The Science of Sustainability

The Unaided Eye

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The Andromeda Galaxy, the most distant object visible
to the unaided eye. Credit: Conrad Jung
Very 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.

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Category: Astronomy, Partners, Physics

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Ben Burress

About the Author ()

Benjamin Burress has been a staff astronomer at Chabot Space & Science Center since July 1999. He graduated from Sonoma State University in 1985 with a bachelor’s degree in physics (and minor in astronomy), after which he signed on for a two-year stint in the Peace Corps, where he taught physics and mathematics in the African nation of Cameroon. From 1989-96 he served on the crew of NASA’s Kuiper Airborne Observatory at Ames Research Center in Mountain View, CA. From 1996-99, he was Head Observer at the Naval Prototype Optical Interferometer program at Lowell Observatory in Flagstaff, AZ. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • Jon Swartz

    Great explanation… exactly what I was looking for -thanks!

  • http://www.usends.com Dale Sanderson

    I realize this article was written quite some time ago… I hope you're still listening. I'm wondering if the unaided eye can observe anything (besides the handful of unaided-eye galaxies) that lies outside of the Orion Arm (or Local Spur) of our galaxy. This article comes closer to answering the question than any other I've found… but to which star in Cassiopeia are you referring? I read elsewhere that Rho Cas is nearly 12,000 ly away… true? Thanks…

  • http://www.chabotspace.org Ben Burress

    Yes, I am still listening! :) Thanks for the question, Dale. And on reviewing my own blog, I found an error in my original numbers…but also, in checking on the facts, I've found a number of other blogs and articles that have slightly different answers to the question, "what's the farthest star visible to the unaided eye?" Below is a link to one of these–Phil Plait's Bad Astronomy page–that's worth a read.

    As for the correction, I think I was mixing two different sources of info. The star I was referring to in Cassiopeia is Rho Cassiopeia–which, in fact, is approximately 12000 light years away. I've also seen different values for its luminosity, everything from 100,000 times the Sun's to 550,000 times. At any rate, it is visible to the unaided eye, and it's a good deal farther away than 4000 light years.

    Another blog I ran across hold up another star in Cass, Hipparcos 5926, as being over 16,000 light years away, and still naked-eye visible.

    Here's an enhanced answer to your question, though. Rho Cass is at least a singular example of a naked eye star that is quite distant, at 12,000 light years, which is almost half the distance from Earth to the center of the Milky Way Galaxy (we're about 27,000 light years from there), which is a healthy distance into the galaxy by any stretch. The "glow" of the Milky Way, which comes from massed distant stars that we don't see as individuals with our eyes, comes, I believe, from a greater distance…but of course when you look at it, you're not looking at singular object.

    Phil's article also mentions that people with really good eyesight under extremely good viewing conditions can see the galaxy M81, which is 12 million light years away…beating even the Andromeda by quite a bit.

    Well, I hope that helps!

  • http://www.usends.com Dale Sanderson

    Thanks so much for your quick response. Maybe I'm being daft, but I didn't see a link to Phil's article. I searched his blog, but don't think I found the article to which you referred. Regardless, I think the gist of your article is still correct, namely: with very few exceptions, what we can see from Earth with the unaided eye "is comprised of about 9000 stars ranging in distance from 4 to 4000 light years". I read elsewhere that the next-nearest spiral arm (the Perseus Arm) is over 6000 LY distant, so I think that means another way of looking at it is: the vast majority of what's visible to the unaided eye is not only within the Milky Way Galaxy, but within our own Local Spur of the galaxy.

  • http://www.chabotspace.org Ben Burress

    Well I THOUGHT I included the link! Let's try that again:
    http://www.badastronomy.com/qandba/transcript_episode3.html

    :)

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