Memento mori

The Helix Nebula is a particularly beautiful example of a planetary nebula: the outer layers shed by a dying star. The death of our Sun will eventually produce a planetary nebula, but only time will tell whether it will be as striking as the Helix Nebula. [Credit: NASA, ESA, C.R. O'Dell (Vanderbilt University), M. Meixner and P. McCullough (STScI)]

The Helix Nebula is a particularly beautiful example of a planetary nebula: the outer layers shed by a dying star. The death of our Sun will eventually produce a planetary nebula, but only time will tell whether it will be as striking as the Helix Nebula. [Credit: NASA, ESA, C.R. O'Dell (Vanderbilt University), M. Meixner and P. McCullough (STScI)]

As deaths go, our Sun’s will be gentle: no supernova, no black hole, no cataclysm. Roughly 5 billion years from now, our host star will exhaust the hydrogen fuel in its core and expand into a red giant. As its core collapses under intense gravity, it will pass through several more stages over the course of millions of years before ultimately shedding most of its outer layers. The remnant will be a planetary nebula, surrounding what was once the Sun’s core and is now a white dwarf.

The image above is of such a planetary nebula. We don’t know what our Sun’s eventual nebula will look like — planetary nebulas come in an amazing variety of shapes. But like a religious memento mori (a portrait including a skull, or some such token) is a symbolic reminder of our individual deaths, the Helix Nebula could stand as a memento mori for all stars like our Sun.

One lesson we learn from studying the Universe: much that happens is destructive, at least in a sense. But that destruction is phenomenally slow on the human scale. A process that lasting a blink of the eye in cosmic terms — the birth or death of a star, the collisions of galaxies — can take places over millions of years, or longer. And often something of beauty is born from destruction. All stars must die, but their deaths are necessary to spread the atomic seeds for new stars and planets. Each new generation of stars is born from the remnants of previous generations, gradually changing the chemistry of the cosmos. The deaths of stars enabled us to exist, and any newborn planet will likewise bear the chemical history of the dead stars that came before it.

Five billion years is a significant amount of time even in cosmic terms: it’s more than one-third of the Universe’s current age of 13.8 billion years. Smaller stars — which are far more abundant even than Sun-like stars — may shine for trillions of years before burning out, and new stars are born all the time. The process can’t go on forever, but the death of stars is still a relative blip compared to the ongoing lives of stars.

We live on a tiny, fragile planet orbiting a nondescript star in a large but otherwise unremarkable galaxy. Our Milky Way contains many stars like our Sun, and possibly an abundance of planets similar to Earth (at least in size and composition, if not habitable). The Universe can seem vast, empty, and meaningless, when viewed from our small perspective. However, I don’t believe that makes our lives meaningless. Just as my inevitable death doesn’t imply I should give up on living a meaningful life in the interim, the eventual death of Earth and the Sun doesn’t mean our collective existence is meaningless.

Eventually all stars will die, and the Universe will go dark. But that event is so far in the future that it is a tragedy only from the standpoint of eternity. The beauty of the cosmos is now.

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