M31?

[Albir phil]

Not sure if this is in the right spot, sorry if it's not.The other day I showed a image of M31to some friends, they asked how far away is it,I told the approx 2.5million L Ys but they meant how far or how near you would have to be to see it like the image with the naked eyes. It was a full image of m31.Anyone got the answer cos I didn't have one for them🤔😵‍💫

[vlaiv]

Depends how close are you standing to the image and the size of the image.

Andromeda spans almost 3 degrees (and maybe a bit more - depends on how you define the "edge" of it). Nicely framed image will most certainly span at least 4 degrees - so let's go with that.

I'm going to give you example that you can use to calculate your particular case.

Let's say I'm sitting 60 cm away from 27" screen and watching above image of M31. 27" screen, if say 16:9 will have width of about 60cm. 60cm away that will form angle of roughly 53.3 degrees. This means that in given conditions M31 has been amplified 53.3 degrees / 4 degrees - or 13.325 times.

You need to be x13.325 closer to it in order to see it at that size like you would when looking at its image 60cm away on 27" screen.

Since it is about 2.5mly away - it would be like looking at it from 2500000 / 13.325 = ~187600 ly away or about 187.6Kly away.

If you look at the same image from 1 meter away - it would be like looking at it from 300Kly away (33.4 degrees or 8.35 times closer than it is).

If you look at the same image from ~8.6 meters away - it will be the same size as looking at it with naked eye in the sky (4 degrees or x1 closer - or rather at the same distance as it is).

Hope this makes sense.

(btw, angle is calculated with a bit of trig - 2 x arctan (half_size / distance) - or using this website: https://www.1728.org/angsize.htm)

[Elp]

I got the impression they meant how close you'd have to be to see it how it appears in those nicely souped up astro images.

[vlaiv]

2 minutes ago, Elp said:

I got the impression they meant how close you'd have to be to see it how it appears in those nicely souped up astro images.

Yes - but how big it appears in any image depends on how close you stand to that image.

It's the same thing as with real galaxy - both M31 and image of it are physical objects in space - and how big they appear depends on how close you stand to them or rather from which distance you observe them.

Here I'm merely talking about size - if you mean brightness - then you are out of luck. Surface brightness remains the same regardless of the distance to the object. It will look equally dim viewed from space craft almost entering it as it is when viewed from earth.

Light intensity falls of as square of distance - meaning when you are twice away only 1/4th of light reaches you. Surface behaves the same - or rather surface angle subtend by object goes down by factor of 4 when you move at twice the distance. Two behave the same so surface brightness remains the same.

This is the reason we don't see brighter image when using binoculars / spotting scope during the day. It would be reasonable to see brighter image because of larger light gathering area of aperture - but due to magnification - surface brightness remains the same.

If this was not the case - milky way would be much much brighter in the night sky than some of the galaxies that can be spotted with the naked eye - because it is much closer.

 

[Albir phil]

1 hour ago, Elp said:

I got the impression they meant how close you'd have to be to see it how it appears in those nicely souped up astro images.

Yes that's what they were asking me😵‍💫

[AstroMuni]

11 hours ago, Elp said:

I got the impression they meant how close you'd have to be to see it how it appears in those nicely souped up astro images.

Is that different to what vlaiv was using as an example? ie. an image on a screen.

[Elp]

48 minutes ago, AstroMuni said:

Is that different to what vlaiv was using as an example? ie. an image on a screen.

Of course it is as the first post was referring to size. Seeing it in person in terms of perceived "beauty" should we say, I'd expect would be a different experience similar to what you get from visual to a finished AP image. Also seeing images on electronic display devices will be all different due to panel technology, how they're setup, the colour gamut and colour space the tech is capable of displaying etc let alone how the image has been post processed in the first place.

[vlaiv]

I'm now a bit confused

@Albir phil

Can you clarify the question? I was under impression that original question is mostly about the size of the galaxy.

If processing is put into equation - then I'm afraid - like I mentioned above - you can never see the galaxy like that. Galaxy will always have the same surface brightness - until you get so close to start resolving stars - even then, you can never resolve all the stars in the galaxy as it is so vast that most parts will be far enough to be unresolvable - think Milky Way - we resolve only "handful" of stars (about 5000 or so?) out of all stars in the galaxy when we look at MW from earth.

So there is that - brightness will remain the same. Other than that - in principle, one can produce image that will resemble what one would see from outer space positioned so that galaxy in the image and real deal coincide in size.

I once did that with one of my images of M31 (taken with lens on star tracker) - it would roughly look like this (minus chromatic aberration and star bloat and I'm not entirely sure about how much color would be seen in galaxy):

Also, that is about as best as M31 can be seen thru a telescope. Seasoned observers that have observed M31 many times can confirm (or refute) this.

[AstroMuni]

15 minutes ago, Elp said:

Of course it is as the first post was referring to size. Seeing it in person in terms of perceived "beauty" should we say, I'd expect would be a different experience similar to what you get from visual to a finished AP image. Also seeing images on electronic display devices will be all different due to panel technology, how they're setup, the colour gamut and colour space the tech is capable of displaying etc let alone how the image has been post processed in the first place.

But the angle (size) would be same, would it not? Hence answering the question of how far should the viewer be