APOD: Milky Way with Airglow Australis (2015 Sep 04)

[APOD Robot]

Milky Way with Airglow Australis

Explanation: After sunset on September 1, an exceptionally intense, reddish airglow flooded this Chilean winter night skyscape. Above a sea of clouds and flanking the celestial Milky Way, the airglow seems to ripple and flow across the northern horizon in atmospheric waves. Originating at an altitude similar to aurorae, the luminous airglow is instead due to chemiluminescence, the production of light through chemical excitation. Commonly captured with a greenish tinge by sensitive digital cameras, this reddish airglow emission is from OH molecules and oxygen atoms at extremely low densities and has often been present in southern hemisphere nights during the last few years. On this night it was visible to the eye, but seen without color. Antares and the central Milky Way lie near the top, with bright star Arcturus at left. Straddling the Milky Way close to the horizon are Vega, Deneb, and Altair, known in northern nights as the stars of the Summer Triangle.

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[Beyond]

Wow! Reminds me of the center of a peach, when you cut it in half and remove the pit. Except that peaches don't normally have a dusty bright streak down the middle.

[MadMan]

The "Discuss" link on today's APOD points to yesterday's discussion. I had to type "http://asterisk.apod.com/discuss_apod.php?date=150904" manually to get here

[Joe Stieber]

Is the apparent curvature of the airglow bands the result of perspective, similar to the effect seen with crepuscular rays? Perhaps emphasized by a very wide-angle lens (judging from the rather small Summer Triangle where the Milky Way meets the horizon)?

[Joe Stieber]

The "Discuss" link on today's APOD points to yesterday's discussion. I had to type "http://asterisk.apod.com/discuss_apod.php?date=150904" manually to get here

From yesterday's discussion page, you could also just click the "Next APOD >>" hotlink under the descriptive text. That brings you to today's discussion page.

[starsurfer]

Would airglow have been less in the sky 500 years ago?

[Ron-Astro Pharmacist]

If we are rotating around the center of the Earth and the Earth is rotating around the center of the Sun and the Sun is rotating around the center of the Milky Way does our local group of galaxies rotate too? There's no large center of mass to rotate around like in a system with a large gravitational center but, as in such systems, the center of motion is slightly offset anyways. (Like Pluto and Charon's wobbly rotation)

http://www.euclideanspace.com/maths/geo ... oundPoint/

I was thinking of it in the context of the Milky Way and Andromeda interaction a few years distant from now. Would the two galaxies be on a direct collision line (only gravitationally interacting with each other) or a slowly forming interaction involving angular momentum?

[neufer]

If we are rotating around the center of the Earth and the Earth is rotating around the center of the Sun and the Sun is rotating around the center of the Milky Way does our local group of galaxies rotate too? There's no large center of mass to rotate around like in a system with a large gravitational center but, as in such systems, the center of motion is slightly offset anyways. (Like Pluto and Charon's wobbly rotation)

http://www.euclideanspace.com/maths/geo ... oundPoint/

I was thinking of it in the context of the Milky Way and Andromeda interaction a few years distant from now. Would the two galaxies be on a direct collision line (only gravitationally interacting with each other) or a slowly forming interaction involving angular momentum?

• Pluto does not observe Charon approaching it;
  Pluto only observes Charon moving tangential or side-ways.
  
  The Milky Way's observation of the Andromeda Galaxy is (essentially) just the reverse:

<<The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second. Up until 2012, there was no way to know whether the possible collision was definitely going to happen or not. In 2012, researchers concluded that the collision is definite after using the Hubble Space Telescope to track the motion of Andromeda between 2002 and 2010. Andromeda's tangential or side-ways velocity with respect to the Milky Way was found to be relatively much smaller than the approaching velocity and therefore it is expected that it will directly collide with the Milky Way in around 4 billion years.

These studies also suggest that M[énage à]33, the Triangulum Galaxy—the third largest and brightest galaxy of the Local Group—will participate in this event. Its most likely fate is to end up orbiting the merger remnant of the Milky Way and Andromeda Galaxy, to merge with it in an even farther future, but a collision with the Milky Way before it collides with the Andromeda Galaxy or being ejected from the Local Group cannot be ruled out.>>

[Ron-Astro Pharmacist]

If we are rotating around the center of the Earth and the Earth is rotating around the center of the Sun and the Sun is rotating around the center of the Milky Way does our local group of galaxies rotate too? There's no large center of mass to rotate around like in a system with a large gravitational center but, as in such systems, the center of motion is slightly offset anyways. (Like Pluto and Charon's wobbly rotation)

http://www.euclideanspace.com/maths/geo ... oundPoint/

I was thinking of it in the context of the Milky Way and Andromeda interaction a few years distant from now. Would the two galaxies be on a direct collision line (only gravitationally interacting with each other) or a slowly forming interaction involving angular momentum?

• Pluto does not observe Charon approaching it;
  Pluto only observes Charon moving tangential or side-ways.
  
  The Milky Way's observation of the Andromeda Galaxy is (essentially) just the reverse:

<<The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second. Up until 2012, there was no way to know whether the possible collision was definitely going to happen or not. In 2012, researchers concluded that the collision is definite after using the Hubble Space Telescope to track the motion of Andromeda between 2002 and 2010. Andromeda's tangential or side-ways velocity with respect to the Milky Way was found to be relatively much smaller than the approaching velocity and therefore it is expected that it will directly collide with the Milky Way in around 4 billion years.

These studies also suggest that M[énage à]33, the Triangulum Galaxy—the third largest and brightest galaxy of the Local Group—will participate in this event. Its most likely fate is to end up orbiting the merger remnant of the Milky Way and Andromeda Galaxy, to merge with it in an even farther future, but a collision with the Milky Way before it collides with the Andromeda Galaxy or being ejected from the Local Group cannot be ruled out.>>

Interesting. So Pluto and Charon interact on a plane (similar to the illustration below)?

Two Masses.jpg (4.46 KiB) Viewed 3460 times

While the two large galaxies will interact directly with the Triangulum Galaxy; a gentler rotational merging still possible. Thank you, Art. I was trying to picture that event and the idea of a rotational merger seemed really, really hard to predict on a decernable time table.

[Dad is watching]

• Pluto does not observe Charon approaching it;
  Pluto only observes Charon moving tangential or side-ways.

Ok, so we understand the first part. But the second statement is a little confusing. We thought that Pluto and Charon were tidally locked together. That is that neither one of them rotates in their common orientation relative to one another. So that Charon will always appear directly overhead of a certain point on Pluto, and vice-versa. If that is the case, then (relative to each other) neither one should appear to be moving to the other at all. Motion would be apparent relative to the rest of the solar system/galaxy/universe, but relative to each other, they should appear static. Yes?

[neufer]

Pluto does not observe Charon approaching it;
Pluto only observes Charon moving tangential or side-ways.

Ok, so we understand the first part. But the second statement is a little confusing. We thought that Pluto and Charon were tidally locked together. That is that neither one of them rotates in their common orientation relative to one another. So that Charon will always appear directly overhead of a certain point on Pluto, and vice-versa. If that is the case, then (relative to each other) neither one should appear to be moving to the other at all. Motion would be apparent relative to the rest of the solar system/galaxy/universe, but relative to each other, they should appear static. Yes?

• Pluto observes Charon moving tangential or side-ways vis-a-vis the background stars:

<<Mach's principle is the name given by Einstein to an imprecise hypothesis often credited to the physicist and philosopher Ernst Mach. The idea is that local inertial frames are determined by the large scale distribution of matter, as exemplified by this anecdote:

• You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?

Mach's principle says that this is not a coincidence—that there is a physical law that relates the motion of the distant stars to the local inertial frame. If you see all the stars whirling around you, Mach suggests that there is some physical law which would make it so you would feel a centrifugal force. There are a number of rival formulations of the principle. It is often stated in vague ways, like "mass out there influences inertia here". A very general statement of Mach's principle is "Local physical laws are determined by the large-scale structure of the universe."

This concept was a guiding factor in Einstein's development of the general theory of relativity.>>

[DavidLeodis]

It's a fascinating image .

It would greatly help (well at least me ) if someone could please post an annotated version to show the stars mentioned in the explanation. Is that likely to be a meteor trail in the Milky Way near to the top?

[Curious Jay]

• Pluto observes Charon moving tangential or side-ways vis-a-vis the background stars:

O.K. But not very obviously to a human eye, right? I mean as Pluto's rotational period is 6.4 times longer than Earth's the stars would move across Pluto's or Charon's sky at a proportionally slower rate. Also, while Charon and Pluto remain fixed in one another's sky from a geographical standpoint, I expect they show phases to each other throughout that day?

[neufer]

Pluto observes Charon moving tangential or side-ways vis-a-vis the background stars:

O.K. But not very obviously to a human eye, right? I mean as Pluto's rotational period is 6.4 times longer than Earth's the stars would move across Pluto's or Charon's sky at a proportionally slower rate. Also, while Charon and Pluto remain fixed in one another's sky from a geographical standpoint, I expect they show phases to each other throughout that day?

Probably easier to track stars moving past a fixed Charon during a long Pluto night
than to track stars moving past a moving Moon during a short Earth night:

View of Charon from Pluto : Sept 3, 2015 17:00 UTC

View of Charon from Pluto : Sept 3, 2015 23:00 UTC

[Curious jay]

Probably easier to track stars moving past a fixed Charon during a long Pluto night
than to track stars moving past a moving Moon during a short Earth night:

View of Charon from Pluto : Sept 3, 2015 17:00 UTC

View of Charon from Pluto : Sept 3, 2015 23:00 UTC

You are correct, sir. Thank you. The web links to the solar system simulator make it pretty obvious. Also it shows the Charon's phase (last quarter?) with respect to Pluto. Fun Stuff! I think I'll spend a little more time playing with that simulator.