Why does the Andromeda galaxy appear so tiny from our perspective?

[Dnj23]

A picture is worth 1000 words: I 'added' pictures I took of the moon and Andromeda. Hope this clears things up.

View attachment 250980

Lol. That would be the most significant, well known celestial object known in the sky if that were even remotely close to such magnitude.

 

[anorlunda]

The editors of APOD must have been reading this thread. They chose today to feature photography of Andromeda.

https://apod.nasa.gov/apod/ap191014.html

Explanation: What does the Andromeda galaxy really look like? The featured image shows how our Milky Way Galaxy's closest major galactic neighbor really appears in a long exposure through Earth's busy skies and with a digital camera that introduces normal imperfections. The picture is a stack of 223 images, each a 300 second exposure, taken from a garden observatory in Portugal over the past year. Obvious image deficiencies include bright parallel airplane trails, long and continuous satellite trails, short cosmic ray streaks, and bad pixels. These imperfections were actually not removed with Photoshop specifically, but rather greatly reduced with a series of computer software packages that included Astro Pixel Processor, DeepSkyStacker, and PixInsight. All of this work was done not to deceive you with a digital fantasy that has little to do with the real likeness of the Andromeda galaxy (M31), but to minimize Earthly artifacts that have nothing to do with the distant galaxy and so better recreate what M31 really does look like.

 

[russ_watters]

Indeed. I was astonished when I first realized that the Galilean moons subtended a perceptible angle to the naked eye.

If they were bright enough to see, collectively they would subtend as much as .175 degrees - that's equivalent to about 2mm wide, at arm's length - on the order of a character on the screen you're reading.

(Callisto's orbit is as much as 1.88 million km in dia. and as little as 588 million km away, for an angular dia. of .175 degrees).

No, that isn't correct. Not even Jupiter or Venus are large enough to see (close though), and the Galilean moons are bright enough if they weren't so near an overpoweringly bright object. I calculate a diameter of about 1.4 arcsec for Ganymede. Jupiter is about 40.

 

[russ_watters]

Well then I must have the worst sky and optics, because I just don't see it. And it gets fairly dark, suburban .

Galaxies are tough even at low magnification.

It is a barely visible point like blob.

That's a self contradiction. is it a point - like a star - or a blob?

And come on, not to make things bigger? Now that's just silly

I agree it was overstated, and would say this: it is a lot easier to make objects bigger than brighter, and/so brighter is more important.

 

[phyzguy]

Well then I must have the worst sky and optics, because I just don't see it. And it gets fairly dark, suburban .

If it's suburban, it's probably just not dark enough. Try going out in the country where it gets really dark.

It is a barely visible point like blob.

Under no circumstances would I describe the Andromeda galaxy as "point-like". I still think you're looking at the wrong thing. Why don't you try posting a sketch of what you see, with nearby stars, so we can see if you're looking in the right place.

 

[russ_watters]

?
So, as a child, when I burned ants with a magnifying glass, I was dreaming?
Or is your assertion true of the image only when it comes back into focus?
(I spent the last 30 minutes doing magnifying glass experiments in my kitchen. I suspect you may be correct.)
I understand that the ants probably weren't being roasted at 5800K.I don't really know much about optics, but "7 meter pupils" makes it sound like I know even less than I did before. Thanks!

Btw, I spent about 2 hours, earlier this morning, trying to figure out where you came up with the "7 meter" figure. No luck.

I have trouble with this one as well, but a little googling tells me the human eye has a focal ratio of about 2.1 at night when the pupil is fully open. That's a really "fast" optical system and pretty much unachievable with a telescope. So any focal ratio above that will produce a dimmer image than with the naked eye.

 

[Buzz Bloom]

I thought this might be of some interest regarding this thread.

https://apod.nasa.gov/apod/ap191014.html​

 

[phyzguy]

I have trouble with this one as well, but a little googling tells me the human eye has a focal ratio of about 2.1 at night when the pupil is fully open. That's a really "fast" optical system and pretty much unachievable with a telescope. So any focal ratio above that will produce a dimmer image than with the naked eye.

Not unachievable at all. The LSST, an 8.3 meter optical telescope currently under construction at Cerro Pachon in Chile, will run at f1.2.

https://www.lsst.org/

 

[Andy Resnick]

So, what would a telescope look like, to be able to allow a human to look into its eyepiece, and see the Andromeda galaxy like that{your image}, in real time?

ps. Fun thread. Thank you, @Dnj23 .

I compared the ratio of luminous flux from a full moon (mag. -13) and Andromeda (mag 3.5), which then gives the (proposed) ratio of entrance pupil areas. From this, the ratio of diameters is 1900, giving a telescope diameter of (about) 15 m, using a 8mm eyeball pupil diameter.

 

[Vanadium 50]

That's a self contradiction. is it a point - like a star - or a blob?

I was going to ask the same question.

Not even Jupiter or Venus are large enough to see (close though)

I can see Venus just fine, thank you.

Seriously, while I don't see a disk, I can see Venus' phases in that sometimes it looks symmetric and sometimes it looks elongated.

 

[Dnj23]

It's point like to the unaided eye, blob like through my telescope.

This is a sky dark enough to see all of Ursa Minor, but none of the Milky Way haze.

For comparison, the Ring Nebula in Lyra is barely a shade lighter than dark. You have to stare at it for moments with adjusted eyesight, knowing that it's there to see it.

 

[russ_watters]

I can see Venus just fine, thank you.

Seriously, while I don't see a disk, I can see Venus' phases in that sometimes it looks symmetric and sometimes it looks elongated.

I have 20/15 vision and have never been able to convince myself of that. If nothing else, it's too bright.

I'll try sunglasses...

[Googles/maths]
Venus's maximum size is 65 arc sec (not corrected for phasing). "normal" 20/20 vision is defined to be 60 arc sec (to discern 300 arcsec letters), so 20/15 should be 45. That's tough.

I wonder, though, if there is an optical blooming effect that makes it possible to see the phases without actually resolving the disk.

 

[sophiecentaur]

Not even Jupiter or Venus are large enough to see

Not large enough to see as discs? I don't know what you are really saying here but any of the planets, when they are visible by eye, look very different from stars. I tell myself that it's because they look like discs but I suppose it could just be the lack of twinkle. My eyesight is pretty rubbishy but I can 'nearly always' tell them apart from stars and that's when I don't know the direction I'm looking in (i.e. in a place where N is not obvs - not blind drunk.)

 

[russ_watters]

Not large enough to see as discs?

Sorry, I meant not large enough to resolve disks with the naked eye.

 

[DaveC426913]

No, that isn't correct. Not even Jupiter or Venus are large enough to see (close though), and the Galilean moons are bright enough if they weren't so near an overpoweringly bright object. I calculate a diameter of about 1.4 arcsec for Ganymede. Jupiter is about 40.

Sorry, I may not have expressed myself adequately. I was talking about their orbits. i.e. if they were bright enough, you could distinguish the Moons as distinctly separated from Jupiter with the naked eye - at their farthest, collectively they would subtend a perceptible angle of the sky.

 

[DaveC426913]

No, that isn't correct. Not even Jupiter or Venus are large enough to see (close though),

??

I assume you mean see "as a disc".

Venus can be seen when in crescent phase by the naked eye.

 

[davenn]

And it gets fairly dark, suburban .

This is a sky dark enough to see all of Ursa Minor, but none of the Milky Way haze.

there you go ... If you can't see the glow of the Milky Way, you are not going to see ( naked eye)
the fuzz of Andromeda

Suburbia is not the place for faint object observations. I have to travel an hour from home to get
far enough out of the city lights to see the elongated fuzz of AndromedaDave

 

[sophiecentaur]

Sorry, I meant not large enough to resolve disks with the naked eye.

Rayleigh criterion can be a bit pessimistic for some aspect of an object. The 3dB point is a bit arbitrary but easy to apply - that's in its favour.

 

[Dnj23]

there you go ... If you can't see the glow of the Milky Way, you are not going to see ( naked eye)
the fuzz of Andromeda

Suburbia is not the place for faint object observations. I have to travel an hour from home to get
far enough out of the city lights to see the elongated fuzz of AndromedaDave

Ok, thanks. This and other replies make sense. I never realized just how star photography can be so deceiving.

Also, you guys have amazing eyesight...Seeing Venus in phases?

Chinese historian of astronomy, Xi Zezong, has claimed that a "small reddish star" observed near Jupiter in 362 BCE by Chinese astronomer Gan De may have been Ganymede, predating Galileo's discovery by around two millennia.

https://en.wikipedia.org/wiki/Galilean_moons
Also interesting

 

[Zeke137]

So, here's another picture which shows the Andromeda galaxy as it would appear to the naked eye in a not very dark sky - it's the fuzzy blob at the center of the red ellipse. The full extent of the outer regions of Andromeda are outlined in red, and Andromeda's dwarf satellite galaxies M110 and M32 are pointed out. The moon is also shown in the correct size on the sky as comparison to the extent of Andromeda:

Hope this helps in visualizing the sizes and appearances on the sky.

 

[Dnj23]

So, here's another picture which shows the Andromeda galaxy as it would appear to the naked eye in a not very dark sky - it's the fuzzy blob at the center of the red ellipse. The full extent of the outer regions of Andromeda are outlined in red, and Andromeda's dwarf satellite galaxies M110 and M32 are pointed out. The moon is also shown in the correct size on the sky as comparison to the extent of Andromeda:

View attachment 251160

Hope this helps in visualizing the sizes and appearances on the sky.

No way dude.

 

[Zeke137]

No way dude.

Ummm, in what way "no way", dude?

 

[Dnj23]

Ok. Where does your picture fall in this top 10?

https://earthnworld.com/top-10-brightest-objects-in-space-that-you-can-see-with-your-naked-eye

 

[Zeke137]

OK, so here's another image, this time from NASA's Astronomy Picture Of The Day site, which overlays an image of the Moon on an image of the Andromeda Galaxy as recorded by a decent telescope, so the galaxy appears much brighter and wider than when seen with the naked eye.:

https://apod.nasa.gov/apod/image/0612/m31abtpmoon_c720.jpg
Note the positions of the two dwarf satellite galaxies M110 and M32...

 

[Zeke137]

Ok. Where does your picture fall in this top 10?

Ask a question that illuminates or adds to the discussion, and you'll get a sensible answer.

 

[snorkack]

So what does the eyepiece of a proper 150x20 scope have to be like? TeleVue offers eyepieces with 100 and 110 degree field of view. Eye does have more than 110 , but peripheral vision is limited resolution. What does te focal length need to be?

 

[phyzguy]

Ok. Where does your picture fall in this top 10?

I don't understand your question. First, do you understand that the moon in the image @Zeke137 posted is there to give you a sense of scale, not brightness? The actual moon would be far, far brighter. Having said that, everything in his image is much dimmer than your "top 10" list.

 

[sophiecentaur]

Also, you guys have amazing eyesight...Seeing Venus in phases?

I think, as with distinguishing between a star and a planet, a 'half Venus' looks different and someone with experience can interpret what's actually going on from a very poor image.
No Elves on PF mate.

 

[Dnj23]

I don't understand your question. First, do you understand that the moon in the image @Zeke137 posted is there to give you a sense of scale, not brightness? The actual moon would be far, far brighter. Having said that, everything in his image is much dimmer than your "top 10" list.

Oh really?

"So, here's another picture which shows the Andromeda galaxy as it would appear to the naked eye in a not very dark sky "

 

[Dnj23]

I think, as with distinguishing between a star and a planet, a 'half Venus' looks different and someone with experience can interpret what's actually going on from a very poor image.
No Elves on PF mate.

Ok.

You can detect differences in light, but you can't actually see the phases.

 

[Dnj23]

What else can you guys see? Neptune white briefs of clouds?

 

[russ_watters]

So, here's another picture which shows the Andromeda galaxy as it would appear to the naked eye in a not very dark sky...

View attachment 251160

Hope this helps in visualizing the sizes and appearances on the sky.

Ummm, in what way "no way", dude?

The way that is worded, it sounds like you are saying that in a not very dark sky you should see Andromeda looking like that. I agree with the "no way". Perhaps in an exceptionally dark sky you might.

 

[Zeke137]

it sounds like you are saying that in a not very dark sky you should see Andromeda looking like that

Well, actually I'm not saying it as such. The background photo comes from Flickr - https://live.staticflickr.com/7016/6467038861_01b1efebf9_b.jpg - so maybe we should all take this up with the person who took the photograph. I do note that neither the image of the tree nor the stars appear to be smeared, so I'd assume this was basically a snapshot of a scene as he/she had seen it.

My own personal experience, however, has always been that Andromeda is a naked eye object only on very dark nights.

Let's just say that the image I posted above shows rather well the relative sizes of Andromeda and the Moon on the sky, and has a nice tree in the foreground as a further aid to visualizing relative sizes. And that the central portion of Andromeda as viewed with the naked eye is just about the size on the sky as portrayed in the picture.

 

[Dnj23]

The way that is worded, it sounds like you are saying that in a not very dark sky you should see Andromeda looking like that. I agree with the "no way". Perhaps in an exceptionally dark sky you might.

Andromeda would be synonymous with the Moon, Venus, Jupiter, Sirius, etc.

 

[Zeke137]

Andromeda would be synonymous with the Moon, Venus, Jupiter, Sirius, etc.

Nice list of point-like or disk objects with no indistinct or fuzzy regions extending well beyond them.

Comparing apples with smoke will get us nowhere.