Views of galaxies

[AdrianPB]

Last week’ s lovely Umbrella N GC4651 galaxy shown on APOD: is there a place in space where, if a human were viewing, presumably form a spaceship, they would see what is seen in this image , or is this image in part determined by the optics etc of telescopes and their capacity to capture light with efficiencies way beyond the capabilities of the naked eye?

[Chris Peterson]

Last week’ s lovely Umbrella N GC4651 galaxy shown on APOD: is there a place in space where, if a human were viewing, presumably form a spaceship, they would see what is seen in this image , or is this image in part determined by the optics etc of telescopes and their capacity to capture light with efficiencies way beyond the capabilities of the naked eye?

We would never see anything at all like this. There is certainly a place in space between here and NGC4651 where the galaxy would have the same size to your naked eye as it appears to have in the image on your monitor, but it would just be a gray, nebulous cloud. In brightness it would look the same as the Milky Way, and if you were decently dark adapted, you might see some faint structure- just as you can in the Milky Way.

The limitation is your eye. Extended objects don't change in surface brightness as you get closer. The total amount of light increases, but so does the size, and so the brightness per unit area is unchanged. And for virtually all non-stellar deep sky objects, that brightness is too little to trigger your cones, which provide high resolution color vision. As humans, we live in a gray universe.

[AdrianPB]

Thanks Chris for the precise and useful information, and the accompanying explanation. Much appreciated. This makes the APOD pictures even more remarkable.
Adrian

[wonderboy]

Aren't there APOD's where the Andromeda Galaxy quite easily viewed. I.E. you can see it from earth without the aid of a telescope and almost see its spiral arms. I'm sure as you got closer it would get clearer, I just can't get my head around why it wouldn't get clearer



Paul

[Chris Peterson]

Aren't there APOD's where the Andromeda Galaxy quite easily viewed. I.E. you can see it from earth without the aid of a telescope and almost see its spiral arms. I'm sure as you got closer it would get clearer, I just can't get my head around why it wouldn't get clearer :?

Sure, the Andromeda galaxy is easily seen from a dark site. I see it all the time- it is several times larger than the Moon. But it just looks like a gray smudge. I don't know exactly what you mean by it getting "clearer". There is absolutely no difference between getting closer and looking at it with higher magnification. That is, if you view this galaxy with a pair of 10X binoculars, your eye will see exactly the same thing as if you were ten times closer. It doesn't get any brighter, but it gets ten times larger. Instead of a small gray smudge, it looks like a big gray smudge. The larger image stimulates more of your retinal cells, and this does enhance detail somewhat. But you'll never see anything with your eye that remotely resembles even the poorest quality image of M31.

[wonderboy]

I don't know. Its hard for me to understand how my eyes wouldn't work the way I would expect them to work. I can understand how the galaxy would appear to be a blur from a distance as I would expect something far away to not be as clear as something close up. By clearer i mean the ability to be able to pick out detail, like dust lanes and so on much like you would see in an APOD picture.

How is it possible to see our own milky way spiral arm from earth but not possible to see another galaxies spiral arms etc as we approach it from afar?



Paul

[Chris Peterson]

How is it possible to see our own milky way spiral arm from earth but not possible to see another galaxies spiral arms etc as we approach it from afar?

How well you can see a galaxy's structure depends on the contrast between the light and dark areas. When you view a galaxy through a telescope (the same thing as getting closer) what you see in terms of structure depends on the galaxy. Some show their spiral structure pretty clearly, some don't. M31, for instance, has a pretty low contrast. It isn't easy to see its structure very well even with good optics. M51, however, is quite contrasty and its spiral structure is evident.

When you see the Milky Way, you are seeing an edge-on section of our galaxy contrasted with space that is outside our galaxy. Even with this extreme case of high contrast, details remain subtle. Most of the internal structure we see in the Milky Way comes from contrast with fairly nearby dust clouds. And dusty spirals are the ones that generally look best to the eye.