The Unaided Eye
The Andromeda Galaxy, the most distant object visible
to the unaided eye. Credit: Conrad JungVery often, the term "naked eye" is used to describe what can be seen with human eyes alone, unaided by tools like telescopes, microscopes, infrared cameras, ultraviolet detectors, and so on. Back in the mid 20th Century, then director of Chabot Observatory, Earl Linsley, coined what he felt was a more accurate (and maybe less provocative?) term to describe what can be seen in the night sky with unaugmented human vision: the unaided eye.
So, without telescope or binoculars, filters or crystal balls, what are we seeing when we look at a night sky full of stars? I mean, what are we really looking at? How much of the Universe meets our unaided eyes? How far into space are we seeing?
In short, not a lot–not very far. Let's ignore the objects of our solar system for the moment–which would include the Sun, the Moon, the five "visible" planets, and the occasional comet. That leaves a multitude of individual stars and some star clusters, the ghostly arch of the Milky Way, the misty blotches of the two nearby dwarf galaxies called the Large and Small Magellanic Clouds (visible from the Southern Hemisphere), and the barely perceptible smudge of the Andromeda Galaxy.
The individual stars you can see, from the brightest beacons to those that barely tickle your dark-adapted retinas, number about 9000 across the entire sky (as seen in every direction, if the Earth were not in the way).
The closest of these, the triple star system Alpha Centauri, is about 4.3 light years away (each light year is equal to about 6 trillion miles).
Within the small group of brightest stellar luminaries (numbering about 25), the most distant is Deneb, the "tail" star of the constellation Cygnus the Swan, which is about 1600 light years away.
The most distant individual star visible to the unaided eye is a little over 4000 light years away, in the constellation Cassiopeia–and though it appears to us as a fairly faint star, it is in reality a supergiant star over 100,000 times more luminous than our Sun.
So, most of the sky is comprised of about 9000 stars ranging in distance from 4 to 4000 light years. As it turns out, the patch of the Universe where these stars exist is a small "bubble" of space set within the much larger expanse of the Milky Way galaxy, which overall is 100,000 light years across and contains at least 200 billion stars…
Put into an analogy, if the entire surface of the Earth represents the expanse of the Milky Way galaxy, then the region encompassing the stars visible to our unaided human eyes would be roughly the size of California–with most of them contained in an even smaller area. In short, most of what you see in the sky are not only the closest things to us in the Universe, they're pretty much the closest things to us merely in our own galaxy!
But not the most distant object visible to the unaided eye, the Andromeda Galaxy.
The Andromeda Galaxy is 2.5 million light years away–so its light reaching us today left there when the earliest hominids were walking the Earth. Sounds pretty far away, but put into perspective, if the entire known Universe were represented by a full-sized football stadium (seating included), then the distance to the Andromeda Galaxy–the farthest we can see with our eyes in the dark of the cosmos–would be about three inches away…
Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.