APOD: Andromeda in Southern Skies (2022 Oct 21)

[APOD Robot]

Andromeda in Southern Skies

Explanation: Looking north from southern New Zealand, the Andromeda Galaxy never gets more than about five degrees above the horizon. As spring comes to the southern hemisphere, in late September Andromeda is highest in the sky around midnight though. In a single 30 second exposure this telephoto image tracked the stars to capture the closest large spiral galaxy from Mount John Observatory as it climbed just over the rugged peaks of the south island's Southern Alps. In the foreground, stars are reflected in the still waters of Lake Alexandrina. Also known as M31, the Andromeda Galaxy is one of the brightest objects in the Messier catalog, usually visible to the unaided eye as a small, faint, fuzzy patch. But this clear, dark sky and long exposure reveal the galaxy's greater extent in planet Earth's night, spanning nearly 6 full moons.

<< Previous APOD

This Day in APOD

Next APOD >>

[FLPhotoCatcher]

Yay for an actual photo!
It may be a little blurry, but I still like it.

[Ann]

Yay for an actual photo!
It may be a little blurry, but I still like it.

I like it too.

Andromeda in Southern Skies
Image Credit & Copyright: Ian Griffin (Otago Museum)

Andromeda as we are used to seeing it in photographs.

---

Today's APOD brings home the brightness of Andromeda's central region and the faintness of its disk. The APOD also demonstrates the yellow color of the center of Andromeda and the extreme faintness of any blue hues in the disk.

For me as a Color Commentator, it is important to note that Andromeda is a quite yellow galaxy. Its color indices, U-B = 0.500 and B-V = 0.920, are decidedly yellow for a spiral galaxy. In fact, Andromeda is a green valley galaxy - as is the Milky Way.

A mock-up of the galaxy color–magnitude diagram with three populations: the red sequence, the blue cloud, and the green valley. Illustration: Joshua Schroeder.

---

Wikipedia wrote:

The galaxy color–magnitude diagram... has three main features: the red sequence, the green valley, and the blue cloud. The red sequence includes most red galaxies, which are generally elliptical galaxies. The blue cloud includes most blue galaxies, which are generally spirals. In between the two distributions is an underpopulated space known as the green valley which includes a number of red spirals...

The Milky Way and the Andromeda Galaxy are assumed to lie in the green valley because their star formation is slowing down due to running out of gas.

Ann

[orin stepanek]

I really wanted a shot for my PC; so i cropped it!!!

[gvann]

So, the camera orientation is stationary relative to the the fixed stars, but the Earth rotates. As a result, the patches of snow on the mountain produce streaks that run approximately horizontal. However, the stars reflected in the lake produce streaks that are perfectly vertical. It is, of course, correct that it should be so, but I must admit that it took me by surprise at first.

The reason for the vertical streaks is that, with a reflecting plane surface, any motion in the plane does not cause a change in the reflected image; it is only the component of motion normal to the plane that causes a change in the position of the reflected image. But it is a bit strange to see different streaks for the earthbound objects v. the reflected stars.

[johnnydeep]

Are the short line segments reflected in the water stars or something else? But if stars, I would have expected them to look like points not lines, like in this old APOD - https://apod.nasa.gov/apod/ap140916.html

[MarkBour]

Are the short line segments reflected in the water stars or something else? But if stars, I would have expected them to look like points not lines, like in this old APOD - https://apod.nasa.gov/apod/ap140916.html

The water surface must have been a little bit in motion relative to the camera.

[johnnydeep]

Are the short line segments reflected in the water stars or something else? But if stars, I would have expected them to look like points not lines, like in this old APOD - https://apod.nasa.gov/apod/ap140916.html

The water surface must have been a little bit in motion relative to the camera.

In motion how? Ripples up and down? Why would the stars all appear spread out in parallel straight line segments like that?

There also seem to be too many very bright stars and not enough other ones compared to the sky above.

[Fred the Cat]

Are the short line segments reflected in the water stars or something else? But if stars, I would have expected them to look like points not lines, like in this old APOD - https://apod.nasa.gov/apod/ap140916.html

The water surface must have been a little bit in motion relative to the camera.

In motion how? Ripples up and down? Why would the stars all appear spread out in parallel straight line segments like that?

There also seem to be too many very bright stars and not enough other ones compared to the sky above.

Does light vibrate atoms or do atoms vibrate light or both?

Or neither when they're measured.

[johnnydeep]

The water surface must have been a little bit in motion relative to the camera.

In motion how? Ripples up and down? Why would the stars all appear spread out in parallel straight line segments like that?

There also seem to be too many very bright stars and not enough other ones compared to the sky above.

Does light vibrate atoms or do atoms vibrate light or both?

Or neither when they're measured.

The first link is interesting I suppose but I'm not sure how (or if) it helps answer my question. From the first link:

Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is re-emitted as a light wave. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and re-emitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted. If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material's surface vibrate for short periods of time and then re-emit the energy as a reflected light wave. Such frequencies of light are said to be reflected.

We are clearly seeing reflected light in the water, but it's the general large scale appearance of the (presumably) point sources that are being reflected that is at issue, not the deeper physics explanation for how reflection in general happens. Or at least that's how I see it after some further reflection.

[MarkBour]

In motion how? Ripples up and down? Why would the stars all appear spread out in parallel straight line segments like that?

There also seem to be too many very bright stars and not enough other ones compared to the sky above.

Does light vibrate atoms or do atoms vibrate light or both?

Or neither when they're measured.

The first link is interesting I suppose but I'm not sure how (or if) it helps answer my question. From the first link:

Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is re-emitted as a light wave. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and re-emitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted. If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material's surface vibrate for short periods of time and then re-emit the energy as a reflected light wave. Such frequencies of light are said to be reflected.

We are clearly seeing reflected light in the water, but it's the general large scale appearance of the (presumably) point sources that are being reflected that is at issue, not the deeper physics explanation for how reflection in general happens. Or at least that's how I see it after some further reflection.

I probably don't have a good answer.
It may have to suffice to say that light reflecting from the surface
of a body of water can be pretty complicated.

I have faith that each ray can be traced to a mirror reflection
with angle of incidence = angle of reflection, but the droplets
of water are the problem. If they are as smooth as glass,
I think you will get the reflection you want. But if they
are in motion, then you can get the reflection you saw.

[johnnydeep]

Does light vibrate atoms or do atoms vibrate light or both?

Or neither when they're measured.

The first link is interesting I suppose but I'm not sure how (or if) it helps answer my question. From the first link:

Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is re-emitted as a light wave. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and re-emitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted. If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material's surface vibrate for short periods of time and then re-emit the energy as a reflected light wave. Such frequencies of light are said to be reflected.

We are clearly seeing reflected light in the water, but it's the general large scale appearance of the (presumably) point sources that are being reflected that is at issue, not the deeper physics explanation for how reflection in general happens. Or at least that's how I see it after some further reflection.

Capture2.PNG

I probably don't have a good answer.
It may have to suffice to say that light reflecting from the surface
of a body of water can be pretty complicated.

I have faith that each ray can be traced to a mirror reflection
with angle of incidence = angle of reflection, but the droplets
of water are the problem. If they are as smooth as glass,
I think you will get the reflection you want. But if they
are in motion, then you can get the reflection you saw.

Interesting assortment of moon reflections. Most are highly elongated and spread out lineally along the line of sight. Can a point source star also look like that? At first I thought that it's because the moon isn't a point source, but here's a nice clear reflection of the moon in water that is not at all spread out:

Lake Super Moon ReflectionLake Cowan Clinton county Ohio 2016 super moon reflection taken late evening moon rising over placid water. - by Randall Branham

---

[MarkBour]

Interesting assortment of moon reflections. Most are highly elongated and spread out lineally along the line of sight. Can a point source star also look like that? At first I thought that it's because the moon isn't a point source, but here's a nice clear reflection of the moon in water that is not at all spread out:

>>> Lake Super Moon ReflectionLake Cowan Clinton county Ohio 2016 super moon reflection
taken late evening moon rising over placid water. - by Randall Branham <<<

Beautiful !

[VictorBorun]

So, the camera orientation is stationary relative to the the fixed stars, but the Earth rotates. As a result, the patches of snow on the mountain produce streaks that run approximately horizontal. However, the stars reflected in the lake produce streaks that are perfectly vertical. It is, of course, correct that it should be so, but I must admit that it took me by surprise at first.

The reason for the vertical streaks is that, with a reflecting plane surface, any motion in the plane does not cause a change in the reflected image; it is only the component of motion normal to the plane that causes a change in the position of the reflected image. But it is a bit strange to see different streaks for the earthbound objects v. the reflected stars.

We have 2 different movements relative to the landscape: the starry sky's rotation and, in the lake, the mirror image's reflected rotation.
The exposure is so short and the frame so narrow that the smear is close to lines of equal length and direction for every star and a horizontally flopped version of the same linear smear for every star's reflected image in the lake.
Like this:
↗
↘

Now add a compensating motion of the camera to make the starry sky stand still.
Now every star is a point and every landscape pixel is smeared, and while the timeline of the landscape's pixel smear is reversed this nuance is lost in the exposed image; every landscape's pixel smear is close to a line of the same length and direction that a star would have:
↙

Now what is the resulting smear of a lake-reflected star?
It's easy. You just give every reflected star two shifts (in any order):

the shift it would have because of the movement relative to the landscape
↘

and the shift added by the camera's motion
↙

Naturally, the result is no horizontal shift and a twice vertical shift
↓
↓