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

In summary, the Andromeda galaxy appears as a tiny fuzzy patch to the naked eye, despite its massive size and distance from Earth. This is due to the limited angle of subtension and the fact that we can only see its center region with higher star density. Even with a powerful telescope, it may be easy to misidentify Andromeda due to its appearance compared to other stars.
  • #1
Dnj23
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TL;DR Summary
Why does the Andromeda galaxy appear so tiny from our perspective
Hi, and thanks for reading. I asked this question in another forum too.

If we agree that Andromeda is 2.5 million light years away, and its diameter around 220,000 light years, I don't see how it just appears as a regular point of light like any other star in the unaided eye.

Something that so extensive is a little over 11 times away from us. For a scale comparison, the Earth is 30 diameters from the moon, or 30 times, and If Andromeda was the diameter of the Earth placed at the moon's distance, it would span at this distance 8 times a full moon.

Where am I going wrong with this?
 
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  • #2
Andromeda subtends an angle that is about 5x that of the moon. You just can't see that much with your naked eyes.
 
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  • #3
Interesting, so if the plane of Andromeda's rotation was orthogonal to the Earth it would appear 8x as large a full moon?
 
  • #4
Andy Resnick said:
Andromeda subtends an angle that is about 5x that of the moon. You just can't see that much with your naked eyes.

So I'm just seeing the galatic center?

I have a 6" reflector scope, I know it's not the best of scopes, but even at 200x all I see is a fuzzy patch.
 
  • #5
Dnj23 said:
Summary: Why does the Andromeda galaxy appear so tiny from our perspective

Hi, and thanks for reading. I asked this question in another forum too.

If we agree that Andromeda is 2.5 million light years away, and its diameter around 220,000 light years, I don't see how it just appears as a regular point of light like any other star in the unaided eye.

Something that so extensive is a little over 11 times away from us. For a scale comparison, the Earth is 30 diameters from the moon, or 30 times, and If Andromeda was the diameter of the Earth placed at the moon's distance, it would span at this distance 8 times a full moon.

Where am I going wrong with this?
 
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  • #6
We get an angle of ##5°## for the entire width of Andromeda, if we could see it in full length. But with the naked eye we only might be able to see the center regions with higher star density, say a third. Then the angle somes down to less than ##2°## which is very small.
 
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  • #7
Dnj23 said:
Where am I going wrong with this?

Here:

Dnj23 said:
it just appears as a regular point of light like any other star in the unaided eye.

It doesn't look like just a star. More like a fuzzball. Might you be confusing it with a nearby star like mu Andromidae?
 
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  • #8
We get an angle of ##5°## for the entire width of Andromeda, if we could see it in full length. But with the naked eye we only might be able to see the center regions with higher star density, say a third. Then the angle somes down to less than ##2°## which is very small.

I don't know how you're defining degrees, and what the overall dimensions of the Galaxy is, IMO it's similar to the shape of a hurricane.

5 degrees?

download.jpeg

 
  • #9
Dnj23 said:
I don't know how you're defining degrees, and what the overall dimensions of the Galaxy is, IMO it's similar to the shape of a hurricane.
1570809543062.png
 
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  • #10
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  • #11
Dnj23 said:
No way
That is exactly what 5 degrees means in this context - it's the angle subtended by the Andromeda galaxy. The diagram isn't drawn to scale because 5 degrees is a fairly tight angle and it's a nuisance drawing the triangle to scale when it's not necessary.
 
  • #12
Nugatory said:
That is exactly what 5 degrees means in this context - it's the angle subtended by the Andromeda galaxy. The diagram isn't drawn to scale because 5 degrees is a fairly tight angle and it's a nuisance drawing the triangle to scale when it's not necessary.

The full moon angular resolution is 1/2 of a degree. This thing is a tiny point of light.
 
  • #13
Dnj23 said:
The full moon angular resolution is 1/2 of a degree. This thing is a tiny point of light.
Which means that we cannot see the full extension by the naked eye.
 
  • #14
Dnj23 said:
I don't know how you're defining degrees, and what the overall dimensions of the Galaxy is, IMO it's similar to the shape of a hurricane.
You started this thread asking about how it looks to the unaided eye. The image you posted came from a very powerful telescope (and depending on the source may also have been artistically enhanced to make it look even cooler). To the naked eye the Andromeda galaxy is just a tiny fuzzy patch, and as @Vanadium50 points out above, it's easy to misidentify it.
 
  • #15
Nugatory said:
You started this thread asking about how it looks to the unaided eye. The image you posted came from a very powerful telescope (and depending on the source may also have been artistically enhanced to make it look even cooler). To the naked eye the Andromeda galaxy is just a tiny fuzzy patch, and as @Vanadium50 points out above, it's easy to misidentify it.
Yes, and it nowhere nearly covers such distance.

measuring-sky-with-hand.png
 
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  • #16
Dnj23 said:
Yes, and it nowhere nearly covers such distance.
That's right, and @Andy Resnick explained why in post #2 of this thread.
 
  • #17
Nugatory said:
You started this thread asking about how it looks to the unaided eye. The image you posted came from a very powerful telescope (and depending on the source may also have been artistically enhanced to make it look even cooler). To the naked eye the Andromeda galaxy is just a tiny fuzzy patch, and as @Vanadium50 points out above, it's easy to misidentify it.

I know that. And I don't know why you edited my post and gave me a warning.
 
  • #18
Dnj23 said:
So I'm just seeing the galatic center?

I have a 6" reflector scope, I know it's not the best of scopes, but even at 200x all I see is a fuzzy patch.

I'm not surprised- your scope probably operates at f/10, so the throughput is low.
 
  • #19
Dnj23 said:
The full moon angular resolution is 1/2 of a degree. This thing is a tiny point of light.

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

Result of 4h_31m_filtered-1.jpg
 
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  • #20
Dnj23 said:
I have a 6" reflector scope, I know it's not the best of scopes, but even at 200x all I see is a fuzzy patch.
That's way too high. Not only will not all of it fit in the field of view, but it gets dimmer when you raise the magnification.

Andromeda's so big and widely varied in brightness it is a pain to take pictures of. Here's an early one of mine, where you can see the bright core is only a few percent of the full width of what is in the photo:

m31-120-9-29-06.jpg
 
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  • #21
russ_watters said:
That's way too high.

Maybe even way, way too high. :smile:

Dnj23, the purpose of a telecope is to make things brighter, not bigger. Magnification is almost an unfortunate side effect of this. A magnification of 200x means you have a field of view of maybe 0.3 degrees. That's smaller than the moon, and much, much smaller than M31. If it fills your whole field of view - and more - you can't tell that it's slightly brighter than the rest of the night sky.

M31 is many times larger than the moon. You don't need any magnification.
 
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  • #22
Dnj23 said:
The full moon angular resolution is 1/2 of a degree. This thing is a tiny point of light.
This comment makes me ask the same question @Vanadium 50 asked. Are you sure you are looking at the Andromeda Galaxy? You need a dark sky to even see it. When I look at it with the naked eye, it is a fuzzy patch, as others have said. It looks nothing like a star and I would never describe it as a "tiny point of light". In binoculars (again, with a dark sky) it is easy to see that the fuzzy patch extends at least a degree or two. I think you must be looking at a nearby star.
 
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  • #23
Andy Resnick said:
A picture is worth 1000 words: I 'added' pictures I took of the moon and Andromeda. Hope this clears things up.

View attachment 250980
That's amazing, I thought Andromeda was a barely visible point like blob because of the vast distance from us
lots of images on pf Our Beautiful Universe and I had no idea of its full size (if we could see it all)
 
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  • #24
Nugatory said:
The image you posted came from a very powerful telescope (and depending on the source may also have been artistically enhanced to make it look even cooler).

That is not really why images are more impressive than telescopic views, the big difference is using long exposure photography vs the mk I eyeball. The camera can collect photons for hours (using many shorter sub-exposures) and is much more sensitive to light in general and color specifically (color vision is very por in low light situations).

The Andromeda galaxy is much too large to image with a large telescope unless you want to do massive mosaics. The (amazing) Hubble mosaic of the Andromeda galaxy has 1.5 billion pixels so it must use at least 100 sub-frames since the Wide Field Channel of the Hubble ACS camera has 16 million pixels.

For us regular folks normal telephoto lenses with focal lengths in the range of 200-400 mm are a perfect match with common DSLR sensor chips sizes (depending on the sensor size and desired framing). I took this one with a tracking mount, a Canon 600D and a 200 mm lens and it is possible to do much better.

m31_st33_8bit.jpg
 
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  • #25
Vanadium 50 said:
Dnj23, the purpose of a telecope is to make things brighter, not bigger.

You have to be careful with terminology here since a telescope cannot increase surface brightness of an object. The telescope can however increase the overall brightness by gathering more light compared to the human eye which is useful if we want to user higher magnification since that lowers the surface brightness.

The OP's 6 inch telescope should have the same surface brightness as naked eye at a magnification of 20x (150 mm/7.5 mm - assuming a young person with a dark adapted pupil of 7.5 mm diameter and no losses in the optics). The extra light the telescope gathers and the loss of light due to magnification is exactly balanced. Under 20x some of the extra light the telescope gathers is wasted because it can't enter the eye and above 20x the loss due to magnification. By 200x the surface brightness is a hundred times dimmer than naked eye or 20x. In practice the "optimal" magnification is more complicated for many reasons, one of which is that the eye works better with extended objects in very dim light (first described by H.R Blackwell in 1946 (pdf)).

In that sense the purpose of a telescope is to make thing bigger while still retaining or at least reducing the loss in surface brightness.
 
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  • #26
glappkaeft said:
That is not really why images are more impressive than telescopic views, the big difference is using long exposure photography vs the mk I eyeball. The camera can collect photons for hours (using many shorter sub-exposures) and is much more sensitive to light in general and color specifically (color vision is very por in low light situations).

The Andromeda galaxy is much too large to image with a large telescope unless you want to do massive mosaics. The (amazing) Hubble mosaic of the Andromeda galaxy has 1.5 billion pixels so it must use at least 100 sub-frames since the Wide Field Channel of the Hubble ACS camera has 16 million pixels.

For us regular folks normal telephoto lenses with focal lengths in the range of 200-400 mm are a perfect match with common DSLR sensor chips sizes (depending on the sensor size and desired framing). I took this one with a tracking mount, a Canon 600D and a 200 mm lens and it is possible to do much better.

View attachment 251045
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 .
 
  • #27
OmCheeto said:
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?
According to several websites I visited, such a telescope would need a primary mirror in the range of between 200 and 350 meters in diameter.

I assumed I wanted to detect sol like stars at a distance of 778,000 parsecs.
I came up with a magnitude of 29 for our sun at that distance.

ps. I checked, and the worlds largest telescope is only 10.4 meters in diameter.
 
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  • #28
OmCheeto said:
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?

Such a telescope is not possible even in theory unless we make the human eye a lot more sensitive and much larger. Passive optics cannot make anything brighter than the source. The image I posted has a field of view comparable to a telescope operated at 10x (cheep eyepiece) to 20x (super expensive wide FOW eyepiece) which would be a perfect match to small 3 inch telescope (10x) or 6 inch telescope (20x).

The total exposure time for the image was 2.5 hours (9000 seconds) and the human eye can't really stack image data like a digital sensor can but a very rough rule of thumb is that our vision system (brain and eye) is equivalent of a 1 second exposure. On top of that CMOS/CCD sensors are many times better at detecting photons than the human eye. The quantum efficiency (the percentage of photons captured) of the eye is around 0.5% while off the shelf digital sensors operate in the 50% to 90% (specialized stuff almost reaches 100%).

Quick BoTE suggests that a telescope in the range you suggested would be about right but would require the human pupil to be around 7 m in diameter to accommodate the light.
 
  • #29
Nugatory said:
To the naked eye the Andromeda galaxy is just a tiny fuzzy patch, and as @Vanadium50 points out above, it's easy to misidentify it.

Andromeda Galaxy can look very different under different conditions. The problem is the distribution of its luminosity. If you can see it with the naked eye in the sky then, with binoculars or a smallish scope, it will not look like a star - it has no twinkle and just looks 'wrong'.

You should try to get dark adapted and even view with a blanket over you head - that's really good value.

If you have a camera (even a reasonable phone) and if you can force a longish (10s+) exposure (needs to be held steady, of course) its image is very obviously not a star and there are two other small galaxies near it in the sky - not hard to spot once you know you have Andromeda.

Warning - this could be the slippery slope into Astrophotography for you if you are not careful. Visual images of Moon, Jupiter and the like can be lovely to see but as soon as you can take photos you can see all sorts of things and they appear in COLOUR! Adaptors for scopes are not very expensive. Things all changed once we didn't need chemical film images. That required dedication.
 
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  • #30
pinball1970 said:
That's amazing, I thought Andromeda was a barely visible point like blob because of the vast distance from us
lots of images on pf Our Beautiful Universe and I had no idea of its full size (if we could see it all)
It's an eye-opener (so to speak). A great many of the nearer galaxies and nebulae are plenty big enough to see with the naked eye if our eyes were only sensitive enough to see such dim objects.

The moon is 30 arcmin, and we can see down to about 1 arcmin. M33 (pinwheel galaxy) is 71 arcmin, M51 (the double-galaxy) is 14 arcmin, M42 is 85 arcmin.

M51 is 37 million light years away!
 
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  • #31
glappkaeft said:
...
Passive optics cannot make anything brighter than the source.
?
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.

Quick BoTE suggests that a telescope in the range you suggested would be about right but would require the human pupil to be around 7 m in diameter to accommodate the light.
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.
 
  • #32
russ_watters said:
It's an eye-opener (so to speak). A great many of the nearer galaxies and nebulae are plenty big enough to see with the naked eye if our eyes were only sensitive enough to see such dim objects.
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).

That realization kind of brought the solar system right into my backyard.
 
  • #33
OmCheeto said:
So, as a child, when I burned ants with a magnifying glass, I was dreaming?

yup, you were just taking a little bit of the Sun's light and concentrating it down to a small point (area)
 
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  • #34
Vanadium 50 said:
Maybe even way, way too high. :smile:

Dnj23, the purpose of a telecope is to make things brighter, not bigger. Magnification is almost an unfortunate side effect of this. A magnification of 200x means you have a field of view of maybe 0.3 degrees. That's smaller than the moon, and much, much smaller than M31. If it fills your whole field of view - and more - you can't tell that it's slightly brighter than the rest of the night sky.

M31 is many times larger than the moon. You don't need any magnification.

I said, even up to 200x, implying everything less.

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

And come on, not to make things bigger? Now that's just silly.
 
  • #35
pinball1970 said:
That's amazing, I thought Andromeda was a barely visible point like blob because of the vast distance from us
lots of images on pf Our Beautiful Universe and I had no idea of its full size (if we could see it all)

It is a barely visible point like blob.
 
<h2>1. Why does the Andromeda galaxy appear smaller than other galaxies?</h2><p>The Andromeda galaxy appears smaller than other galaxies because it is farther away from us. This distance makes it appear smaller in our field of view, just like how a car in the distance appears smaller than a car right in front of us.</p><h2>2. How far away is the Andromeda galaxy from Earth?</h2><p>The Andromeda galaxy is approximately 2.5 million light years away from Earth. This is equivalent to about 14.7 quintillion miles.</p><h2>3. Why does the Andromeda galaxy appear as a spiral shape?</h2><p>The spiral shape of the Andromeda galaxy is due to its rotation. The galaxy is spinning and the arms of the spiral are created by the gravitational pull of the stars and gas within the galaxy.</p><h2>4. Can we see the Andromeda galaxy with the naked eye?</h2><p>Yes, the Andromeda galaxy is one of the few galaxies that can be seen with the naked eye. However, it appears as a small, fuzzy patch in the night sky and can be difficult to spot in areas with light pollution.</p><h2>5. How long will it take for the Andromeda galaxy to collide with the Milky Way?</h2><p>Scientists predict that the Andromeda galaxy will collide with the Milky Way in about 4 billion years. This collision will result in a new galaxy, which has been named "Milkomeda".</p>

1. Why does the Andromeda galaxy appear smaller than other galaxies?

The Andromeda galaxy appears smaller than other galaxies because it is farther away from us. This distance makes it appear smaller in our field of view, just like how a car in the distance appears smaller than a car right in front of us.

2. How far away is the Andromeda galaxy from Earth?

The Andromeda galaxy is approximately 2.5 million light years away from Earth. This is equivalent to about 14.7 quintillion miles.

3. Why does the Andromeda galaxy appear as a spiral shape?

The spiral shape of the Andromeda galaxy is due to its rotation. The galaxy is spinning and the arms of the spiral are created by the gravitational pull of the stars and gas within the galaxy.

4. Can we see the Andromeda galaxy with the naked eye?

Yes, the Andromeda galaxy is one of the few galaxies that can be seen with the naked eye. However, it appears as a small, fuzzy patch in the night sky and can be difficult to spot in areas with light pollution.

5. How long will it take for the Andromeda galaxy to collide with the Milky Way?

Scientists predict that the Andromeda galaxy will collide with the Milky Way in about 4 billion years. This collision will result in a new galaxy, which has been named "Milkomeda".

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