If Andromeda Were Brighter, This is What You’d See

Andromeda is the closest large galaxy to us.

At 140,000 light years across, it’s 40% bigger than our 100,000 light year diameter Milky Way.

Andromeda is 2.5 million light years away from us, or about 25 Milky Way diameters. Here’s a picture to scale:

Distance to Andromeda

Light takes 2.5 million years to pass between the two galaxies, so if a fancy Andromeda alien is viewing us with a telescope right now, it’s seeing a bunch of Australopithecus walking around being unappealing.

In kilometers, Andromeda is 250 quintillion km away from us. The moon is 400,000 km away, so if you had a ruler that stretched from the Earth to the moon, you’d need 625 trillion of them to reach Andromeda.

And yet, in the scheme of galactic distance, Andromeda is our next door neighbor and one of the only galaxies close enough to be moving toward us—because its “short” distance away means the force of gravity can overpower the movement caused by the expansion of the universe, which in almost all other cases pulls all galaxies away from all other galaxies.

And in about 4 billion years, Andromeda will selfishly collide with the Milky Way and the two will form an extra-huge galaxy. You won’t be here.

Andromeda is also one of the very few objects outside our Milky Way that you can see in the night sky with our naked eye—if you know where to look. It looks like a fuzzy star:

andromeda in sky

But the thing is, it’s so far away that only the super-condensed nucleus of Andromeda is bright enough to be visible to our eye, so that’s all we’re seeing. If the whole galaxy were bright enough, this is what you’d see at night (with the moon as a reference)1:

If Andromeda Were Brighter

Huge! And given that it’s 2.5 million light years away, the surprising amount of sky space Andromeda takes up speaks to just how ridiculously large a 140,000 light year diameter galaxy is.

Here’s one more delicious composite photo that shows the relative size Andromeda would be in the sky, courtesy of NASA:

Andromeda composite (NASA)


1 – Composite by Tom Buckley-Houston. Original photo of moon by Stephen Rahn.

  • Anita

    Wow…now that is something to think about! Thanks for those pictures!

  • Abhishek

    That was… interesting. But not very elaborate. Are you planning on doing two articles this week?

    • Wait But Why

      Yeah this is just a mini-post in the Shed. Big post coming in a couple days. I did this post purely to avoid working on the daunting big post.

      • Abhishek

        Cool. Looking forward to the Big Post!

      • Abercrombie

        Sounds like you could really benefit from reading that post about procrastination!

  • wobster109

    The pictures are cool, but the being close enough to gravitationally collide is really uncool!

    Statistically, based on the density of the two galaxies, how likely is it to cause another star to collide into the sun, eventually?

    • Taco

      Very unlikely. Galaxies, in terms of star density, are mostly empty space. Disco mentions this below.

  • Poncho

    Statistically? 100%

    The two galaxies will engage in a dance of death, it will be total chaos with objects from both galaxies hitting each others’ over and over again, all caused by the gravitational forces of both until they finally Settle into one big galaxy.

    Not that it matters to us much 🙂 by then the sun’s heat would have made Life on Earth impossible.

    • disco

      This is wrong. Despite the fact that two enormous galaxies will be merging, it’s staggering to think there’s STILL so much empty space between stars that it’s highly unlikely any one star (like our Sun) is going to hit another one. Here’s a good National Geographic article about it: http://phenomena.nationalgeographic.com/2014/03/24/scientists-predict-our-galaxys-death/

      And here’s a quote excerpt from it:
      “The good news is that, as Tyson says, stars are so far apart that even though galaxies are colliding, the probabilities of stellar collisions are small. So the sun and its planets will likely survive the birth of Milkomeda, though Earth will no longer be able to call the Milky Way home. “

      • Taco

        Thanks for bringing that up, disco. I was going to say the exact same thing but you did it for me. Glad there are other people out there that know about this stuff. 🙂

      • electropath

        Um, don’t jump the gun, disco. Poncho said “objects” which can include large, diffuse things like nebulae and galactic molecular clouds. These will certainly collide and trigger a bright rash of new starbirth.

  • SuperIT

    This is perfect procrastination material. 🙂

  • SOMARA556

    Amazing to look at Andromeda and think that there are probably many planets with intelligent life there.

  • David R

    The really amazing thing is that if the Milky Way was a 0ne metre ball Andromeda would only be 25 metres away!

  • Vince Burden

    space may be mostly empty…but i have a hard time believing that a trillion and a half stars will somehow all miss each other when the average distance between them is 4 light years or less and they will be whipping all over the place in all directions.

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