APOD: Spiral Galaxy NGC 4038 in Collision (2018 May 23)

Comments and questions about the APOD on the main view screen.
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MarkBour
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Re: APOD: Spiral Galaxy NGC 4038 in Collision (2018 May 23)

Post by MarkBour » Thu May 24, 2018 4:26 pm

DL MARTIN wrote: Wed May 23, 2018 6:50 pm Perhaps Chris Peterson can enlighten me as to exactly what has happened to the Antennae during the last 60 million years. Or, can he be absolutely certain the Sun is still shining?
Whew! You had me worried there. But I checked 8 minutes and 20 seconds later. The Sun was indeed still shining. I think it at least a little irresponsible to put such fears into reader's minds, don't you?
Mark Goldfain

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Ann
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Re: APOD: Spiral Galaxy NGC 4038 in Collision (2018 May 23)

Post by Ann » Thu May 24, 2018 5:33 pm

rlking wrote: Thu May 24, 2018 11:33 am Lovely picture. It looks so dramatic and cataclysmic, and the caption is very melodramatic. Looking at pictures like this, one is tempted to think that any life there is likely to be totally snuffed out in what appears to be a sort of cosmic fireball, sterilised in a bath of deadly electromagnetism.

And yet, my suspicion is that sentient beings living in the colliding galaxies would likely be just fine, and if they hadn't yet developed significant observational capabilities like ours they really wouldn't notice at all.

Consider this: a simple calculation suggests a supernova explosion (100 billion times as bright as the sun) at a distance of 5 light years from the sun would appear about the same brightness as the sun - though with presumably a far higher output of X rays, gamma rays etc. That would certainly be enough to totally destabilise, perhaps even destroy, earth's atmosphere. But what if it were 100 light years away? It would then only be one four-hundredth as bright; and at 1000 light years only one forty-thousandth: it would be several times brighter than the full moon, and a magnificent sight, but no danger to us at all.

In the colliding galaxy scenario, the rate of star formation and supernova explosions would be greatly increased. But even if it increased by 100 times, that would still only be on average about 1 per year. For a medium-sized galaxy with a diameter of 50,000 lightyears, it would take a couple of thousand years for every point to have a supernova within 1000 lightyears, and very much longer to have one dangerously close. Of course these collisions are non-uniform events, so there would probably be regions where the destructive potential is much higher than others. But overall, I suspect that a substantial proportion of life-bearing planets would be unscathed. But naked-eye astronomy would be rather more spectacular for their inhabitants than for us.

As Douglas Adams famously said, 'Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space'.

And that bigness helps enormously to keep us safe from even the most violent cosmic events. But the most magnificent thing is that gravity, an almost incomprehensibly weak force, spreads its tentacles everywhere and overcomes that vastness, to give us this wonderful universe.
Very well put, all of it.

As you said, nearby supernova progenitors (and nearby supernovas) are rare. I googled "List of supernova candidates" and found that, according to Wikipedia, the nearest known progenitor candidate is IK Pegasi, about 150 light-years away. If IK Pegasi does indeed go out in a burst of glory, it is going to become a Type Ia supernova, and I read somewhere that Type Ia's may be nastier than the core-collapse ones, because the latter emit most of their energy as harmless neutrinos. Type Ia ones don't.

So if IK Pegasi goes pop while humans are still around, the blast might prove to be an extremely nasty and potentially deadly surprise to our descendants. But then again, chances are that the Sun and IK Pegasi have drawn apart so much by the time when the explosion occurs - which may quite likely not happen for millions of years - that it poses no danger to the Earth.

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Thierry Legault
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Re: APOD: Spiral Galaxy NGC 4038 in Collision (2018 May 23)

Post by Thierry Legault » Thu May 24, 2018 6:32 pm

very nice picture, but the antennas have been published here about once per year since 2010...would there be a lack of (more) original pictures proposed in starship asterisk? :wink:

DL MARTIN

Re: APOD: Spiral Galaxy NGC 4038 in Collision (2018 May 23)

Post by DL MARTIN » Thu May 24, 2018 6:59 pm

First to Ann: I spent about 50 years worrying about patients in such circumstance as heart surgery, transplants ... so get off the comment on my scientifically referenced use of the word 'troubled'. What you find worrisome looking through a telescope does not compare with the reality of surgery where there is no eraser on the end of a scalpel.
Secondly, I have the greatest respect for astronomers and astronomy. I wish I could understand it all. What concerns me, however, is when expressions are used such as 'it doesn't matter' or 'one shouldn't care'. To me, science is the very essence of caring and everything matters. In addition, speculation is valid only when supported by evidence. And I don't see anything but retrospective analysis as to what, for example, happens when light leaves Andromeda and arrives here; Despite the calls that 'this or that' explains that 2.5 million years of Andromeda's evolution doesn't matter, well I am troubled by the seeming lack of scientific rigor in that perspective. in fact, I'm wondering if astronomy should not be relegated to a branch of archaeology. Perhaps the considerable monies spent on divining the next nebula, might better be invested in more Earthly concerns.

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Chris Peterson
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Re: APOD: Spiral Galaxy NGC 4038 in Collision (2018 May 23)

Post by Chris Peterson » Thu May 24, 2018 7:06 pm

DL MARTIN wrote: Thu May 24, 2018 6:59 pm First to Ann: I spent about 50 years worrying about patients in such circumstance as heart surgery, transplants ... so get off the comment on my scientifically referenced use of the word 'troubled'. What you find worrisome looking through a telescope does not compare with the reality of surgery where there is no eraser on the end of a scalpel.
Secondly, I have the greatest respect for astronomers and astronomy. I wish I could understand it all. What concerns me, however, is when expressions are used such as 'it doesn't matter' or 'one shouldn't care'. To me, science is the very essence of caring and everything matters. In addition, speculation is valid only when supported by evidence. And I don't see anything but retrospective analysis as to what, for example, happens when light leaves Andromeda and arrives here; Despite the calls that 'this or that' explains that 2.5 million years of Andromeda's evolution doesn't matter, well I am troubled by the seeming lack of scientific rigor in that perspective. in fact, I'm wondering if astronomy should not be relegated to a branch of archaeology. Perhaps the considerable monies spent on divining the next nebula, might better be invested in more Earthly concerns.
If you truly appreciate science, than you should appreciate that special relativity tells us that "now" is different for different locations. In a very rigorous sense, "now" is when we observe something. It truly does not matter what processes have occurred after the light left an object, since the Universe prevents any of that from having any impact on us at all, through any mechanism at all. We are, in every meaningful sense, observing these galaxies as they are now.
Chris

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Ann
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Re: APOD: Spiral Galaxy NGC 4038 in Collision (2018 May 23)

Post by Ann » Thu May 24, 2018 7:31 pm

Thierry Legault wrote: Thu May 24, 2018 6:32 pm very nice picture, but the antennas have been published here about once per year since 2010...would there be a lack of (more) original pictures proposed in starship asterisk? :wink:
NGC 2623. Hubble Legacy Archive, ESA, NASA; Processing: Martin Pugh
Well, there are other merging galaxies too. A nice example is NGC 2623 in Cancer. The two galaxies that make up this galactic train wreck have come so far in their merging process that they can no longer be identified as two separate galaxies. But they do have antennae, or at least tails, like the real Antennae galaxies.

And for those of you who find the picture at left too colorful, fret not: Here is a much more wan and color-equanimous version of the same image.

Ann
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