Western Slope Skies - The Starry Dark

Nov 13, 2020

Credit NASA/ESA/Hubble

Have you ever tried to count the stars on a crisp, clear night, far from city lights? It is not for the numerically faint of heart.

Astronomers estimate a few thousand are typically visible to the naked eye, with another few million readily seen through optical telescopes. Overall, roughly one septillion (i.e., 1 followed by twenty-four zeroes) are estimated to exist. Given this, you may marvel that the sky is largely dark. Where is all that starlight? You would not be the first to ask.

The dark night sky question has a long pedigree. In sixth-century Egypt, Greek monk Cosmas Indicopleutes wondered why the myriad stars do not incinerate the sky. In 1576, English astronomer Thomas Digges formalized the question in terms of a universe of infinite age, volume, and star population; Johannes Kepler briefly took it up in 1610. The idea was popularized in the 1700s as Olbers’ Paradox, after German astronomer Heinrich Olbers, though several contemporary astronomers (including Edmond Halley of cometary fame) also grappled with it.

In an 1848 essay, Edgar Allen Poe suggested that in a sufficiently large universe, not all starlight will have had time to reach us, an idea also advanced by Lord Kelvin in an obscure 1901 paper. Both anticipated the modern understanding that we see a limited range of the cosmos -- the Observable Universe -- due to the finite speed of light traversing space.

We know now that the universe is expanding, flinging galaxies further apart. If the expansion continues unchecked, the Observable Universe will gradually shrink, as ever less starlight manages to reach us. We also know that the universe was once much smaller, denser, and hotter than today. Back then, space was blindingly opaque. In the billions of years since, this energy has been stretched and cooled by cosmic expansion into an extremely low-frequency microwave background. We cannot see it with our eyes, so space remains dark and transparent; but with highly sensitive detectors, astronomers can observe a uniformly faint shimmer.

Perhaps the marvel is not the darkness of the sky, but rather that we see stars at all. Humanity fortunately exists in a cosmic Goldilocks epoch, between the universe’s seething past and potentially dim future. Next time you look up at the heavens, literally count your lucky stars that we do.

Western Slopes Skies is produced by the Black Canyon Astronomical Society.  This feature was written and voiced by Michael T. Williams.