APOD: Andromeda over Patagonia (2020 Nov 25)

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APOD: Andromeda over Patagonia (2020 Nov 25)

Post by APOD Robot » Wed Nov 25, 2020 5:05 am

Image Andromeda over Patagonia

Explanation: How far can you see? The Andromeda Galaxy at 2.5 million light years away is the most distant object easily seen with your unaided eye. Most other apparent denizens of the night sky -- stars, clusters, and nebulae -- typically range from a few hundred to a few thousand light-years away and lie well within our own Milky Way Galaxy. Given its distance, light from Andromeda is likely also the oldest light that you can see. Also known as M31, the Andromeda Galaxy dominates the center of the featured zoomed image, taken from the dunes of Bahía Creek, Patagonia, in southern Argentina. The image is a combination of 45 background images with one foreground image -- all taken with the same camera and from the same location within 90 minutes. M110, a satellite galaxy of Andromenda is visible just below and to the left of M31's core. As cool as it may be to see this neighboring galaxy to our Milky Way with your own eyes, long duration camera exposures can pick up many faint and breathtaking details. Recent data indicates that our Milky Way Galaxy will collide and combine with the similarly-sized Andromeda galaxy in a few billion years.

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by samcon1@iinet.net.au » Wed Nov 25, 2020 11:37 am

I have always wondered what the Milky way galaxy and Magellanic clouds would look like from Andromeda. An artists impression with some valid orientations would satisfy my thirst!

Harry Wheeler

Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Harry Wheeler » Wed Nov 25, 2020 12:24 pm

I've wondered too, what we would look like from M31. My guess is that they would see Lupis and/ or Sagittarius about to collide with the Milky Way. The LMC and SMC would have been a swirl of stars. Perhaps they would see 1 or 2 small galaxies settling into the Milky Way disc.

Tszabeau

Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Tszabeau » Wed Nov 25, 2020 1:00 pm

So... presumably, M110 is nearer us than Andromeda?

Robolt

Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Robolt » Wed Nov 25, 2020 1:12 pm

Is this how we would see Andromeda if our eyes were as sensitive as the camera? Would its span across the sky be that broad?

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by orin stepanek » Wed Nov 25, 2020 1:19 pm

M31Horizon_Ferrarino_1080.jpg


Oh what a beautiful picture! Wouldn't it be wonderful if we could
see Andromeda that clearly? :thumb_up:
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by XgeoX » Wed Nov 25, 2020 1:46 pm

Robolt wrote:
Wed Nov 25, 2020 1:12 pm
Is this how we would see Andromeda if our eyes were as sensitive as the camera? Would its span across the sky be that broad?
Oh yeah, it’s about six times as wide and twice as tall as the moon!

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by XgeoX » Wed Nov 25, 2020 1:54 pm

Tszabeau wrote:
Wed Nov 25, 2020 1:00 pm
So... presumably, M110 is nearer us than Andromeda?
No, M110 is about 150,000-200,000 light years more distant. There is some uncertainty in measuring how far those galaxies are but 110 is definitely more distant.

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by skyping » Wed Nov 25, 2020 1:59 pm

My question is since we are headed for a collision a billion years or so from now, would this "zoomed photo" look like the actual sky someone would see while standing on planet Earth maybe three quarters of a billion years from now as Andromeda nears us? (and yes I'm aware that it might be tough to stand on planet Earth at that point, after the Sun begins it end cycle, but none the less...)

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Chris Peterson » Wed Nov 25, 2020 2:01 pm

Robolt wrote:
Wed Nov 25, 2020 1:12 pm
Is this how we would see Andromeda if our eyes were as sensitive as the camera? Would its span across the sky be that broad?
No. You cannot really compare an image to a naked eye view. How far are you from the screen? And the choice of lens focal length and the distance chosen to the foreground determines how large or small the foreground and background appear with respect to each other. The image spans perhaps 15°. If you were standing in the same place, you'd be seeing a field five or ten times wider (meaning that Andromeda would look relatively smaller by that amount).
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Chris Peterson » Wed Nov 25, 2020 2:02 pm

skyping wrote:
Wed Nov 25, 2020 1:59 pm
My question is since we are headed for a collision a billion years or so from now, would this "zoomed photo" look like the actual sky someone would see while standing on planet Earth maybe three quarters of a billion years from now as Andromeda nears us? (and yes I'm aware that it might be tough to stand on planet Earth at that point, after the Sun begins it end cycle, but none the less...)
Keep in mind that no matter how close Andromeda gets to us, it's never going to look any brighter than the Milky Way. Just a fuzzy gray spot.
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by orin stepanek » Wed Nov 25, 2020 2:07 pm

About how Andromeda looks as it gets nearer! None the less; I'm not going to worry about it; unless I'm reincarnated somehow!! :mrgreen: :wink:
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Chris Peterson » Wed Nov 25, 2020 2:13 pm

orin stepanek wrote:
Wed Nov 25, 2020 2:07 pm
About how Andromeda looks as it gets nearer! None the less; I'm not going to worry about it; unless I'm reincarnated somehow!! :mrgreen: :wink:
Make sure you are reincarnated on Europa, which ought to be about in the middle of the Goldilocks zone by then.
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by skyping » Wed Nov 25, 2020 2:20 pm

Chris L Peterson -Thanks for the reply(replies). David

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by orin stepanek » Wed Nov 25, 2020 2:26 pm

Chris Peterson wrote:
Wed Nov 25, 2020 2:13 pm
orin stepanek wrote:
Wed Nov 25, 2020 2:07 pm
About how Andromeda looks as it gets nearer! None the less; I'm not going to worry about it; unless I'm reincarnated somehow!! :mrgreen: :wink:
Make sure you are reincarnated on Europa, which ought to be about in the middle of the Goldilocks zone by then.

:mrgreen:
+
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Ann » Wed Nov 25, 2020 6:42 pm

samcon1@iinet.net.au wrote:
Wed Nov 25, 2020 11:37 am
I have always wondered what the Milky way galaxy and Magellanic clouds would look like from Andromeda. An artists impression with some valid orientations would satisfy my thirst!
I like the question, and I think the smart people here at Starship Asterisk* should be able to provide at least a small part of the answer!

Of course, we can't say what the actual spiral arm pattern of the Milky Way would look like from Andromeda, since we can't get a bird's eye view of our galaxy and determine once and for all what its features actually look like.

No, but we should be able to say if the Milky Way would be seen face on, edge on or "in between" as seen from Andromeda!























Astrophotographer David Malin once said that from the vantage point of galaxy NGC 253 (at right above), the Milky Way would be a face-on spiral. I guess that has to mean that the south pole of the Milky Way faces directly towards one of the poles of NGC 253.

What about Andromeda? Does one of its poles face one of the poles of the Milky Way? Or not? Is there a way to figure it out?

I note that Andromeda and NGC 253 (from the vantage point of which the Milky Way would be a face-on spiral) have similar right ascensions. The right ascension of Andromeda is 00 hours, 42 minutes and 42.6 seconds. The right ascension of NGC 253 is 00 hours, 47 minutes and 33.1 seconds. Does that have anything to do with how the Milky Way would appear from the Andromeda Galaxy?

Hey, you math whizzes who frequent Starship Asterisk*, can you help?

Finally, let me have a stab at guessing what the Milky Way might look like if we could see it face on. I think it may look somewhat similar to NGC 5371. NGC 5371 is classified as an SBbc galaxy, and the Milky Way is also believed to be an SBbc galaxy. The "S" means "spiral", the "B" means "bar", and the "bc" means that the galaxy is midway between an Sc spiral (with a small bulge and wide-ranging arms) and an Sb spiral (with a larger bulge and more tightly wound arms).

I believe that the bar of the Milky Way is longer than the bar of NGC 5371. But in other respects, it is possible that NGC 5371 resembles the Milky Way.

Do note that NGC 5371 is not seen exactly face on. It is somewhat inclined, which is why it looks elongated.

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Old Grandma » Wed Nov 25, 2020 7:13 pm

I've just found the discussion page.
It is amazing
When I read about the photos it my mind was scrambled.
Good to know other people have similar questions
Thankyou

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by De58te » Wed Nov 25, 2020 8:34 pm

So if Andromeda Galaxy is the oldest light my unaided eye can see, my inquisitive mind naturally wonders, what is the youngest light that my unaided eye can see? It is not daylight since that is some 8 and a half minutes old. Could be even 100,000 years old since they say a photon can take that long to reach the surface of the Sun. Can it be Moonlight at 2 seconds old? No since the Moon only reflects light so Moonlight is still 8 and a half minutes old. So we learn that the greater the distance light travels, the older it is. So the youngest light must form the closest to our eyeballs. My guess is the light from the flashlight that the optometrist shines in our eyes from a distance of about an inch away during an eye exam would be the youngest. .

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by johnnydeep » Wed Nov 25, 2020 8:40 pm

Ann wrote:
Wed Nov 25, 2020 6:42 pm
samcon1@iinet.net.au wrote:
Wed Nov 25, 2020 11:37 am
I have always wondered what the Milky way galaxy and Magellanic clouds would look like from Andromeda. An artists impression with some valid orientations would satisfy my thirst!
Astrophotographer David Malin once said that from the vantage point of galaxy NGC 253 (at right above), the Milky Way would be a face-on spiral. I guess that has to mean that the south pole of the Milky Way faces directly towards one of the poles of NGC 253.

What about Andromeda? Does one of its poles face one of the poles of the Milky Way? Or not? Is there a way to figure it out?
Not sure what you are getting at as regards the poles. I think that spiral galaxy A will appear "face on" to any observer in galaxy B (which could be of any shape) if and only if the plane of the disk of galaxy A is tangent to the sphere centered on the core of galaxy B and intersecting the core of galaxy A. If I could draw a picture I'd include it here :ssmile:

[ EDIT: I got hung up on the core of galaxy B being important. It's not! It would have been simpler to use the sphere centered on the observer, wherever s/he is, in a galaxy or not. In order to be able to use the core of galaxy B as the center and not care where the observer is within it, we have to assume that A and B are far enough apart for the diameter of B to be much smaller than the distance between A and B. ]
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Chris Peterson » Wed Nov 25, 2020 9:07 pm

De58te wrote:
Wed Nov 25, 2020 8:34 pm
So if Andromeda Galaxy is the oldest light my unaided eye can see, my inquisitive mind naturally wonders, what is the youngest light that my unaided eye can see? It is not daylight since that is some 8 and a half minutes old. Could be even 100,000 years old since they say a photon can take that long to reach the surface of the Sun. Can it be Moonlight at 2 seconds old? No since the Moon only reflects light so Moonlight is still 8 and a half minutes old. So we learn that the greater the distance light travels, the older it is. So the youngest light must form the closest to our eyeballs. My guess is the light from the flashlight that the optometrist shines in our eyes from a distance of about an inch away during an eye exam would be the youngest. .
Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by johnnydeep » Wed Nov 25, 2020 9:11 pm

De58te wrote:
Wed Nov 25, 2020 8:34 pm
So if Andromeda Galaxy is the oldest light my unaided eye can see, my inquisitive mind naturally wonders, what is the youngest light that my unaided eye can see? It is not daylight since that is some 8 and a half minutes old. Could be even 100,000 years old since they say a photon can take that long to reach the surface of the Sun. Can it be Moonlight at 2 seconds old? No since the Moon only reflects light so Moonlight is still 8 and a half minutes old. So we learn that the greater the distance light travels, the older it is. So the youngest light must form the closest to our eyeballs. My guess is the light from the flashlight that the optometrist shines in our eyes from a distance of about an inch away during an eye exam would be the youngest. .
Interesting thought experiment. The photons from the display of a smart phone, or a luminous watch face pressed against your eye socket might also qualify. [ EDIT: I see Chris beat me to a much better - and truer - answer! ]

As for solar photons, I thought I read somewhere that it takes 50 million years for a photon generated in the core to exit the photosphere, but that seems to be wrong, since Wikipedia says your 100000 year figure is closer to the truth. From https://en.wikipedia.org/wiki/Solar_cor ... y_transfer:
The high-energy photons (gamma rays) released in fusion reactions take indirect paths to the Sun's surface. According to current models, random scattering from free electrons in the solar radiative zone (the zone within 75% of the solar radius, where heat transfer is by radiation) sets the photon diffusion time scale (or "photon travel time") from the core to the outer edge of the radiative zone at about 170,000 years. From there they cross into the convective zone (the remaining 25% of distance from the Sun's center), where the dominant transfer process changes to convection, and the speed at which heat moves outward becomes considerably faster
Not sure where I got my 50 million year figure from!

[ EDIT: here's one source for the 50 million year number, but perhaps this reflects outdated knowledge: https://courses.lumenlearning.com/sanja ... r/the-sun/ :
The radiative zone, just outside the core, has a temperature of about 7 million degrees C. The energy released in the core travels extremely slowly through the radiative zone. A particle of light, called a photon, travels only a few millimeters before it hits another particle. The photon is absorbed and then released again. A photon may take as long as 50 million years to travel all the way through the radiative zone.
But since these numbers are all only estimated averages, perhaps there really are some photons that take 50 million years or more to make it out!
]
Last edited by johnnydeep on Wed Nov 25, 2020 9:22 pm, edited 2 times in total.
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Ann » Wed Nov 25, 2020 9:17 pm

Chris Peterson wrote:
Wed Nov 25, 2020 9:07 pm

Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.
Chris? Any thoughts on how much the Milky Way would be inclined if viewed from the Andromeda Galaxy?

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by johnnydeep » Wed Nov 25, 2020 9:31 pm

Chris Peterson wrote:
Wed Nov 25, 2020 9:07 pm
De58te wrote:
Wed Nov 25, 2020 8:34 pm
So if Andromeda Galaxy is the oldest light my unaided eye can see, my inquisitive mind naturally wonders, what is the youngest light that my unaided eye can see? It is not daylight since that is some 8 and a half minutes old. Could be even 100,000 years old since they say a photon can take that long to reach the surface of the Sun. Can it be Moonlight at 2 seconds old? No since the Moon only reflects light so Moonlight is still 8 and a half minutes old. So we learn that the greater the distance light travels, the older it is. So the youngest light must form the closest to our eyeballs. My guess is the light from the flashlight that the optometrist shines in our eyes from a distance of about an inch away during an eye exam would be the youngest. .
Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.
A-ha! So how is it that glass and other transparent materials seem not to change the direction of the light passing though it? Yes, I know about refraction, but even so, I would have thought that any photon re-emitted by an atom after being absorbed would be in some random direction? or is it a conservation of momentum thing?

Also, I never understood how neutrinos can apparently pass unimpeded through ludicrous thicknesses of material. I'm guessing they don't get absorbed and re-emitted, which means they somehow are able to wind their way through an obstacle course of electrons and atomic nuclei!
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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Chris Peterson » Wed Nov 25, 2020 9:37 pm

Ann wrote:
Wed Nov 25, 2020 9:17 pm
Chris Peterson wrote:
Wed Nov 25, 2020 9:07 pm

Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.
Chris? Any thoughts on how much the Milky Way would be inclined if viewed from the Andromeda Galaxy?

Ann
Well, Andromeda has a galactic latitude of about -10°. This is the angle below the galactic plane referenced to the Sun, not the galactic center, but they're not going to be very different. So that means that from Andromeda, the Milky Way looks like an edge-on galaxy tipped about 10°. So even more flattened than Andromeda appears to us.
Chris

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Re: APOD: Andromeda over Patagonia (2020 Nov 25)

Post by Chris Peterson » Wed Nov 25, 2020 9:39 pm

johnnydeep wrote:
Wed Nov 25, 2020 9:31 pm
Chris Peterson wrote:
Wed Nov 25, 2020 9:07 pm
De58te wrote:
Wed Nov 25, 2020 8:34 pm
So if Andromeda Galaxy is the oldest light my unaided eye can see, my inquisitive mind naturally wonders, what is the youngest light that my unaided eye can see? It is not daylight since that is some 8 and a half minutes old. Could be even 100,000 years old since they say a photon can take that long to reach the surface of the Sun. Can it be Moonlight at 2 seconds old? No since the Moon only reflects light so Moonlight is still 8 and a half minutes old. So we learn that the greater the distance light travels, the older it is. So the youngest light must form the closest to our eyeballs. My guess is the light from the flashlight that the optometrist shines in our eyes from a distance of about an inch away during an eye exam would be the youngest. .
Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.
A-ha! So how is it that glass and other transparent materials seem not to change the direction of the light passing though it? Yes, I know about refraction, but even so, I would have thought that any photon re-emitted by an atom after being absorbed would be in some random direction? or is it a conservation of momentum thing?

Also, I never understood how neutrinos can apparently pass unimpeded through ludicrous thicknesses of material. I'm guessing they don't get absorbed and re-emitted, which means they somehow are able to wind their way through an obstacle course of electrons and atomic nuclei!
There are different scattering mechanisms, but yes, there is very little change of direction for interactions of optical wavelength photons and the electrons in transparent media.

Neutrinos interact only very weakly with baryonic matter, which is another way of saying they don't often get absorbed. Most likely that's what makes dark matter dark, as well.
Chris

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