APOD: M100: A Grand Design Spiral Galaxy (2015 Feb 11)

[Nitpicker]

I'm most certainly thinking in 3D (plus time) but my intuition is betraying me and I'm not following you. Or maybe you're wrong. I can't tell. Ha.

I'm not wrong. [poking tongue emoticon]

I mentioned "3-D" only because you said you don't understand how it could appear to us with a 150 ly radius after only 4 years. That, to me, suggested you were thinking only in 2-D. I will accept I might have been wrong about that. You might have been thinking in 3-D in the incorrect way. [poking tongue emoticon]

So, I'll repeat what I now assume to be my most pertinent point, as far as I assume your understanding goes:
When we see an initial pulse of light from afar, we are also seeing anything that it illuminated along the way, in the direct line of sight. As time progresses, anything else illuminated by the initial pulse, that is not in our direct line of sight with the initial light source, will also appear to us, based on how much further the reflected light has to travel to us, and how long that takes.

[geckzilla]

If you're going to work so hard trying to explain it, you may as well draw a picture. Even a very crude drawing not to scale with simple arrows and some numbers would be easier to understand.

[Nitpicker]

Does this help?:

The sum distance from the supernova, to any point on the parabola, then on to the observer, is constant.

If we define the focus of the parabola (location of the supernova) to be (0,0), and the y-direction towards the observer, then the equation of the parabola is as follows:
y = x²/4p - p

Substituting 2 light years for p and 150 light years for x, yields y = 2810.5 light years, meaning that the illuminated dust is almost 3000 light years in front of the supernova, from our perspective.

[Nitpicker]

And this is a much more realistic diagram of the particular light echo in the APOD, in terms of a realistic scale:

(The dust cloud is probably wider than shown, extending further back towards the supernova.)

[geckzilla]

All this time I thought you were using a 3d dust volume.

[Nitpicker]

All this time I thought you were using a 3d dust volume.

Well, I kinda was initially, back when I was talking about my hypothetical spherical dust shell around the supernova (similar to the light echo through V838 Mon, which is within the Milky Way and only 20,000 light years away, with a maximum dust radius of about 3 light years).

But then I looked more closely at this APOD, and made some (rough) measurements, and realised the dust must be much further from the supernova and is probably not even related to the pre-supernova system. So, in this case, we wouldn't expect the illuminated dust to contract back down to a dot.

(I suppose one could imagine a spherical dust shell around SN2006X, with a radius of almost 3000 light years, but then we wouldn't have a hope of seeing the far side of the sphere illuminated for nearly 6000 years.)

[LocalColor]

Beautiful. Goosebumps!!

[Donnageddon]

I think I get it, Nitpicker.

The ring we are seeing is a part of the same surrounding gas cloud we saw when the SN was initially discovered, AND it was hit by the light of the SN at the same time as the dust we saw during the initial nova outburst, BUT because that part of the dust cloud is 4 light years more distant from our perspective, it takes 4 more years to reach us, but the size of the dust cloud will make it seem like the light from the ring traveled FTL, but not at all. It is just a part the geometry.

[starsurfer]

It appears to have two cores....a main brighter one, and the other next to it....


This is a slendid shot...but wish it was a tad wider....

:---[===] *

This image by Adam Block shows the whole galaxy!

[MarkBour]

Yeah, I am just not getting this light echo thing.

The light of the light echo we see in the APOD (outer radius ~150 light years), took 4 years longer to reach us than the original supernova burst. This is because the light that bounced off the illuminated dust we see, had to travel 4 light years further, because it did not travel in a straight line to us.

Haha, you're making it worse. 150 light years? How'd it get so big in four years? And still struggling with the radius being twice as wide as expected... I need to tell Universe Sandbox developers to please add a light echo function to its light pulse feature so I can see this happening. My poor brain is just failing left and right on this.

NitPicker, as I read this thread, although I did not go to the trouble to check your calculations, I think you are right and have given a nice exposition of the matter, both your calculations and diagrams.

If I may, I am thinking that part of what you said early on is what threw off Geckzilla. It was your comments about apparent diameter or radius. On the contrary, your comment in the above excerpt is the helpful one:

IF we see this light echo 4 years after the supernova, AND IF it is a light echo from that initial burst, THEN we know the path these photons took to get to us is 4 LY longer than the path of the photons we initially saw (which we assume to have come straight to us).

For Geckzilla: the ring we now see could actually have appeared to expand faster than the speed of light only if one makes the mistaken interpretation that we are seeing a ring around the original star. Instead, we are seeing the reflection off of dust that is nowhere near the star, it is much nearer to us than the star, so it appears too big to have gone straight out to the side and then bounced to us. Certainly, it did not do that. So, the "apparent radius", or "apparent diameter" of the echo is a nearly meaningless idea. (Nitpicker's avatar is quite apropos here. His thumb would have an immense "apparent radius", held up in front of this galaxy, even though light can bounce from one side of his thumb to the other in about 2/(3*10^10) sec.)

And I'm struck by the realization that a light echo is always a rather personal thing. It depends on the location of the light source, the location of the reflective materials, and most of all is only observed by us in our location. Any other place in the universe might see it unfold very differently, or not see any echo at all.

[DavidLeodis]

Through a link to the image in Judy's 'Geckzilla' website I learnt a new word - unported - that is in the Creative Commons license. Mind you, after trying to find the meaning of unported I am still unsure what it means (it is also used in the Creative Commons website). I'm but that's likely my fault in not understanding. I don't know if I should be or not in not knowing the word.

[geckzilla]

Through a link to the image in Judy's 'Geckzilla' website I learnt a new word - unported - that is in the Creative Commons license. Mind you, after trying to find the meaning of unported I am still unsure what it means (it is also used in the Creative Commons website). I'm but that's likely my fault in not understanding. I don't know if I should be or not in not knowing the word.

This website has a good explanation: http://www.oerafrica.org/copyright-and- ... licence-cc

[DavidLeodis]

Through a link to the image in Judy's 'Geckzilla' website I learnt a new word - unported - that is in the Creative Commons license. Mind you, after trying to find the meaning of unported I am still unsure what it means (it is also used in the Creative Commons website). I'm but that's likely my fault in not understanding. I don't know if I should be or not in not knowing the word.

This website has a good explanation: http://www.oerafrica.org/copyright-and- ... licence-cc

Thanks Geckzilla (Judy) for that link which is appreciated and has helped me understand what unported means.

[Nitpicker]

I think I get it, Nitpicker.

The ring we are seeing is a part of the same surrounding gas cloud we saw when the SN was initially discovered, AND it was hit by the light of the SN at the same time as the dust we saw during the initial nova outburst, BUT because that part of the dust cloud is 4 light years more distant from our perspective, it takes 4 more years to reach us, but the size of the dust cloud will make it seem like the light from the ring traveled FTL, but not at all. It is just a part the geometry.

I am not sure if any dust was detected at the same time as the very bright supernova outburst was observed. I imagine the light echoes off the dust are at least an order of magnitude less bright.

The total distance from the supernova, to the illuminated echo dust we see in the APOD, to us, is 4 light years longer than the direct distance from the supernova to us.

[Nitpicker]

NitPicker, as I read this thread, although I did not go to the trouble to check your calculations, I think you are right and have given a nice exposition of the matter, both your calculations and diagrams.

If I may, I am thinking that part of what you said early on is what threw off Geckzilla. It was your comments about apparent diameter or radius. On the contrary, your comment in the above excerpt is the helpful one:

IF we see this light echo 4 years after the supernova, AND IF it is a light echo from that initial burst, THEN we know the path these photons took to get to us is 4 LY longer than the path of the photons we initially saw (which we assume to have come straight to us).

For Geckzilla: the ring we now see could actually have appeared to expand faster than the speed of light only if one makes the mistaken interpretation that we are seeing a ring around the original star. Instead, we are seeing the reflection off of dust that is nowhere near the star, it is much nearer to us than the star, so it appears too big to have gone straight out to the side and then bounced to us. Certainly, it did not do that. So, the "apparent radius", or "apparent diameter" of the echo is a nearly meaningless idea. (Nitpicker's avatar is quite apropos here. His thumb would have an immense "apparent radius", held up in front of this galaxy, even though light can bounce from one side of his thumb to the other in about 2/(3*10^10) sec.)

And I'm struck by the realization that a light echo is always a rather personal thing. It depends on the location of the light source, the location of the reflective materials, and most of all is only observed by us in our location. Any other place in the universe might see it unfold very differently, or not see any echo at all.

Well, the term apparent size/radius/diameter is used a lot in astronomy and is typically an angular measurement. If you know the distance to an object (or the size of an equidistant object in the same image), its measured angular/apparent size can be converted to a size. I think I confused matters by talking about the measured radius of the light echo apparent in the APOD and the (different) radius of a hypothetical, mostly un-illuminated, spherical dust shell around the supernova.

But yes, the fact that echoes are personal things, based on the location of the observer, is an important point to grasp.

[Nitpicker]

I might also add that my rough equivalence of 4000 pixels representing 100,000 light years (upon which all my measurements are based) could easily be out by a factor of 2. A more accurate number might be 50,000 light years.

[bkebackstrom]

What do the dustlines consist of? Is it the remains of older exploded stars or what? How does a Young galaxy look?

[Nitpicker]

What do the dustlines consist of? Is it the remains of older exploded stars or what? How does a Young galaxy look?

I dunno really, but here is a newspaper article:
http://www.washingtonpost.com/news/spea ... r-factory/

I imagine the dust and gas is a mix of material of stellar and non-stellar origin. There was dust and gas before the first star ever formed, as far as I know.

[Ann]

What do the dustlines consist of? Is it the remains of older exploded stars or what? How does a Young galaxy look?

I dunno really, but here is a newspaper article:
http://www.washingtonpost.com/news/spea ... r-factory/

I imagine the dust and gas is a mix of material of stellar and non-stellar origin. There was dust and gas before the first star ever formed, as far as I know.

The way I understand it, there was no dust before the first stars came into existence. Dust consists for the most part of elements heavier than hydrogen and helium, and these heavier elements were cooked up in stars.

Pristine hydrogen and helium gas, by contrast, must have existed before the first stars.

Ann

[Nitpicker]

What do the dustlines consist of? Is it the remains of older exploded stars or what? How does a Young galaxy look?

I dunno really, but here is a newspaper article:
http://www.washingtonpost.com/news/spea ... r-factory/

I imagine the dust and gas is a mix of material of stellar and non-stellar origin. There was dust and gas before the first star ever formed, as far as I know.

The way I understand it, there was no dust before the first stars came into existence. Dust consists for the most part of elements heavier than hydrogen and helium, and these heavier elements were cooked up in stars.

Pristine hydrogen and helium gas, by contrast, must have existed before the first stars.

Ann

You're right Ann. You made me look it up, too, where I also found this interesting morsel:
http://en.wikipedia.org/wiki/Interstellar_medium

By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements. The hydrogen and helium are primarily a result of primordial nucleosynthesis, while the heavier elements in the ISM are mostly a result of enrichment in the process of stellar evolution.

So, the majority of the interstellar medium is of non-stellar origin.

[Chris Peterson]

So, the majority of the interstellar medium is of non-stellar origin.

That's certainly true. But 100% of the dust is from stellar nucleosynthesis or supernova nucleosynthesis. Likewise for the stuff like oxygen, nitrogen, carbon, silicon, and iron. In other words, none of the really interesting stuff is primordial.

[Nitpicker]

So, the majority of the interstellar medium is of non-stellar origin.

That's certainly true. But 100% of the dust is from stellar nucleosynthesis or supernova nucleosynthesis. Likewise for the stuff like oxygen, nitrogen, carbon, silicon, and iron. In other words, none of the really interesting stuff is primordial.

Indeed, but I found it interesting that the vast majority of the ISM isn't "really interesting".

(Personally, I find hydrogen and helium to be really interesting stuff.)

[starsurfer]

So, the majority of the interstellar medium is of non-stellar origin.

That's certainly true. But 100% of the dust is from stellar nucleosynthesis or supernova nucleosynthesis. Likewise for the stuff like oxygen, nitrogen, carbon, silicon, and iron. In other words, none of the really interesting stuff is primordial.

Indeed, but I found it interesting that the vast majority of the ISM isn't "really interesting".

(Personally, I find hydrogen and helium to be really interesting stuff.)

At least the ISM looks interesting in a deep image with maybe Ha? I also love planetary nebulae interacting with the ISM, they're so awesome!