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

#### Dnj23

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?

Related Astronomy and Astrophysics News on Phys.org

#### Andy Resnick

Andromeda subtends an angle that is about 5x that of the moon. You just can't see that much with your naked eyes.

#### BWV

Interesting, so if the plane of Andromeda's rotation was orthogonal to the Earth it would appear 8x as large a full moon?

#### Dnj23

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.

#### PeroK

Homework Helper
Gold Member
2018 Award
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?

#### fresh_42

Mentor
2018 Award
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.

Staff Emeritus
Where am I going wrong with this?
Here:

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?

#### Dnj23

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?

#### fresh_42

Mentor
2018 Award
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.

#### Dnj23

No way
[Edited by a mentor]

Last edited by a moderator:

#### Nugatory

Mentor
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.

#### Dnj23

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.

#### fresh_42

Mentor
2018 Award
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.

#### Nugatory

Mentor
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.

#### Dnj23

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.

Mentor

#### Dnj23

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.

#### Andy Resnick

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.

#### Andy Resnick

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.

#### russ_watters

Mentor
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:

Staff Emeritus
That's way too high.
Maybe even way, way too high.

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.

#### phyzguy

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.

#### pinball1970

Gold Member
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)

#### glappkaeft

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.

#### glappkaeft

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.

Last edited: