Take a look at this photo.
I shot it a few weeks ago 70 miles outside of Phoenix, AZ. It's a long exposure photo of the stars, taken on a tripod, with a wide-angle lens, a wide aperture, and a high ISO. I set the camera to take an exposure over 20 seconds.
But I think the exposure is really much longer than that.
Taking photos of the stars is something I've fallen in love with over the past couple of months. It's something I discovered relatively by accident one night, when I pointed the camera at the sky and was surprised to find a picture with many more stars than I saw above me. The idea that the camera, with so little effort, could capture so much more than we could ever possibly see with our eyes was intoxicating.
I spent the next several weeks researching and learning techniques, going out at night to take practice shots, investing in better lenses and intervalometers and laser pointers and the lot. The fervor culminated in my first real test of my skills, an impromptu trip to Arizona, during which I saw some of the most breathtaking scenes I've ever laid my eyes on. You just don't get stars like that in the city.
One of the things I learned, though (which probably should have been obvious, but wasn't), is that composition is just as important with photos of the stars as it is with photos of anything else. Photos of a field of nameless stars seem breathtaking when you take them, but are unbelievably boring once you get them into Lightroom — it's a bunch of little white dots on a black screen. Foreground objects are important, and sometimes, so are the stars and bodies you choose to put into your photo.
There's a great free app called Stellarium that a lot of people use to assist in just such an endeavor. Give it a location, a date, and a time, and it'll give you a real-time 3D preview of exactly what the sky will look like. I've used it many-a-time for identifying objects in my photos after the fact, as well, and the Arizona photo above was no exception.
If you look closely, you can match up some of the more prominent stars with the ones in the photo. I set to work doing some of it myself:
After some time, though, I just started tapping around in Stellarium, looking at random stars at all corners of my photos. Two of them happened to catch my eye.
The stars were Eta Cassiopeiae (η Cas), also known as Achird, a fairly prominent star in the Cassiopeia constellation; and a tiny, almost indistinguishable star known as HIP 16149. They are, respectively, the nearest and farthest stars I could find that are readily identifiable in the photo.
How far apart are they, you might ask? η Cas is a mere 19.42 lightyears away from us. HIP 16149 on the other hand…
… is over 108,000 lightyears away.
So let's take a step back and think about how a digital photograph gets made. A shutter inside the camera body opens on command, and photons, passing through a lens, bombard a rectangular sensor, which registers their intensity and, ultimately, converts that intensity into a digital value that gets written into the RAW file. A photo is, then, an impression made by the interaction of light with a sensor.
When you take a photo during the daytime in bright sunlight, if you leave the shutter open for 20 seconds, besides having a completely overexposed photo, you will have captured the impressions of 20 seconds worth of photons bombarding the sensor. The light bouncing off the objects nearest to your camera will reach the sensor nearly instantaneously; the light from the objects farthest from your camera will arrive right behind. Because the speed of light is so great, and relative to that, the distance between the nearest and farthest objects in your photos will be so short — even a photo spanning a mile or two will cover an insignificant distance from light's perspective — you have captured a photo that records, near as makes no difference, 20 seconds of history.
At night, in the desert, though, things change a bit. The light from your foreground subjects will still reach the sensor nearly instantaneously, but if stars are in your frame, their light takes much longer. The light from η Cas in the photo above? It left the surface of the star when I was five years old.
Perhaps even more significant, though, is that the relative distance between the stars themselves is also massive — massive enough that the light from each star is capturing a 20-second snapshot from an entirely different period in history. I may have been playing in the elementary schoolyard when the photons from η Cas started making their way to Earth, but when HIP 16149 emitted its light, the Lost City of Atlantis was still a thriving civilization! If, of course, Plato is to be believed.
Looking up at the stars, in a very real sense, allows us to look into the past; taking a photo allows us to see even farther. What we capture in a single exposure is no longer just a single moment in time, nor even 20 seconds in time, but a span of time stretching from the age of the light from the nearest object in our photo, to the age of the light from the farthest.
So that photo above: the shutter may have only stayed open for 20 seconds, but the light in the exposure? It forms a canvas over 100,000 years in the making.
This post was inspired by a quote found online:
Not all starlight is the same age, some of it is millions or even billions of years apart even though it hits your eye at the same time. So the night sky isn't a single moment in the universe, but rather a patchwork of time billions of years in breadth.
And of course, the obligatory xkcd cartoon.