Starship Asterisk*

NGC 2403 in Camelopardalis

Explanation: Magnificent island universe NGC 2403 stands within the boundaries of the long-necked constellation Camelopardalis. Some 10 million light-years distant and about 50,000 light-years across, the spiral galaxy also seems to have more than its fair share of giant star forming HII regions, marked by the telltale reddish glow of atomic hydrogen gas. The giant HII regions are energized by clusters of hot, massive stars that explode as bright supernovae at the end of their short and furious lives. A member of the M81 group of galaxies, NGC 2403 closely resembles another galaxy with an abundance of star forming regions that lies within our own local galaxy group, M33 the Triangulum Galaxy. Spiky in appearance, bright stars in this colorful galaxy portrait of NGC 2403 lie in the foreground, within our own Milky Way.

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APOD Robot wrote:
the long-necked constellation Camelopardalis

Joking aside, this is a very beautiful picture. This smallish galaxy - about M33-sized, indeed - really sparkles with star formation. And Martin Pugh has created a lovely RGB (+ Ha, I guess) portrait of it.

Ann

Edit: Okay. I don't have to guess. It really is an HaRGB image.

May I have indications on the instrumental setup and exposure times for the different filters? thanks.

When our Earthly view of a galaxy is at an oblique angle, is it true that we're seeing the far edge as it was tens of thousands of years before the light we're getting from the near edge ? Easy to think we're looking at the entire galaxy as it was 50 million years ago, but is there a temporal gradient across the diameter of the disc, also, such that different parts of our view of it appear as they were at different times in the past ? If so, does that usually matter to astronomers ?

roberto_nesci wrote:May I have indications on the instrumental setup and exposure times for the different filters? thanks.

Try emailing Martin. You can find the address in the footer of his website. I'm not sure he reads the forum.
http://www.martinpughastrophotography.id.au/

chuckster wrote:When our Earthly view of a galaxy is at an oblique angle, is it true that we're seeing the far edge as it was tens of thousands of years before the light we're getting from the near edge ? Easy to think we're looking at the entire galaxy as it was 50 million years ago, but is there a temporal gradient across the diameter of the disc, also, such that different parts of our view of it appear as they were at different times in the past ? If so, does that usually matter to astronomers ?

Galaxies have a diameter in the order of about 100 thousand lightyears. So, yes, the light of the far edge needs a few 100 thousand light years more to reach us then the that from the near edge. I am not an astronomer, but I would think that it does not matter that much, since usual time scales are much larger (age of stars is 100 million years to billion years; revolution time of the sun around the galactic center about 200 million years).

But this raises another question to me: when simulating the orbit of stars in a galaxy, do retardation effects need to be taken into account? The time dependent gravitational field of a accelerating mass can only propagate at the speed of light, and would take about a year (on average) to reach the nearest star. A similar reason would also apply to the solar system (distance sun-neptune 4 light hours). As far as I know, these retardation effects are not taken into account. Does anyone know the reason why?

Markus Schwarz wrote: ... The time dependent gravitational field of a accelerating mass ...

What exactly are you referring to here?

Markus Schwarz wrote: But this raises another question to me: when simulating the orbit of stars in a galaxy, do retardation effects need to be taken into account? The time dependent gravitational field of a accelerating mass can only propagate at the speed of light, and would take about a year (on average) to reach the nearest star. A similar reason would also apply to the solar system (distance sun-neptune 4 light hours). As far as I know, these retardation effects are not taken into account. Does anyone know the reason why?

Thanks for making me think! I'm a professional astronomer, and I've watched these simulations grow incredibly more sophisticated over the last decade (or two; time flies when you're having fun), but your question never occurred to me. The simulators are VERY smart people, however, and I am confident they do it right. I hope!

FloridaMike wrote:

Markus Schwarz wrote: ... The time dependent gravitational field of a accelerating mass ...

What exactly are you referring to here?

Stars orbit the center of the galaxy in more-or-less elliptical orbits, hence they are constantly accelerated. And since the size of a galaxy extends over 100 light years, I wonder if one should take retardation effects into account as well.

I am think along the lines of classical electrodynamics: the Coulomb field of an electric field is static and extends infinitely in space (with decreasing magnitude, of course). But once the charge accelerates, the information that the location of the charge has changed can only propagate at the speed of light. This means that an observer always detects the retarded field of the charge.

The Newtonian gravitational field of a star behaves in the same way as the Coulomb field of an electric charge. Even though the time dependence is described by Einstein's equations instead of Maxwell's, by causality, the effect of an accelerating source can only propagate at the speed of light.

Ann wrote:

APOD Robot wrote:
the long-necked constellation Camelopardalis

Joking aside, this is a very beautiful picture. This smallish galaxy - about M33-sized, indeed - really sparkles with star formation. And Martin Pugh has created a lovely RGB (+ Ha, I guess) portrait of it.

Ann

Edit: Okay. I don't have to guess. It really is an HaRGB image.

It is a lovely HaLRGB image! Ha exposures greatly enhance the emission nebulae in the spiral arms. Another good example where Ha makes a big difference is NGC 300.

Markus Schwarz wrote:But this raises another question to me: when simulating the orbit of stars in a galaxy, do retardation effects need to be taken into account?

I can't speak to whether this effect is important in galaxy simulations (but I would guess that it is). However, it is an important component of the sort of simulations I and other space dust researchers carry out in order to model comet dust trails over time. Particle ejections are simulated and their positions modeled by numerical integration over thousands of years, subject to gravitational perturbations of all the planets, and end up with positions calculated to accuracies better than a thousand kilometers. We have to consider the real-time gravitational field of the Solar System.

A similar reason would also apply to the solar system (distance sun-neptune 4 light hours). As far as I know, these retardation effects are not taken into account. Does anyone know the reason why?

When you're talking about a two-body calculation, the retardation doesn't affect the orbit, only where we see the bodies. This basically means considering the aberration of light - where the body appears because of light lag versus where it is because of gravity driven orbital dynamics. What you're discussing in a simulation is different, but closely related, and probably handled the same way. Solar System ephemerides are established by at least three different methods. Some are based on empirically derived series solutions, some are based on precise observations, and some are based on numerical simulation. These are regularly checked against each other. I don't see how the simulated solutions could avoid considering the dynamical gravitational field, given that they predict the positions of the terrestrial planets to sub-kilometer accuracy.

Chris Peterson wrote:Solar System ephemerides are established by at least three different methods. Some are based on empirically derived series solutions, some are based on precise observations, and some are based on numerical simulation. These are regularly checked against each other. I don't see how the simulated solutions could avoid considering the dynamical gravitational field, given that they predict the positions of the terrestrial planets to sub-kilometer accuracy.

Thanks Chris! After some search on "ephemeris" I found on Wikipedia

http://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris#Construction wrote: The physics [used in the construction of the ephemeris] included the mutual Newtonian gravitational accelerations and their relativistic corrections (a modified form of the Einstein-Infeld-Hoffmann equation) [...]

So, it appears that they do take retardation (and other post-Newtonian corrections) into account.

Markus Schwarz wrote:

Markus Schwarz wrote:
when simulating the orbit of stars in a galaxy, do retardation effects need to be taken into account? The time dependent gravitational field of a accelerating mass can only propagate at the speed of light, and would take about a year (on average) to reach the nearest star. A similar reason would also apply to the solar system (distance sun-neptune 4 light hours). As far as I know, these retardation effects are not taken into account. Does anyone know the reason why?

Stars orbit the center of the galaxy in more-or-less elliptical orbits, hence they are constantly accelerated. And since the size of a galaxy extends over 100 light years, I wonder if one should take retardation effects into account as well.

I am think along the lines of classical electrodynamics: the Coulomb field of an electric field is static and extends infinitely in space (with decreasing magnitude, of course). But once the charge accelerates, the information that the location of the charge has changed can only propagate at the speed of light. This means that an observer always detects the retarded field of the charge.

The Newtonian gravitational field of a star behaves in the same way as the Coulomb field of an electric charge. Even though the time dependence is described by Einstein's equations instead of Maxwell's, by causality, the effect of an accelerating source can only propagate at the speed of light.

The Earth responses gravitationally to where it thinks the Sun is now and NOT to where the Sun was sitting 8 minutes ago. If the Earth responded gravitationally to where the Sun was sitting 8 minutes ago then the Earth would be constantly accelerated forward and spiral out of the Solar System. The Earth doesn't seem to do that.

The classic example of retarded Einsteinian gravitational interaction is what happens if the Sun were to suddenly disappear. As you probably already know: the Earth would continue to orbit (in a helical ellipse) to the old (moving!) position of the Sun for 8 minutes until a powerful gravitational signal finally reaches the Earth at which point the Earth would fly off into space along a straight line.

The Einsteinian gravitational field of a star behaves in the same way as the Maxwell field of an electric charge in that one sometimes has to take into account 1) relativistic effects and/or 2) the propagation of waves.

However, the vast majority of many-body gravitational simulations are 1) highly nonrelativistic and 2) the propagation of gravitational waves in them is negligible. Inaccuracies in the calculations come rather from dynamic computer algorithm digital approximations such as round off error and chaos.

It seems much of our understanding of the universe has always been about what drives celestial mechanics. First the drama surrounding the way our solar system was actually arranged. It was followed by the understanding that our galaxy wasn't the only one. Then that the gas in those galaxies and that clusters of galaxies rotated faster than expected.

http://www.learner.org/courses/physics/ ... 0&secNum=2

All along we are becoming more sophisticated with our abilities to detect motions and use the analysis of those motions to cleverly figure out what is causing the unexpected results. The simulations we devise though are only as good as the information that is used to build them. Throughout the history of astronomy very few totally unique thoughts pass the test of time through scientific validation. I find it curious that many were foretold way before the validation was possible or socially acceptable. With today's social media the ability for many to express brand new ideas has never been so widely available. I have to applaud the APOD discussion forum for "walking the tightrope" between both walls of the chasm separating extreme scientific conservatism and flights of fantasy that may allow good new ideas to flourish potentially blossoming into unexpected fruits that depict the way our universe really is put together.
And by the way – I really like how Martin Pugh "does his thing!"

neufer wrote:However, the vast majority of many-body gravitational simulations are 1) highly nonrelativistic and 2) the propagation of gravitational waves in them is negligible. Inaccuracies in the calculations come rather from dynamic computer algorithm digital approximations such as round off error and chaos.

All accurate dust trail simulations must include multiple relativistic effects or they produce meaningless results over scientifically interesting simulation periods.

(FWIW, modeling gravitational waves is not the same thing as modeling the dynamical gravitational field.)

Chris Peterson wrote:
(FWIW, modeling gravitational waves is not the same thing as modeling the dynamical gravitational field.)

What exactly does "advanced" & "retarded" refer to if not gravitational waves?

neufer wrote:

Chris Peterson wrote:(FWIW, modeling gravitational waves is not the same thing as modeling the dynamical gravitational field.)

What exactly does "advanced" & "retarded" refer to if not gravitational waves?

Gravitational waves are transient distortions (usually periodic or semi-periodic) of spacetime, which are only understood in the context of GR (specifically, a perturbation of the metric tensor). In the context of simulating a gravitational system like the Solar System, a galaxy, or a cometary dust stream, all we care about is the dynamical gravitational field, which is solved classically. It just means that we have to consider the propagation of the field, which introduces an apparent lead or lag in apparent position, depending on frame. Gravitational fields are not treated as propagated by gravitational waves.