Living at the edge of the Milky Way

[noozoo]

I have a fairly simple theoretical question. If our Sun were at the very edge of our galaxy, how would our night sky look? I asked this question on Yahoo forums and got answers like "We ARE at the edge of our galaxy." I'm sure the people in this forum are much better informed...

My thinking is that one half of the sky (which half depends on the orientation of the axis of the earth's rotation) would be completely black to the naked eye, except for some distant galaxies. In other words, on a dark night, with no moon or stars, we might see other galaxies with the naked eye. As you looked away from the darkness, the density of visible stars would gradually increase, until you were looking toward the galactic center.

If the earth's axis were pointed directly toward the galactic center, and we called that north, then in the northern hemisphere you'd have no starless area of the sky - only almost-starless near the horizon. But in the southern hemisphere you'd have a largely starless area above you, and at the south pole it would always be starless. At 45 degrees south you'd see a small density of stars near the horizon (for 45 degrees) when looking north.

Does that seem right?

[Chris Peterson]

My thinking is that one half of the sky (which half depends on the orientation of the axis of the earth's rotation) would be completely black to the naked eye, except for some distant galaxies. In other words, on a dark night, with no moon or stars, we might see other galaxies with the naked eye. As you looked away from the darkness, the density of visible stars would gradually increase, until you were looking toward the galactic center.

There is no edge as such, the stellar density just decreases as you go outward. So unless you were well outside the galaxy, you'd always have some local stars around, and there would be no direction that was starless. You can see a number of galaxies with the naked eye from our current location, and that situation would be unchanged if we were farther out. In terms of seeing deep sky objects, the sky would be no darker there than here. The darkness of the sky is actually limited by very local conditions: scatter off of dust in the Solar System and skyglow from upper atmosphere ionization. Those would still be present if we were farther from the galactic center.

If the earth's axis were pointed directly toward the galactic center, and we called that north, then in the northern hemisphere you'd have no starless area of the sky - only almost-starless near the horizon. But in the southern hemisphere you'd have a largely starless area above you, and at the south pole it would always be starless. At 45 degrees south you'd see a small density of stars near the horizon (for 45 degrees) when looking north.

I don't think that is correct. The galaxy would not look wider from farther out than it does from here, inside it. In fact, it should look somewhat narrower. So you'd still see the Milky Way, looking almost the same as it does now, except instead of being a complete ring it would only span part of the sky- a smaller part of the sky as you get farther and farther out.

Now, there is another "edge" we could consider, and that would be the edge of the galactic bulge, fairly close to the center. That is, imagine you fly out of the galaxy, from the center, directly along the axis of rotation. In that case, as you get to where the density starts really dropping, you actually would have a LOT of stars in one direction (towards the galaxy) compared to the other. And from that view, the galaxy wouldn't look like a band, but like a big circle, almost filling an entire hemisphere of the sky.

[Ann]

What would our galaxy look like if we were at our current distance from the center of the Milky Way, but instead of orbiting inside the thin disk we would be in the halo, actually looking "straight down" on the nucleus of the Milky Way? I guess that our galaxy would appear to have a much "lower" surface brightness than it does now that we see it edge on. Edge-on galaxies are often relatively bright and obvious, such as almost edge on Andromeda galaxy and, say, NGC 4565.

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NGC 4565, bright and obvious.

Face-on galaxies, on the other hand, often have low surface brighnesses and are hard to spot. Good examples are M 33 and M 74.

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M 74.Where's the galaxy?

Some face-on galaxies, however, have very bright centers. Two examples are M 100 and M 94:

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M 100.

What would our galaxy look like face-on? Would the arms be faint and hard or impossible to spot without optical aid? Would the nucleus be bright and star-like?



This is an image of the center of the Milky Way. The bright white object is a large cluster of massive stars orbiting the central black hole. Surely this cluster is the brightest individual cluster in the Milky Way? Wouldn't we see it if we could see the Milky Way face on?

Ann

[Céline Richard]

Hello,

And if we were living in a nebula, what would look like our sky? Would be there many colors in the sky?
Could a solar system exist into a nebula, like into the Horse nebula?

Does it exist a place in the Universe, from where we couldn't see any stars?
When we admire a starry night, we experiment some sort of hope; when we see a flying star, we even make a wish. Thus i hope there is starry light for all places in the Universe, although maybe i should say, for all places where there is life.

Have a very good day

Céline

[Chris Peterson]

And if we were living in a nebula, what would look like our sky? Would be there many colors in the sky?

No colors, I'm afraid. Just a diffuse gray glow, like the Milky Way.

Could a solar system exist into a nebula, like into the Horse nebula?

Sure.

Does it exist a place in the Universe, from where we couldn't see any stars?

Possibly, in the space between distant galaxies. Stars and galaxies would still be visible telescopically, but maybe not with eyes (our eyes, anyway).

[Céline Richard]

Hello

No colors, I'm afraid. Just a diffuse gray glow, like the Milky Way.

I hope i don't ask too many things, sorry, but... why couldn't we see colors, while we see so wonderful colors, thanks to telescopes?

Thank you a lot for all your answer Chris It interests me a lot.

As far as our eyes are concerned, I forgot eagles can see further than human beings, for instance.
Moreover, I guess other species (from other planets!) could see further
Actually, it would be so strange if the Earth would be being the only planet to conceal life, in the Universe. I hope Giordano Bruno was right to think many other inhabited worlds than the Earth conceal life, already in the XVI th century, all the more since he was executed for advocating such ideas, by the Inquisition.

Sorry for my grammar mistakes
Thank you for your answers Chris, have a very nice day!

Céline

[VoijaRisa]

why couldn't we see colors, while we see so wonderful colors, thanks to telescopes?

The answer is quite simple. It's due to the amount of light being received and how our eyes perceive it.

While it's true that moving closer to the source of light increases its intensity significantly, I think you underestimate just how diffuse nebulae are. They only appear to have nice edges and contours because of distance. In reality, they're tens to hundreds of light years across and extremely low density. We only see them as "bright" because we're looking through so much of a column of gas that's all contributing. Moving closer increases the intensity in one manner, but it also makes the angular size that much larger which has the opposite effect: it spreads light out. Like putting too little paint over too large a canvas, it becomes weaker and diluted. Thus, you can never reach a really good intensity that's strong enough for our eyes to really pick up as color.

The reason for this is that our eyes have two types of receptors: Cones and rods. One works great for high intensities of light and has a tri-chromatic subset of receptors that our brain fuses together into our perception of colors. The other works well for low levels of light, but can't perceive color. No matter what, you're never going to be able to get a sufficiently high flux of photons to stimulate the receptors that register color without a stronger, denser source.

So why can telescopes perceive color?

Two reasons:
1) They have a significantly higher potential to gather light. The collecting area for our eyes is determined by our pupils and generally, in low light conditions, have a surface area of ~1 cm^2. Telescopes, meanwhile, have surface areas thousands of times larger for big ones. Thus, more light can be gathered and focused, which can reach the threshold of stimulating color receptors.

2) Telescopes are also often connected to cameras. Our eyes nearly instantaneously dump the information they've received to the brain which doesn't allow much time to collect much light. Cameras on the other hand, can have their shutters left open and collect light as long as we'd like them to allowing for long exposure times. Additionally, we can produce cameras that are far more sensitive to colors at low light than our own eyes are.

I hope that helps!

[Céline Richard]

Thank you so much!!!
It is even more interesting than all what i was expecting to be
I was so surprised to get an answer at once! It took me rather a long time to reply because i was trying to understand it well in french.
Thank you, have a very nice day,
Céline

[VoijaRisa]

I'm glad it made sense. I studied French for three years back in high school, but I've forgotten most of what I knew and I'm certain I didn't know most of the technical terms I use anyway.

[Chris Peterson]

I hope i don't ask too many things, sorry, but... why couldn't we see colors, while we see so wonderful colors, thanks to telescopes?

The problem is that extended objects don't get brighter as you get closer, nor do they get brighter when you use a telescope. What happens is they get bigger. If you get closer, or magnify an object with a (suitably large) telescope, you do get more total light. But that light is spread out over a larger area. For instance, get twice as close and you receive four times more light (due to the inverse square law), but the area of the object is now four times larger as well. So the surface brightness (intensity per unit area) remains unchanged. That's why nothing is ever brighter through a telescope than it is with the naked eye, and nothing will ever be brighter as you get closer than you can see it from here. And since objects like nebulas and galaxies are too faint to trigger human color vision, they will still be too faint, even when nearby.

We only see color in images, where it is possible to collect orders of magnitude more light than the eye can use (the eye only integrates for about 100 ms; color astroimages are made from integrations lasting many minutes or hours).

Moreover, I guess other species (from other planets!) could see further :)

Certainly. But how well they could see astronomical objects is uncertain. Such objects are generally very faint, meaning they don't produce many photons at the receptor. While a very good eye might be about ten times more sensitive than our eyes (by making use of every photon, instead of just one in ten), the only way to get better beyond that would be to have a long integration time. Certainly, a biologic eye could do that, but it would be at the expense of seeing things move rapidly. So whether an eye like that would evolve is an interesting question. Certainly not in an environment with fast prey, or fast predators!

[Céline Richard]

Hi

Thank you Voija Risa, yes it made sense a lot. If you learned French in high school and for 3 years, so you speak very well.
Actually, I think it is easier to understand technical terms, in a language which is not our native language, than to express oneself directly in this foreign language.
Indeed, most of the time, there are similarities, analogies, between technical terms from French to English, and vice-versa (ex: cones=cônes, although rods=bâtonnets).
However, what I write there is wrong, if we compare Indo-European languages with Chinese, or with Russian, for instance

Thank you a lot Chris. I understand so better now!
It is funny, because I have the impression I have the law you speak about, isn't it: Brightness=Intensity/(4*Pi*d*d) ? This one is in my little workbook of "introduction to astronomy", and it seemed so logical to me... while I needed to read, again and again, your sentence "they don't get brighter as you get closer", because it was shocking my intuition!
Are words more powerful than math, sometimes, to understand physics? Math need abstraction, but words too because words need concepts... Maybe I understand the law better, because we were thinking about traveling into a nebula This is imagination, while images are more direct representations into our minds.

While a very good eye might be about ten times more sensitive than our eyes (by making use of every photon, instead of just one in ten), the only way to get better beyond that would be to have a long integration time.

So when ten photons come to us, we just receive one in ten… I remember Voija Risa said there were rods and cones in our eyes, why do they work only for one photon out of ten?

A biologic eye could do that, but it would be at the expense of seeing things move rapidly. So whether an eye like that would evolve is an interesting question. Certainly not in an environment with fast prey, or fast predators!

Yes, indeed!! This is deeply interesting...
Cats are able to hunt by night. They are very quick, while the moonlight is very faint... Of course, they don't see far away like owls. Moreover, I doubt neither nor cats could see colors in the darkness, because I believe they just detect motion. So I don’t think my cat could detect the motion of a nebula, or I should introduce her to the scientific community as soon as possible!!!

Have a very good day

Céline

[noozoo]

Céline, think of when you are in the mountains and you see a cloud ahead, on the road. It looks like a big white ball of cotton. As you get closer, it starts to appear translucent, and once you're inside it you are surprised that you can see the road in front of you, to a point. The cloud is now all around you - so you are in it, but it looks less solid.

Perhaps a poor analogy, but I hope it helps.

[noozoo]

If the earth's axis were pointed directly toward the galactic center, and we called that north, then in the northern hemisphere you'd have no starless area of the sky - only almost-starless near the horizon. But in the southern hemisphere you'd have a largely starless area above you, and at the south pole it would always be starless. At 45 degrees south you'd see a small density of stars near the horizon (for 45 degrees) when looking north.

I don't think that is correct. The galaxy would not look wider from farther out than it does from here, inside it. In fact, it should look somewhat narrower. So you'd still see the Milky Way, looking almost the same as it does now, except instead of being a complete ring it would only span part of the sky- a smaller part of the sky as you get farther and farther out.

What I was trying to say here was not that the galaxy would look wider. As you say, it would look narrower. On the edge of the galaxy, if our galactic center were directly above when sitting on the north pole, the Milky way would be above and all around, but the stars would be less dense the closer you looked toward the horizon, at least on two (opposite) sides of the horizon. Perpendicular to those sides the density might still be high, as you are looking along the edge of the 'disc'.

[Chris Peterson]

So when ten photons come to us, we just receive one in ten… I remember Voija Risa said there were rods and cones in our eyes, why do they work only for one photon out of ten?

A photon can land between retinal cells, or if it penetrates a cell, it may miss striking any pigment. Furthermore, our eyes are rather badly "designed", in that the sensitive cells actually lie behind the nerve and blood supply layers, so some photons are absorbed before even reaching the sensitive part of the retina.

Cats are able to hunt by night.

Cats record more photons. They have a reflective layer at the back of the eye which sends missed photons back through the retina for a second chance at being caught. Most nocturnal mammals have this reflective layer, which is why you can see them so well at night in the beams of your headlights or with a flashlight.

[Céline Richard]

Thank you a lot Chris for i am learning very interesting things about the constitution of the eye.

So i suppose if most photons weren't absorbed by blood supply layers, or by the optical nerf, then we could see better. But we might more often need sunglasses, although it is snowing in France by now.

About cats, so they have a reflecting layer at the back of the eye, which enable them to see well in the dark. However, i know my cat doesn't see well by day: when i push open the curtains of the windows, she believes she can jump already to find herself outside... Unfortunately, she hits the glass windows, while she has a reflecting layer, which looks strange.
If her reflecting layer enables her to see better than me, instead of just detecting motions, do you know why she sometimes hits the window?

Have a very good day thank you,

Céline

[Chris Peterson]

About cats, so they have a reflecting layer at the back of the eye, which enable them to see well in the dark. However, i know my cat doesn't see well by day: when i push open the curtains of the windows, she believes she can jump already to find herself outside... Unfortunately, she hits the glass windows, while she has a reflecting layer, which looks strange.
If her reflecting layer enables her to see better than me, instead of just detecting motions, do you know why she sometimes hits the window?

My cats don't do that. Maybe yours is not so clever? <g> Seriously, I guess it has more to do with the brain than the eye. Cat's didn't evolve around glass windows, after all. And anybody who has cats knows they are not far removed from their wild cousins.

A cat's eye isn't necessarily as "good" as a human eye. It doesn't see color, and has a distribution of sensor cells that lie horizontally, so it doesn't have so much resolution above and below that center line (think about what a cat needs to see best when hunting on a savanna). The reflecting membrane at the back of the eye doesn't provide any additional acuity, only better sensitivity in dim light.

[Céline Richard]

Hi Chris,

My cats don't do that. Maybe yours is not so clever?

What?! My cat isn't clever

Seriously, I guess it has more to do with the brain than the eye. Cat's didn't evolve around glass windows, after all. And anybody who has cats knows they are not far removed from their wild cousins.

There is a forest near my house: except in winter, my cat often goes rather far away from her cousins

Thank you a lot for your explanation about the disposition of cells in cats' eyes,
Have a very good day

Céline