by MarkBour » Wed Jan 04, 2017 8:48 pm
That's a beautiful image!
Tszabeau wrote:Assuming that the interstellar cirrus clouds shown are associated with the Milky Way and not Andromeda... where are the ones that are, surely, associated with it? Just too faint to be photograhed?
https://www.nasa.gov/audience/foreducat ... Faint.html explains that any object is going to be fainter, the farther it is away from you, by an inverse square law. If you look at a typical galactic cirrus cloud in today's APOD and imagine that it is in the Milky Way, then it is at most about 90,000 light years away from us, or you could say typically 50,000 light years away would be a reasonable initial guess. And I'm assuming it is somewhere in or near the visible disk one would see from a distant view of our galaxy.
Now Andromeda is 2,500,000 light years away. A galactic cirrus cloud in Andromeda then would be an object in or near its disk, and would be 2,500,000 / 50,000 = 50 times as far away, roughly. Such a cloud, then, would be 50x50 = 2500 times as faint as one in our galaxy, and also 2500 times as small in apparent extent (area).
It would be interesting to learn more of these clouds we are seeing here.
Ann wrote:I'm going to stick my neck out and guess that they may, indeed, be a remnant of a past outburst of Nu Andromeda, the brightest blue star in the picture, located just below center.
If, as Ann is guessing, they were created by the star(s) Nu Andromedae, then they are somewhere near it, at only 620 light years distant from Earth. So, if what you are seeing is a cloud at only 600 light years, now a comparable cloud in Andromeda itself would be 2,500,000 / 600 = 4167 times as far away. So a similar cloud in Andromeda would be (very roughly) 4000x4000 = 16 million times as small in area and 16 million times as faint!
One other thought on the illusion. If you go look at the moon on a night where conditions are right and there are a few (earth-atmosphere) cirrus clouds "around" it, they will definitely tend to look like they are somehow being affected by the Moon. The Moon will be so bright that it will shine right through the clouds and it will often appear as if the clouds have parted around it, because of this. This effect is even more pronounced in camera images. If one of the clouds in today's APOD appeared to run right across and in front of Andromeda, it would help us see it correctly. But the image of Andromeda is bright enough, I think, that it is preventing that, in the same way that the Moon does to earthly clouds. (See
https://www.metabunk.org/explained-why- ... oon.t7084/).
That's a beautiful image!
[quote="Tszabeau"]Assuming that the interstellar cirrus clouds shown are associated with the Milky Way and not Andromeda... where are the ones that are, surely, associated with it? Just too faint to be photograhed?[/quote]
[url]https://www.nasa.gov/audience/foreducators/9-12/features/F_How_Far_How_Faint.html[/url] explains that any object is going to be fainter, the farther it is away from you, by an inverse square law. If you look at a typical galactic cirrus cloud in today's APOD and imagine that it is in the Milky Way, then it is at most about 90,000 light years away from us, or you could say typically 50,000 light years away would be a reasonable initial guess. And I'm assuming it is somewhere in or near the visible disk one would see from a distant view of our galaxy.
Now Andromeda is 2,500,000 light years away. A galactic cirrus cloud in Andromeda then would be an object in or near its disk, and would be 2,500,000 / 50,000 = 50 times as far away, roughly. Such a cloud, then, would be 50x50 = 2500 times as faint as one in our galaxy, and also 2500 times as small in apparent extent (area).
It would be interesting to learn more of these clouds we are seeing here.
[quote="Ann"]I'm going to stick my neck out and guess that they may, indeed, be a remnant of a past outburst of Nu Andromeda, the brightest blue star in the picture, located just below center.[/quote]
If, as Ann is guessing, they were created by the star(s) Nu Andromedae, then they are somewhere near it, at only 620 light years distant from Earth. So, if what you are seeing is a cloud at only 600 light years, now a comparable cloud in Andromeda itself would be 2,500,000 / 600 = 4167 times as far away. So a similar cloud in Andromeda would be (very roughly) 4000x4000 = 16 million times as small in area and 16 million times as faint!
One other thought on the illusion. If you go look at the moon on a night where conditions are right and there are a few (earth-atmosphere) cirrus clouds "around" it, they will definitely tend to look like they are somehow being affected by the Moon. The Moon will be so bright that it will shine right through the clouds and it will often appear as if the clouds have parted around it, because of this. This effect is even more pronounced in camera images. If one of the clouds in today's APOD appeared to run right across and in front of Andromeda, it would help us see it correctly. But the image of Andromeda is bright enough, I think, that it is preventing that, in the same way that the Moon does to earthly clouds. (See [url]https://www.metabunk.org/explained-why-clouds-appear-behind-the-sun-and-moon.t7084/[/url]).