Andromeda Galaxy question

the_monkey

Hi,

This was on the cool pics forum

Is this true ?

So the smudge we see with binocs or naked eye is just the center ?

or is the below pic bollocks ?

namloc1980

Yeah it's true. Considering it is 2,500,000 light years away it gives an idea of the vast size of it. Courtesy of NASA:

http://apod.nasa.gov/apod/image/0612/m31abtpmoon.jpg

Link

mickmackey1

Yeah it's true, and Andromeda is by no means the biggest. M87 for example has 3 thousand billion stars.

the_monkey

wow ... I would have thought it be smaller - appearing smaller I mean , since it's 2.5m lya ...

namloc1980

the_monkey wrote: »

wow ... I would have thought it be smaller - appearing smaller I mean , since it's 2.5m lya ...

It's also c.200,000 light years in diameter.

coylemj

To answer the original question, the photo is bollocks. The galaxy would need to be much closer to us to subtend that angle in the sky. The person who produced the photo is suggesting that the galaxy is much bigger than it is.

mickmackey1

Angular diameter of moon - 31 arcminutes
Angular diameter of Andromeda - 178 arcminutes

The galaxy would subtend an angle almost 6 times greater than the moon so the pic is not far off at all:

El Weirdo

mickmackey1 wrote: »

Angular diameter of moon - 31 arcminutes
Angular diameter of Andromeda - 178 arcminutes

The galaxy would subtend an angle almost 6 times greater than the moon so the pic is not far off at all:

Looks like a row of yokes.

ps200306

mickmackey1 wrote: »

Angular diameter of moon - 31 arcminutes
Angular diameter of Andromeda - 178 arcminutes

The galaxy would subtend an angle almost 6 times greater than the moon so the pic is not far off at all:

The photo is both accurate and misleading. Yes, Andromeda is the right size in relation to the moon. But the whole field of view is only 18 degrees wide. With the naked eye you can see almost 180 degrees at a glance. A photo capturing that angular width would show the moon like this:



Very few people guess that the correct-sized moon for a view of half the horizon is the one in the bottom left.

mickmackey1

ps200306 wrote: »

...the whole field of view is only 18 degrees wide. With the naked eye you can see almost 180 degrees at a glance.

Well yes, but that doesn't really have anything to do with the question raised by the photo. The angular relationship between moon and galaxy holds regardless of the field of view.

Rubecula

come back in the future and it will look a lot bigger

ps200306

mickmackey1 wrote: »

Well yes, but that doesn't really have anything to do with the question raised by the photo. The angular relationship between moon and galaxy holds regardless of the field of view.

I think Andromeda was compared to the moon in order to impress you with its angular size. My point was the angular width of the moon is pretty tiny and so is Andromeda's. Photo's that make the moon look big have a small field of view themselves. That's why people invariably are disappointed by the tiny appearance of the moon when they snap it with, say, a 50mm lens... especially when it is close to the horizon and the large naked eye appearance is due to an optical illusion.

Thargor

Something the width of 5 or 6 moons in the sky would still be absolutely gigantic though, its still a good meme, and fairly accurate.

Maximus Alexander

ps200306 wrote: »

I think Andromeda was compared to the moon in order to impress you with its angular size. My point was the angular width of the moon is pretty tiny and so is Andromeda's. Photo's that make the moon look big have a small field of view themselves. That's why people invariably are disappointed by the tiny appearance of the moon when they snap it with, say, a 50mm lens... especially when it is close to the horizon and the large naked eye appearance is due to an optical illusion.

The moon is perceived as being large to the eye though, regardless of the fact that it's objectively small. So, the point being made, I believe is that Andromeda would be perceived as enormous in the sky, were it fully and easily visible to the naked eye.

Huntergonzo

Personally I find Andromeda the most fascinating object in the night sky that's visible to the naked eye, although it can be quite tricky to find for a first timer. Now I know experienced star gazers and astronomers could probably point to many other more interesting objects but I just love the fact that it's a whole separate Galaxy to ours yet it's still visible even at a mind boggling distance, containing many billions of stars and probably trillions of planets, and of course that's where the possibilities get even more fascinating.

ps200306

vastly

Thargor wrote: »

Something the width of 5 or 6 moons in the sky would still be absolutely gigantic though, its still a good meme, and fairly accurate.

Maximus Alexander wrote: »

The moon is perceived as being large to the eye though, regardless of the fact that it's objectively small. So, the point being made, I believe is that Andromeda would be perceived as enormous in the sky, were it fully and easily visible to the naked eye.

Ok, I take those points. I suppose I find it even more interesting that our size perception can be so illusory. Andromeda would occupy only seven square degrees on the sky even if it were face-on to us. In practice it's a good deal less than half that -- less than three square degrees out of 41,000 in the sky. If we brightened it just enough to be able to see it's proper extent, it would still be vastly less bright than the moon and not nearly as conspicuous.

I used Stellarium to produce this approximation of its actual size on the Sky if we could see all of it -- Andromeda is the white squiggle at the tip of the arrow:



(The Sun and star sizes are obviously vastly exaggerated here, being designed to show relative brightnesses and not size; the moon would be one third of the Andromeda height. The actual position is as of right now on 5th May).

Carawaystick

Nasa found M31 is much bigger
http://www.nasa.gov/feature/goddard/nasa-s-hubble-finds-giant-halo-around-the-andromeda-galaxy/