I love looking up in the sky on a clear night (partly cloudy nights with lots of moon are nice too, but that’s another blog post entirely) and just knowing that each of those pinpoints of light is billions of miles away. As you gaze into the star-studded blackness, photons from those exact stars are entering your eyes, and they’ve been traveling for anywhere from tens to millions of years, to get there. Each photon, each piece of light that hits your eye and sends a signal to your brain, started its journey by leaping boldly from the surface of its star and diving into the void of space. Because that little packet of light actually completed its arduous journey through the Universe to your eyeball, you know that it never got absorbed along the way – it just headed straight for you.
Now here’s a brain-bender: because the light took so many years to reach you, i.e. you see the star as it was when its photons left it, you are literally looking back in time. Keep that in the back of your head awhile, because we’ll come back to it soon.
Our photon’s journey didn’t just begin when it “leapt from the surface of the star” – no no no, that wasn’t nearly the beginning at all. Let’s say it was born in the core, where hydrogen with its one proton is being fused into helium with its two protons and two neutrons, thus releasing energy in the form of photons. Gradually, each photon fights its way out of the dense, busy interior of the star, bouncing from one atom to the next, messing with orbits of electrons in the process. It could take as long as years for a photon from the core to reach the surface, being absorbed and ejected all along the way. As it makes its way to the star’s exterior, the going gets a little easier, until finally it is in empty space, and the rest of its voyage to your eyeball is spent in harmonious peace, gliding along through the void, past other stars, its journey watched and guided by the surrounding infinitely-extending curtain of lights.
And when you see light from another galaxy? Wow, that’s seriously cool, because even though it looks like a teeny point or perhaps a faint smear on the sky, that light is from all of the stars in that galaxy! And thinking about how long ago those photons were liberated from their stars really makes one wonder what those stars and galaxies look like now: are they even still there? Perhaps they aren’t, but the light from their demise has not yet reached us. Only time will tell.