## visible galaxy

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JonBanes
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Joined: Thu May 28, 2009 8:31 pm UTC

### visible galaxy

Ok, here's one for you.

How close would a galaxy have to be for us to see it in the night sky as big as the moon? Also what would it look like? Most of those pictures you see are IR and not visible spectrum pictures, i think.

Lets say it's an average sized spiral galaxy oriented so that we can see the spiral.

BlackSails
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### Re: visable galaxy

Which class is this for?

antonfire
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### Re: visable galaxy

About ten or twenty times as far away as these two. (Though, to be fair, they're not "average-sized spiral galaxies".)
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JonBanes
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### Re: visable galaxy

First off, not for a class. I'm a molecular biologist. Way to jump to conclusions, it is possible for someone to be curious outside of a classroom.

Doing the simple math a spiral galaxy of 200,000 lightyears wide would need to be about 22 mil lightyears away to appear the sizeof the moon in the sky.

What would this look like? Would you be able to see the arms?

JonBanes
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### Re: visable galaxy

antonfire wrote:About ten or twenty times as far away as these two. (Though, to be fair, they're not "average-sized spiral galaxies".)

how big is this in seconds/minutes as seen from earth and how does it compare to the moon?

phlip
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### Re: visable galaxy

JonBanes wrote:
antonfire wrote:About ten or twenty times as far away as these two. (Though, to be fair, they're not "average-sized spiral galaxies".)

how big is this in seconds/minutes as seen from earth and how does it compare to the moon?

I had to do a lot of hunting around, but I eventually found these two pages (which totally haven't been linked to yet), which give:
Small Magellanic Cloud page wrote:Apparent dimensions (V) 5° 20′ × 3° 5′
Large Magellanic Cloud page wrote:Apparent dimensions (V) 10.75° × 9.17°
Also, from here:
Moon page wrote:Angular diameter 29.3 to 34.1 arcminutes

It's possible for someone to be curious outside of a classroom, but typically such curiosity would be paired with actually looking up the answers to simple questions... whereas just blindly asking them of some forum implies the sort of disinterest that comes with homework.

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Tass
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### Re: visable galaxy

Well the Andromeda Galaxy is significantly larger in the sky than the moon, but it is only barely visible with the naked eye.

Galaxies are not so bright.

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### Re: visible galaxy

Those numbers confuse me. The Apparant dimension of Andromeda is given as 190 degrees. That's over half the sky. That's only possible if are actually inside it.
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phlip
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### Re: visible galaxy

190 minutes, I think you'll find. A bit over three degrees.

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ThinkerEmeritus
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### Re: visible galaxy

Actually 190 minutes of arc[ ' is minutes, not degrees], so a little over 3 degrees.
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rflrob
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### Re: visible galaxy

For comparison, by the way, the moon and sun are both typically around half a degree (30 arcminutes) in apparent dimension.

If you're thinking, "that's crazy, the moon and sun can't possibly be the same size in the sky!" remember that we have total solar eclipses every once in a while, where the moon covers the sun almost perfectly. Sometimes the moon is slightly farther away (hence appears smaller), so it's slightly smaller than the sun, so we end up with an annular eclipse.
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seridos
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### Re: visible galaxy

Andromeda would appear much larger, but only its bright central region is bright enough to be visible.
From wikipedia:
"though it appears more than six times as wide as the full moon when photographed through a larger telescope, only the brighter central region is visible with the naked eye."

Sir_Elderberry
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### Re: visible galaxy

Am I right in suspecting that this question is fueled by that Carl Sagan song that's been circulating?
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Charlie!
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### Re: visible galaxy

Apparent size of something some large distance away: 2*pi*R/(4*pi*distance^2)

distance of the moon.
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Mr Jack
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### Re: visible galaxy

Sir_Elderberry wrote:Am I right in suspecting that this question is fueled by that Carl Sagan song that's been circulating?

I strongly suspect so.

And that song is awesome...

Vieto
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Joined: Sun Jun 22, 2008 10:44 pm UTC

### Re: visible galaxy

Mr Jack wrote:
Sir_Elderberry wrote:Am I right in suspecting that this question is fueled by that Carl Sagan song that's been circulating?

I strongly suspect so.

And that song is awesome...

yes it is awesome.

It caused me to watch all 13 episodes of Carl Sagan's Cosmos in 3 days.

Kow
Posts: 195
Joined: Sat Apr 19, 2008 12:37 pm UTC

### Re: visible galaxy

Vieto wrote:
Mr Jack wrote:
Sir_Elderberry wrote:Am I right in suspecting that this question is fueled by that Carl Sagan song that's been circulating?

I strongly suspect so.

And that song is awesome...

yes it is awesome.

It caused me to watch all 13 episodes of Carl Sagan's Cosmos in 3 days.

Ditto. There need to be more things like that made.