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If in the distant future a ship were somehow able to move beyond the galaxy and into intergalactic space, how far would it have to go before we could look out a window and see the Milky Way in its entirety, rather than just edge-on as we see it from Earth?

temporary_user_name
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  • This is a better question for astronomy than here, but I'm not going to migrate it there because as this question stands, it isn't very well defined. And I don't think it's the kind of question that can be well defined, unfortunately. The answer is a very long ways away, but it largely depends on which direction you go. – PearsonArtPhoto Jul 19 '16 at 16:40
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    I don't think it's that unclear. He's basically asking how far away you would have to be in order to see the Milky Way like we see pictures of other galaxies. It's an angular size question, and it just depends on how large you want it to appear. Ultimately, it comes out to ~50,000 light years. – Phiteros Jul 19 '16 at 16:43
  • @PearsonArtPhoto I was actually writing out an answer to this when you closed it. I agree, though, it might be better in Astronomy. – Phiteros Jul 19 '16 at 16:43

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We can calculate the distance we need to be from an object in order for it to be any arbitrary angular size using the formula $D=\frac{d}{2\tan(\frac12\delta)}$. In this case, $D$ is the distance from the object, $d$ is its diameter, and $\delta$ is the angular size. The Milky Way's diameter is 100,000 light years. If we want the Milky Way to appear as large as the Sun appears from the Earth (0.5$^\circ$), we would need to be 5,700,000 light years away. If we want it to appear much larger, we could be closer. For the Milky Way to be the size of a 1 meter circle held at arm's length (90$^\circ$), we would need to be 50,000 light years away.

Phiteros
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    Hmm....you said the we'd have to be 5.7 million ly away before it'd appear the same size as the sun from Earth, but the Andromeda Galaxy is only 2.5 million lightyears away and is twice the estimated width of the Milky Way, and we can't see it clearly without telescopes! – temporary_user_name Jul 19 '16 at 17:03
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    Yes, Andromeda is about 2.97 degrees across from the Earth. So we would be able to see it, but it would be rather dim. – Phiteros Jul 19 '16 at 17:08
  • Sorry, what do you mean? By my reasoning, according to your math, shouldn't it be even larger than the sun? – temporary_user_name Jul 19 '16 at 17:37
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    The Andromeda Galaxy's angular size is larger than the Sun. However, because it is so far away, it has an apparent magnitude (how bright we see it) of 3.44 (lower means brighter). In comparison, the star Vega has an apparent magnitude of 0, and the Sun has an apparent magnitude of -26.

    In terms of absolute magnitude (how bright it appears from 10 parsecs away), Andromeda is -21.5, Vega is 0.582, and the Sun is 4.83. So yes, the Andromeda Galaxy takes up more space on the sky than the sun, but it is much, much dimmer.

     – Phiteros Jul 19 '16 at 17:44
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    @Aerovistae See this article, which includes a nice image depicting how large a "bright" andromeda would be in our sky. – zibadawa timmy Jul 19 '16 at 18:39
  • Math is amazing.. – NuWin Jul 19 '16 at 23:47
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    The field of view for binocular vision is approximately $114^\circ$. So I think $\frac{100\ 000\text{ ly}}{2\tan(\frac{1}{2} 114^\circ)} = 32\ 370\text{ ly}$ belongs in this answer. – KRyan Jul 20 '16 at 03:02
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    By binocular vision means "with two eyes" not "with binoculars" (binoculars have a field of view of 5-10 degrees). We only have detailed vision for 20-40 degrees. Also, accuracy: your data is accurate to one or two significant figures, hence your answers should be similarly rounded. – James K Jul 20 '16 at 05:28
  • @JamesKilfiger Ah, I see. I'll just take that bit out. – Phiteros Jul 20 '16 at 15:01
  • could give a physical example: – Jack R. Woods Jul 25 '16 at 17:51
  • I didn't see mention of the thickness of the galactic bulge nor the direction of travel in the answers or comments. The bulge extends 15-20000 light years above the galactic plane and I would think that "straight up" along the Z axis of galactic coordinates would be the direction to go. That being said, 50000 light years sounds good. – Jack R. Woods Jul 26 '16 at 05:04
  • @JackR.Woods I was assuming he wanted to see the Galaxy face-on, so there is no need to consider the galactic bulge. The Galaxy is just modeled as a circle. – Phiteros Jul 26 '16 at 05:11
  • I interpreted the question as "What is the shortest distance a starship can go to see the Milky Way as a galaxy (ie. spiral structure)?". – Jack R. Woods Apr 10 '17 at 23:52
  • @Aerovistae Rethinking the answer to my interpretation of this question: 95% of disk stars (and nearly all star formation) are in the thin disk which extends only to about 1000 LY above the galactic plane. Thus, I would guess that you might be able to start seeing structure at about 10000 LY due galactic north. – Jack R. Woods Apr 11 '17 at 00:02
  • Of course, the bulge would still obscure the far side. – Jack R. Woods Apr 11 '17 at 00:03