Deep-sky objects may appear static throughout our lifetime but by carefully “blinking” archival and current images we can discern real changes in their appearance.
Amateur astronomers are constantly aware of the passage of time from the split-second disappearance of a star occulted by the Moon to the trillions of years it takes for the Sun to evolve from black dust to black dwarf. We so often rub shoulders with ancient things we're keenly aware of time's wide spectrum and the tiny slot we occupy called now. Regrettably, we can't be around long enough to witness changes on a vast scale but have a good sense of what has been and will be by blending what science has revealed with happy hours under the star-specked sky.
Variable stars, novae, supernovae, and a small number of double and large-proper-motion stars (e.g., Barnard's Star) show changes within the scope of a human lifetime. The rest of the universe is essentially a series of still-life paintings. The Andromeda Galaxy looks the same to my floater-filled eyes today as it did during my clear-eyed youth. Just about everything is too far away and evolves too slowly for the human eye to grasp. What I wouldn't give to live to be a million years old — and keep my health (and health coverage). That would be enough time to see wholesale changes in the outlines of the constellations, star clouds ripen into stars, and maybe even a stunning Milky Way supernova.
Wishful thinking aside, we can use time-lapse photography to see how a few cosmic objects have changed in the past 100-plus years and in a few cases in even less time. Changes in the structure of planetary nebulae, emission and reflection nebulae, supernova remnants, and novae debris clouds are all fair game. Several of these then-and-now videos use Hubble Space Telescope imagery. Amateur astronomers have created others, including several superb examples by Tom Polakis, observer and research assistant at Lowell Observatory. Polakis has created a time-lapse gallery of deep-sky objects, variable stars, and supernovae by melding old and new images into animated gifs that reveal real changes in their appearance. It's not an easy process.
"The alignment can be difficult, even with software that uses multiple stars," said Polakis. "But what is more difficult is balancing brightness levels and sharpness in the image pairs. The worse image of the two is the lowest common denominator that I try to somewhat match by adjusting histogram levels and sometimes even blurring the better image."
Planetary nebulae typically expand at the rate of around 40 kilometers per second (94,000 miles/hour) and supernovae remnants at (initially) 10,000 kilometers per second (22.4 million miles/hour). This makes them good targets for detecting expansion and other structural changes over nearly a century and a half. Time-lapses also reveal changes in the brightness levels and positions of small cloudlets in emission nebulae. The cameras started rolling when amateur astronomer Henry Draper took the first deep-sky photo in 1880 of the Orion Nebula, and images have been pouring in ever since.
The creators of these stunning animations have done their best to equalize variations in exposure and equipment. I hope you find them as eye-opening as I did. Be sure to visit Tom's gallery to revel in more. Until we've figured out the secret to immortality these compressed views reveal a universe of continual change. Of course, we knew that, but to see it is quite another thing.