How to See the Farthest Thing You Can See

Step by step, we explore the farthest things visible with the naked eye, from the most distant star to the hinterlands of the Andromeda Galaxy.

Close enough to be neighbors

Saturn, the most distant, bright naked-eye planet, lies 950 million miles from Earth at mid-month.
Efrain Morales

Whenever I point out the planet Saturn to the public and tell them it's nearly a billion miles away, they're aghast. It doesn't seem possible to see that far. And yet at that distance, we're barely out the door, cosmically speaking. The unaided eye can do so much more.

Back on Earth, we're lucky to see beyond 12 miles (20 km) at sea level due to haze, dust, and the curvature of the Earth getting in the way. To peer further, we must go higher. From the top of Mt. Everest it's theoretically possible to see 210 miles (339 km) to the horizon.

But once we lift our gaze, we easily see the space station at night 250 miles up, the edge of the auroral oval 500 miles away, the Moon at 240,000 miles, and of course the planets — all the way to Saturn — with the naked eye. On dark nights, some even manage to snare a look at dim Uranus 1.7 billion miles away.

When we depart the Solar System to plumb the starry depths, the tremendous distances to the stars make using miles and kilometers cumbersome. Astronomers prefer the light-year, the distance light travels in one year's time moving at 186,000 miles per second (300,000 km/sec).

By seconds, minutes, and years to the Andromeda Galaxy

The light-year is not only a more practical way to describe distances, it also gives a clue as to how long light has traveled to reach our eyes. This scale starts close to home but takes us all the way out to the Andromeda Galaxy, the most distant object most people can see with the unaided eye.
Bob King

Since there are about 31,536,000 seconds in a year, that tallies up to just shy of six trillion miles in one year — a distance well beyond the most remote planets and asteroids but only a quarter the way to the nearest star system beyond the Sun, Alpha Centauri.

Using light-years also allows us to more fully appreciate the vastness of the universe. When we gaze at Alpha Centauri, we see the light that departed the star 4.4 years ago and traveled across a chasm of space 26 trillion miles wide before finally touching our retinas.

Deneb, which heads up the Northern Cross or marks the tail of Cygnus, is one of the intrinsically brightest stars in the sky. Located approximately 1,550 light-years away, the light you see tonight left on its journey to Earth around 465 A.D. during the sacking of Rome.

Distant supergiant lair

Three of the most distant naked-eye stars appear near one another in the northern sky on fall nights: Mu and Nu Cephei and Rho Cassiopeiae, perhaps the most distant star visible without optical aid.
Stellarium

But Deneb's distance pales to that of the most remote stars visible with the unaided eye. Several of the most distant are easily visible from the outer suburbs and countryside on fall nights in the northern sky. If you turn your gaze toward the familiar W of Cassiopeia, you'll find the dim point of light Rho Cassiopeiae 2.5° west of Beta. Currently at magnitude +4.7, nothing about its visual appearance offers a hint of the star's true nature.

Too big for the room

The Sun shrinks to a minute disk next to the enormity that is Rho Cassiopeiae. The star, 450 times bigger than our Sun, is a likely candidate to go supernova within the next 50,000 years. Click image to learn more about this amazing ball of nuclear fire.
Gauloiq / Wikipedia Français

Yet from a careful study of its light, we know that Rho is a hypergiant star radiating 550,000 times more light than the Sun with a girth 40% larger than the orbit of Mars! Indeed, this is the reason we can see it with the naked eye in first place despite its spectacular distance of 8,200 light years.

Nearby Mu Cephei, better known as the "Garnet Star," and Nu Cephei (magnitudes +3.9 and +4.3 respectively), are both grand supergiants visible from remote realms of the Milky Way galaxy. Can we go further?

Of course! Just squeeze a bunch of stars into ball called a globular star cluster and your eyes can telescope across 25,000 light-years. The Milky Way's largest globular cluster, Omega Centauri, visible from the far southern U.S. and points south, packs 10 million into a spherical swarm 150 light-years wide some 15,000 light years away. At 4th magnitude, the cluster's plainly visible to the naked eye as a patch of glowing mist nearly as big as the Full Moon.

Swarms of light

Globulars Omega Centauri (left) and the Hercules Globular M13 (right). Each is visible with the naked eye across distances of 15,000 and 25,000 light-years, respectively.
ESO (left) / N. A. Sharp, REU program, NOAO, AURA, NSF

Similarly, the Great Globular in Hercules (M13) lies 25,000 light-years from Earth and contains up to 300,000 stars. At magnitude +5.8, it shines just a hair above the standard naked-eye limit of +6.0, but I've seen it many times from dark, country skies as a small, milky patch with averted vision. As you whirl your scope in its direction and revel in its blizzard of stars, know that your ancestors were chipping away at projectile points 50,000 years ago during the Stone Age when the photons from those stars began their incredible journey to your eye.

25,000 light-years takes us a quarter way across the galaxy. To make the next leap, we must journey to the southern hemisphere, where the Milky Way's two brightest satellite galaxies, the Magellanic Clouds, reside. This duo of irregular dwarf galaxies orbit our own galaxy; the Large Magellanic Cloud lies more than six times farther away than the Hercules cluster at about 160,000 light-years, while the Small Magellanic Cloud is around 200,000 light-year away.

Shoot an arrow to Andromeda

If you use the top half of the W of Cassiopeia — the Alpha, Beta, and Gamma stars — as an "arrow", it points directly to the Andromeda Galaxy, a small, 4th-magnitude fuzzy patch of light close northwest of Nu Andromedae. Map the sky facing east-northeast around 9:30 p.m. local time. See photo below.
Stellarium

Impressive as these distances are, we can still do better. Much better. A fist and a half below the Cassiopeia's W, we finally arrive at the stopping point of human vision. Like so many objects in the astronomical world, its naked-eye appearance is deceiving. Just a bit of fuzzy fluff like a shard of Milky Way gone adrift. But that little spot has a name that says it all: Andromeda Galaxy.

Use the W to double your pleasure

This photo shows how Cassiopeia makes easy work of finding Andromeda. You can use the bottom half of the W to wander over to another wonderful sight — the Double Cluster in Perseus. Shining around magnitude +3.5, the clusters appear like a brighter fuzzy spot.
Bob King

Its enormous disk, some 220,000 light-years across, or more than twice the size of our galaxy, lies 2.5 million light-years from Earth. Amazingly, we can see it without optical aid from a moderately dark sky. Why? Andromeda's close as galaxies go, and a trillion stars cram its fuzzy disk. That's a lot of candlepower. But all those suns are so distant, they blend into a smooth, unresolved haze, even in large amateur telescopes.

When you've found the galaxy, spend a few metaphysical minutes pondering your place in the cosmic vastitude. Consider any number of potential Andromedid life forms looking back at your spiral galaxy, the Milky Way.

Home sweet home

If we could find a suitable location equidistant from both the Milky Way Galaxy (left) and Andromeda Galaxy (right), we'd be able to appreciate their true sizes. Andromeda is more than twice the size of our home. Both are spiral galaxies with majestic, star-forming arms wound around dense cores of older stars.
Bob King

As you soak in the view, you may notice a bit of structure to the galaxy even even without optical aid. The center, where more of Andromeda's stars are concentrated, appears distinctly brighter than the more lightly-populated outer disk. Use averted vision — looking to this side and that rather than directly at the object — to appreciate its large apparent size. Under dark skies, the galaxy spans nearly 3° or six side-by-side full Moons.

Simply magnificent!

The Andromeda Galaxy (M31) in all its stellar glory! The two smaller, fuzzy glows are two of its satellite galaxies.
Frank Barrett / celestialwonders.com

Some keen-eyed amateurs under the darkest skies have netted even more distant prey like the Triangulum Galaxy (2.7 million light-years) and even the M81–M82 pair in Ursa Major (11 million light-years), but most of us ordinary folk hit our limit at Andromeda. To go beyond requires optical aid, and that would spoil the fun.

Or would it? Next week, we'll return to Andromeda and discover how much we can discover there using only a pair of binoculars.


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Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. Check out my forthcoming book "Night Sky with the Naked Eye" (on Amazon and BN) about all the great things you can see at night without any special equipment.

7 thoughts on “How to See the Farthest Thing You Can See

    1. Bob KingBob King Post author

      Anthony,
      Thank you very much for your kind words. I hope your friends go out for a look, too!

  1. Karl

    Then there was GRB 080319B, a naked eye object for a short while in 2008… at 7.5 billion light years (3000 times the distance of Andromeda)! It wasn’t much to look at, but a GRB is NOT something you want to witness close up. That light had been on it’s way for almost twice the age of the Earth…

    1. Karl

      I should add, while I’m sure quite a few people “saw” it, I doubt anyone noticed it. It was only bright enough to see in a very dark sky for less than a minute.

      1. Bob KingBob King Post author

        Hi Karl,
        I remember that explosion, but I don’t recall any naked eye sightings. Not because it wasn’t bright enough (magnitude +2) but because no one was paying attention in that brief span when it was visible. As you suggest, some may have “seen” it, but they never noticed.

  2. Bruce Dickson

    My long distance record is NGC5128 (aka Centaurus A / the cosmic hamburger.) At around 6.8m and 12.4 million light years, it requires an exceptional site, good seeing and averted vision. The galaxy subtends about 20′ – provided you know where to look, it’s quite possible.
    If memory serves, SJO’M once did a piece in S&T on long distance observing – probably 20 years back. It’s fuzzy but I think he and a friend observed galaxies in CVn or Vul from a mountaintop. I seem to recall two targets – one at 8 million light years and the other at around 12 or 14.

    1. Bob KingBob King Post author

      Thanks Bruce – that’s amazing! Wish I could have a shot at that one, but it sits exactly on my southern horizon when on the meridian. I was able to spot M33 two nights ago under a very dark sky at 2.7 million l.y.

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