How To See The Farthest Thing You Can See

Comet Hartley 2 approaches the "Pacman" Nebula in Cassiopeia last night as seen from Gizerka in the Czech Republic. Credit: Martin Gembek

As long as we’re spending time with Comet Hartley this month and its host constellation Cassiopeia, let’s use Cassiopeia to point us to the Andromeda Galaxy, the farthest object most of us will ever see with the naked eye. Astronomers use the ‘light year’ as a standard when measuring distances to stars and galaxies because they’re all so far away miles and kilometers simply aren’t adequate. Light travels at 186,000 miles per second (300,000 km/sec), and since there’s about 31,536,000 seconds in a year, that adds up to some six trillion miles of travel in one year. That’s well beyond the most distant planets and asteroids but only about 1/4 the way to the nearest star system after the sun, Alpha Centauri. When we gaze at Alpha, the light we see left it 4.4 years ago and traveled across a chasm of space 25 trillion miles wide before finally reaching our eye.

The farthest star we can see with our naked eye is V762 Cas in Cassiopeia at 16,308 light-years away. Its brightness is magnitude 5.8 or just above the 6th magnitude limit. While that seems remote, we can delve much deeper, but we’ll need to leave the realm of stars and move outward to galaxies beyond the Milky Way to do so.

A time-exposure photo reveals the beauty of Andromeda. All the individual stars are foreground stars in our own Milky Way. The dark swirls are lanes of dust shed by earlier generations of stars. You can also see two other fuzzy spots - those are satellite or companion galaxies of Andromeda. Credit: Frank Barrett

The Andromeda Galaxy is similar to our Milky Way in size and structure; both are spiral galaxies containing several hundreds of billions of stars. Every star you see in the sky on a clear night belongs to our galaxy. Picture Andromeda as another Milky Way 2.5 million light years from Earth. Amazingly, we can see this without optical aid from a moderately dark sky. Why? While that distance may seem enormous, it’s close as far as galaxies go, plus Andromeda is large and packs a bounty of stars.

To find the Andromeda Galaxy, start with the familiar W of Cassiopeia, now high in the northeastern sky around 9 o'clock. Use the right side of Cassiopeia as an "arrow" to point you to the galaxy, located one outstretched fist to its lower right. You can also use the Square of Pegasus above Jupiter to guide you to Beta Andromedae. The galaxy is two "fingers" above Beta. Created with Stellarium

If you lived on a planet revolving around a star in Andromeda, the sky would be filled with stars just like it is at home. They’d be strewn about in completely different patterns, and you’d be free to create your own constellations. Through a telescope, you’d be able to look at entirely different nebulas and star clusters scattered across the alien sky. Even better, if you were located in the thick disk of the galaxy as the sun and planets are in the Milky Way, you could look up and see a ribbon of hazy light cutting across the sky – the Andromeda Way. While its contours would vary, it would bear an uncanny resemblance to how the Milky Way looks from a dark sky on Earth.

A photographic finder map for the galaxy. If you have difficulty seeing Andromeda with your naked eye, try binoculars. Binoculars will also show its considerable extent as well as the bright nucleus. Photo: Bob King

Thank goodness for imagination. We need it, because with all that distance between us and Andromeda, the scenes described above are reduced to little more than a wisp of cloud below Cassiopeia. That’s what the galaxy looks like to the eye – a fuzzy patch with a brighter center where the majority of Andromeda’s stars are concentrated. It’s still one of the coolest things you’ll ever see; an entire galaxy right before your eyes. All those stars, planets and nebulas blend together to form an unresolved haze even in larger telescopes. At 2.5 million light years, its stars are not only too faint, but we can’t resolve them from one another, because they appear close together from such a great distance.

The Milky Way and Andromeda are the two largest galaxies in a ragtag assemblage of more than 30 called the Local Group. We’re part of a small cluster held together by our mutual gravities. On an even broader scale, the Local Group is a small piece of a vastly larger supercluster of galaxies centered in the constellation Virgo 60 million light years away.

We’ve come along way for one day. When you succeed in finding the Andromeda Galaxy, consider how deep you’re looking into space. It took the light you see 2.5 million years to travel the distance to your eye. Those are some old photons! With a little imagination, you can play time traveler and picture how Earth and life may have looked so long ago.

26 Responses

  1. alliu

    Wow..reading this makes me feel so igsinificant to the rest of the universe. The time it takes for light to travel here is great, yet to us on earth light travels faster than anything. Meaning that the light we see from the sun must have been somewhat old bcuz of the time it takes from the sun to earth to travel i wrong??!!

  2. maggie

    thanks for such a simple yet elegant discussion. as a geologist the fossils i work with are hundreds of millions of years old – and yet the astrological distances you discuss are more thought-provoking and awe-inspiring than most of my easily handled samples. to think that we can see an entire galaxy from our humble planet is truly wondrous. perhaps someday far in the future we’ll posess the technology to actually go to the Andromeda (or another) galaxy. fantastic!

  3. lance

    I think the fact that our distance to the sun 93,000,000 miles divided by the speed of light 186,000 miles per second equals 500 seconds. Trippy huh?

    1. astrobob

      Agreed. And if the sun is ever snuffed out, we’d be clueless for those 500 seconds until the last ray of sunshine reached us.

      1. james

        Hi bob you put it so simply and elegantly. I was wondering how i would explain this concept to my son and i think i will use your prose. The greatest miracles are out there for us all to see, if we can be bothered to look :)

  4. Pedro

    Astrobob, thanks for your guide to viewing Andromeda – i was trying to look this evening, but the moon was so strong that I couldn’t pick-up much even with the binoculars. I then got to thinking what is the furthest thing you can see with the naked-eye within our galaxy. So I was so pleased to read your blog above. I’d be pleased if you could give some instruction on how to view V762 Cas in Cassiopeia? Is it quite obvious to see? I live in a small city so there’s a bit of light pollution, would that cause issues in viewing V762?

    1. astrobob

      Hi Pedro,
      V762 Cas is 6th magnitude, so it would be easy to see in binoculars but you’d need a dark, moonless sky to see with the naked eye. It might be visible from your town if the light pollution’s not bad, since Cassiopeia is currently high in the sky. There are a lot of similar brightness stars near V762, so you’d need a good star chart to identify it. If you have a planetarium-style sky charting program, you can make your own chart. Your comment has got me thinking about doing a map and short piece on V762 for an upcoming blog. I’ll consider this once the moon is out of the sky again. In the meantime, here’s its position: RA 01 16 11.9, Dec +71 44 38

    1. astrobob

      Hi Victor,
      I’d have to make a map to point the star out to you. It’s faint. Maybe I’ll do that in a future blog. I really should have made one in the original!

  5. Charles

    What is the furthest star in the Milky Way Galaxy seen using a telescope? Like have we seen the closest end that we live end and seen the furthest end? How many black holes are in the Milky Way?

    1. astrobob

      I’ve heard that the most distant star in the Milky Way is some 160,000 light years from Earth. The closest star is Proxima Centauri (4.24 light years), a member of the Alpha Centauri triple system. The Milky Way has around 100 million stellar-mass black holes and one monster supermassive black hole at its center.

  6. Katnea

    Thank you so much for writing this article! I found it not only easy to understand, but very informative as well! I recall a friend of mine quizzing me years ago on what the most distant object known to man is, and yet can still be seen with the naked eye? I of course didn’t have the correct answer then and now it looks like he apparently didn’t either! (hah) Because he had told me that it was the “Andromeda Galaxy!” But after reading your article here, I understand now that its actually the star V762 Cas in Cassiopeia, that is the most distant object and yet can be observed with the naked eye!

    I’ve also read that our Universe is approximately 13.7 Billion years old and yet the distance to the most distant detectable objects, (via telescope) is about 46 billion (46,000,000,000) light-years away? What?!…. Yeah, it doesn’t take much to confuse me! (grin)

    1. astrobob

      Hi Katnea,
      Thanks for writing and I’m glad you enjoyed the article. Your friend is correct! For most of us, the most distant object visible with the naked eye is indeed the Andromeda Galaxy. My article briefly mentioned Andromeda as the most distant object but focused on the most distant STAR visible with the naked eye. The Andromeda Galaxy is much further away – about 2.5 million light years.

    1. astrobob

      Excellent question Rob. I got my figure of 16,308 light years from previously published online science articles and discussion groups. Looking back now, I see those figures still out there, but I also came across a parallax measurement made with the Hipparcos satellite. Here’s the data:
      V762 Cas (HIP 5926)
      parallax = 0.00022 +- 0.00059
      This implies a distance of 15,000 light years. Notice though the significant uncertainty, so to be honest, we don’t know the exact distance to this star. It’s far away, but it could be considerably closer than 15,000 light years or go the other way and turn out to be even farther!

      1. Rob Jeffries

        Bob, that’s what I thought. The revised Hipparcos parallax to this star from van Leeuwen (2007) is 1.18 +/- 0.45 milli-arcsecs. i.e. A distance of about 850 +/-350 pc. i.e. A just-about significant measurement and vastly closer than 5kpc.

        I was just wondering whether you had some other source for the larger estimate based on a spectral type or something?

        1. astrobob

          No, I was using previously published material. I had not come across the revised estimate when I wrote the article. Matter of fact, the Hipparcos estimate which I shared with you on the previous comment could put the star even farther than what was written up in the blog. Can you point me to the link for the new estimate? Many thanks Rob!

  7. Rob Jeffries

    Here are the top 4 with “reliable” parallaxes in the van Leeuwen (2007) catalogue (Astronomy and Astrophysics 2007, 474, 653). By reliable, I mean with parllaxes >2.5 times their uncertainties.

    Name Hpmag | Plx (mas) | e_Plx (mas) | Distance (pc) | Name/Spectral Type
    HIP 107418 4.39 0.48 0.14 2080 nu Cep A2 I
    HIP 22783 4.29 0.52 0.19 1920 alpha Cam 09.5 I
    HIP 54463 4.09 0.52 0.17 1920 chi Car G0 I
    HIP 107259 3.91 0.55 0.20 1820 mu Cep M2 I

    However, missing from this catalogue is Eta Carina. This is (currently) a naked eye object with V=5. The best distance estimates put is at 2200-2900 pc. (Recall that 1 parsec is about 3.1 light years).

  8. Interesting article, Bob, especially about V762 Cas. I had no idea that Hipparchos could measure distances that far out so accurately. I thought it could get to only a couple thousand light years, tops.

    I’m curious about something though, not as if you would know the answer. But since V762 is 5.8 magnitude, why didn’t Bayer see it and assign it a Greek letter? Or why didn’t Flamsteed see it and assign it a number? Especially when those guys had zero light pollution back in the day and could see 6th magnitude stars very easily. Where does the name V762 come from?

    1. astrobob

      Hi Jon,
      Thanks! Bayer’s original list included only 1,564 stars, only a fraction of the stars visible to magnitude +6. I doubt there are any 6th magnitude stars with Greek letter (Bayer) names. Since there are so many 6th mag. stars even though Flamsteed numbers are found on some, they’re not on others. There’s no Flamsteed number for V762 Cas. Its name instead is a variable star designation. It’s the 762nd variable star discovered in Cassiopeia, hence V762 Cas.

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