Tributaries of Time: Autumn Leaves

Across North America, as the transition toward winter begins, we see symptoms in the changing colors of tree leaves. The lively green hue of summer gives way to yellows, oranges, reds and purples. Living cells inside the leaves have instructions for how to react to cooler and cooler environments: this reaction reduces the production of the green pigment, chlorophyll, allowing other colors (caused by the pigments of the carotenoids and anthocyanins) to dominate. When spring returns, so do leaves, newly filled with oxygen-producing chlorophyll.

Year after year, we watch the cycle of death and rebirth in the life-giving foliage around us.

But what if we were insects? What if, like the mayfly, we lived for only a day or two? Would we have any way of understanding the immense tapestry evolving around us? Imagine for one day on Earth, looking at leaves all over the globe – in different environments and in various states of health and age. With just this one day to create a coherent picture, could we piece together the clues of color, environment and the internal workings?

This is the challenge we face in understanding the life-cycle of galaxies, the leaves on our cosmic tree of matter and light. To these celestial objects, we are indeed the mayfly, living for only a blink of an eye in cosmic time.

Consider the cornucopia of dusty swirls in the image above, their colors spanning the entire visible rainbow and beyond. Each puff of light contains billions of stars. Through our telescopes, images and spectrographs, we learn about the kinds of chemicals, of matter that reside within galaxies. Through an understanding of physics, we link this information to the possible physical processes, from gravity to quantum mechanics.

Similar to that of tree leaves, the colors of galaxies are the result of the chemical constituents, and they reflect their ages. Blue galaxies, still young, are cold enough to be forming stars, because young stars and the gas enshrouding them release bluer light to the cosmos. Red galaxies have had their star-formation extinguished: their gases are too warm for the force gravity to collapse them into energy-generating balls of fusion. These ‘red and dead’ galaxies represent the end of the galactic life-cycle.

While we have ways of peering inside galaxies to reveal some of their guts, we still have no way to watch an entire galaxy come into being, much less live out a full life. Each galaxy represents its own tributary of time, its own puzzle piece in the delta of the cosmic web.

Det. B. Nord

Image: Dark Energy Camera [Edited and logged by Det. M. Murphy]