearest star?
The closest star to Earth is of course our Sun at 93 million miles distant. It takes sunlight roughly 8 minutes to reach us here on Earth. The next closest star is Proxima Centauri, which is one of the 3 stars often referred to as Alpha Centauri. If you were to again use our coin model from above, Proxima Centauri (and any planets orbiting around it that might comprise it's own solar system) would be in the center of another quarter, over two soccer fields away.
As you look up into the sky on any clear night, you will see mostly stars within our own galaxy, but surprisingly, many points of light that appear to be stars, are actually other farther distant galaxies. They are so far away they only appear to be single points of light.
The nearest galaxy to Earth is M31, or the Andromeda Galaxy. It is slightly smaller in mass than the Milky Way and about 2.5 million light years from Earth. If you know where to look on a clear night, you can actually see M31 with your naked eye. With a decent telescope, Andromeda is a spectacular sight.
How far is it to the farthest star?
It is impossible to detect the farthest star in our universe. We can't see more than roughly 13 billion light years into the far reaches of space with our most powerful telescopes.
However, the farthest known object in the universe to date was reported in October 2010 as a primitive galaxy, called UDFy-38135539 (UDF stands for Hubble Telescope's Deep Field Identifier). No one can claim that's a very sexy name for a galaxy.
But what has scientists excited, is that it is also the youngest galaxy they have spotted to date. Estimated at only 600 million years old, this would also be the closest object to the time of the "Big Bang" (the theoretical beginning of the universe) we have on record to date. UDFy-38135539 is estimated at over 13 billion light years from Earth.
How big is the Universe?
It is estimated that the observable universe may be more than 93 billion light years across. Even for Star Trek fans, who are comfortable with warp factor 9 (by various estimates over 1000x light speed), that is a mind boggling distance which would be impossible to traverse.
But even if 93 billion light years across sounds big to you, and it is, imagine now that the edge of the universe is still expanding at roughly the speed of light as well - in all directions (it may be, but we calculate expansion of the visable part of the universe closest to us technically using a calculation referred to as the Hubble Constant, which is most recently estimated to be almost 74 kilometers per second and the farther objects are away from us the faster their acceleration).
Not only that, but we've learned that expansion is also accelerating! Our estimates of the distance from one end to the other then, are just that - a pretty good guess. Some might call it a WAG, or wild-ass-guess.
So, the truth is we have no idea how big the universe is. But it's pretty fun to think about how far out there really is, don't you think?
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