Quantities like 100 trillion kilometers are extremely unwieldy to write (100,000,000,000,000), let alone use, but when can we use that big of a number anyway?
Traveling at over 300,000 kilometers per second, light travels around 10 trillion kilometers in one year. This figure (the distance travelled by a ray of light in one year or the so called 'light year') - has become the standard yardstick for measuring distance outside the Solar System. The nearest star of all, Proxima Centauri, lies at a distance of 4.2 light years.
The astronomer William Herschel likened the stellar distribution to a giant millstone - a flat, disc-shaped structure that was much wider than it was thick. The Milky Way galaxy is a disc of starts, dust, and gas around 100,000 light years in diameter but only 2,000 light years thick. This disc swells in the middle into a crowded region known as the 'bulge', containing billions of ancient stars and a supermassive black hole with a mass equivalent to 4 million Suns, while several spiral arms (dense lanes of stars and dust) swirl out from the center towards the rim of the disc. The Sun is just one of between 200 and 400 billion stars in the Milky Way, and it is situated in the disc around 27,000 light years from the galactic center, on the edge of the Orion Arm. If the Milky Way is a city of stars, our Sun is in the outer suburbs.
Just as the planets of the Solar System orbiting around the sun, everything in the Milky Way is orbiting around its center of mass, which lies at the supermassive black hole in the middle of the galaxy. Constant motion is the natural state of affairs within the Solar System and within the Milky Way and other galaxies, so it comes as no surprise that the galaxies themselves are also moving. Our immediate galactic environment is a throng of over 50 other galaxies known as the Local Group, which is roughly 10 million light years across.
The Local Group is also moving through space (no surprise). However, the Local Group is rather a modest one and utterly dwarfed by its neighbor, a collection of over 1,000 galaxies (Local Group has only ~50 galaxies) called the Virgo Cluster. Collectively, these giant clusters, along with the Local Group itself, are known as the Laniakea Supercluster. Laniakea is 520 million light years across and its borders encompass as many as 100,000 individual galaxies. Appropriately its name comes from a Hawaiian word meaning 'immeasurable heaven'.
This distance will increase overtime and the fairly astounding explanation for this was demonstrated by Edwin Hubble in 1929: our universe is expanding. The space between everything in our universe is constantly stretching out (search for cosmological redshift if you'd like to know more). In the instant of a Big Bang, space itself exploded into existence and the universe created the volume into which is expanded. This might make the perfect mathematical sense but it is hard for our brains to visualize it; as a rough analogy - although in 2 dimension instead of 3 - we can imagine that it's like spots painted on the surface of rubber balloon. As the balloon is inflated the rubber between the spots expands. Pick any spots as your reference point and it will seem as though all of the others are receding from it. This is what happens to our own three-dimensional universe. We might see more of it than our future generation as the distance will become even larger and larger, and harder to be seen.
Is our universe finite? The answer is we don't know. The part of the universe we can observe is certainly finite, and we refer to this as the Observable Universe. The most distant object we can see are objects which light has been able to travel during 13.8 billion years, which is the age of the universe. Because we know how much the universe has expanded, we can calculate the diameter of the Observable Universe today, and it is over 93 billion light years.
I really like the word Laniakea, it makes us feel insignificant yes, but it's a revelation that the universe out there is immeasurable (limitless?) and there still is a lot of possibilities we don't know. There lies our sense of wonder.
