APOD: NGC 891 Edge On (2013 Oct 11)

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APOD: NGC 891 Edge On (2013 Oct 11)

Post by APOD Robot » Fri Oct 11, 2013 4:06 am

Image NGC 891 Edge On

Explanation: This sharp cosmic portrait features NGC 891. The spiral galaxy spans about 100 thousand light-years and is seen almost exactly edge-on from our perspective. In fact, about 30 million light-years distant in the constellation Andromeda, NGC 891 looks a lot like our Milky Way. At first glance, it has a flat, thin, galactic disk and a central bulge cut along the middle by regions of dark obscuring dust. The combined image data also reveal the galaxy's young blue star clusters and telltale pinkish star forming regions. And remarkably apparent in NGC 891's edge-on presentation are filaments of dust that extend hundreds of light-years above and below the center line. The dust has likely been blown out of the disk by supernova explosions or intense star formation activity. Faint neighboring galaxies can also be seen near this galaxy's disk.

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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Beyond » Fri Oct 11, 2013 4:12 am

The filaments of dust extend 100's of light years... They look so small. That's because galaxies are so damn BIG :!:
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Chris Peterson » Fri Oct 11, 2013 4:46 am

Beyond wrote:The filaments of dust extend 100's of light years... They look so small. That's because galaxies are so damn BIG :!:
No, galaxies are incredibly small. And distances of 100s of light years are insignificantly small. The only thing that is big is nearly empty space.
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Ann » Fri Oct 11, 2013 5:37 am

Chris Peterson wrote:
Beyond wrote:The filaments of dust extend 100's of light years... They look so small. That's because galaxies are so damn BIG :!:
No, galaxies are incredibly small. And distances of 100s of light years are insignificantly small. The only thing that is big is nearly empty space.
That depends on what you compare these structures with, doesn't it, Chris?

Compared with the Higgs particle, the filaments of dust are pretty damn humongous.

(And then, so are we, although on a completely different scale than the dust filaments, but still...)

Anyway, even vast stretches of nearly empty space may be tiny compared with the combined size of the multiverse, if such a thing exists. In any case, even infinities come in different sizes!!!

When I first heard of the theory of relativity, I thought that this was what it meant. That some things that look so big are small when compared with something that is even bigger, and vice versa.

Only the speed of light is one and the same and unchanging. Or so I think anyway. Image

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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Beyond » Fri Oct 11, 2013 6:40 am

Chris Peterson wrote:No, galaxies are incredibly small. And distances of 100s of light years are insignificantly small. The only thing that is big is nearly empty space.
No, the only reason nearly empty space seems so big, is because it's expanding. It started off insignificantly small, as compared to what it appears to be today, and todays bigness will seem insignificantly small, in perhaps only a few billion years, or less, at the rate it is expanding now.
But still, galaxies are damn BIG compared to measly 100's of light years long dust filaments :!:
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by SaraBiga » Fri Oct 11, 2013 10:58 am

Why is there this tiny voice inside my head saying: space is VAST, matter is BIG. Somehow I cannot bring myself to describing nearly empty space as 'big'...

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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by FloridaMike » Fri Oct 11, 2013 1:26 pm

Even a block of lead is mostly empty space.
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Chris Peterson » Fri Oct 11, 2013 2:04 pm

Ann wrote:That depends on what you compare these structures with, doesn't it, Chris?
Of course. That was really my point, although perhaps it was a bit too indirect. All sizes are relative, and "big" and "small" don't mean anything except by comparison.

In the ordinary course of my day I'm accustomed to dealing with both the very small (microscopic) and the very large (cosmological). As a consequence, I've naturally come to shift my scale depending on what I'm considering. I no longer think of galaxies as large at all. A few tens of thousands of light years when I'm in that mode seem to me natural and normal, with 100s of light years a small number and the distance between galaxies as a large one.

In a sense, I think my scaling system has gone from linear (which is probably what we are born with, or at least, learn quite early) to logarithmic. I don't just think of things in terms of powers of ten, I feel them that way now. So things lie on some scale of only a few tens negative to a few tens positive... much more manageable than trillionths of trillionths to trillions and trillions.
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by BDanielMayfield » Fri Oct 11, 2013 2:53 pm

So NGC 891 is about the same size as the Milky Way, but how about the galactic bulge? 891 looks like it barely has a bulge at all. Wouldn’t our galaxy, viewed from the same perspective, show much more of a bulge?
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by geckzilla » Fri Oct 11, 2013 3:04 pm

BDanielMayfield wrote:So NGC 891 is about the same size as the Milky Way, but how about the galactic bulge? 891 looks like it barely has a bulge at all. Wouldn’t our galaxy, viewed from the same perspective, show much more of a bulge?
That's something we have a lot of pictures of. It doesn't look much different from NGC891 to me. http://en.wikipedia.org/wiki/File:Milkyway_pan1.jpg
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by bystander » Fri Oct 11, 2013 3:27 pm

Beyond wrote:No, the only reason nearly empty space seems so big, is because it's expanding. It started off insignificantly small, as compared to what it appears to be today, and todays bigness will seem insignificantly small, in perhaps only a few billion years, or less, at the rate it is expanding now.
But still, galaxies are damn BIG compared to measly 100's of light years long dust filaments :!:
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Chris Peterson » Fri Oct 11, 2013 3:36 pm

Beyond wrote:No, the only reason nearly empty space seems so big, is because it's expanding.
I don't agree with that statement. By the time the Universe was a picosecond old, it was already a substantial fraction of its current size. Cosmological expansion proceeds at a snail's pace compared with what happened during the inflationary period. For as long as there has been a universe with stable laws of physics, it has been largely empty space, so I'd say that space has always been big compared with its contents.
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by FloridaMike » Fri Oct 11, 2013 3:53 pm

Chris Peterson wrote: ... By the time the Universe was a picosecond old, it was already a substantial fraction of its current size....
In the old days, the inflationary period brought the size of the universe from microscopic to "grapefruit" sized. To the best of my recollection anyway. When did that change?
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Chris Peterson » Fri Oct 11, 2013 3:54 pm

FloridaMike wrote:
Chris Peterson wrote: ... By the time the Universe was a picosecond old, it was already a substantial fraction of its current size....
In the old days, the inflationary period brought the size of the universe from microscopic to "grapefruit" sized. To the best of my recollection anyway. When did that change?
The increase from its original size to grapefruit sized is many, many orders of magnitude larger than from grapefruit sized to the present size.
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by FloridaMike » Fri Oct 11, 2013 3:57 pm

Chris Peterson wrote:
FloridaMike wrote:
Chris Peterson wrote: ... By the time the Universe was a picosecond old, it was already a substantial fraction of its current size....
In the old days, the inflationary period brought the size of the universe from microscopic to "grapefruit" sized. To the best of my recollection anyway. When did that change?
The increase from its original size to grapefruit sized is many, many orders of magnitude larger than from grapefruit sized to the present size.

Ah, There you go again with the scales....
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Cousin Ricky » Fri Oct 11, 2013 4:40 pm

One thing about this thread: it's fun to read! :D

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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Beyond » Fri Oct 11, 2013 5:28 pm

Chris Peterson wrote:...so I'd say that space has always been big compared with its contents.
Of Course! Otherwise It wouldn't be able to hold all it's contents, and what is taking place within it, that we mostly can't see, but have a very small perception of through various means, wouldn't be able to take place if it leaked out beyond space. The conditions would be all wrong for it, and great disorder would occur. As what is going on increases, space expands, to allow it to continue.
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Chris Peterson » Fri Oct 11, 2013 5:43 pm

Beyond wrote:
Chris Peterson wrote:...so I'd say that space has always been big compared with its contents.
Of Course! Otherwise It wouldn't be able to hold all it's contents, and what is taking place within it, that we mostly can't see, but have a very small perception of through various means, wouldn't be able to take place if it leaked out beyond space. The conditions would be all wrong for it, and great disorder would occur. As what is going on increases, space expands, to allow it to continue.
Well, a jug half filled with water is able to hold its contents, but I wouldn't say it is particularly big compared with them. But for the vast majority of the existence of the Universe, space has been big compared with its contents in the way a jug is big compared to an atom of water inside it.
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by rstevenson » Fri Oct 11, 2013 6:10 pm

Chris Peterson wrote:
FloridaMike wrote:
Chris Peterson wrote: ... By the time the Universe was a picosecond old, it was already a substantial fraction of its current size....
In the old days, the inflationary period brought the size of the universe from microscopic to "grapefruit" sized. To the best of my recollection anyway. When did that change?
The increase from its original size to grapefruit sized is many, many orders of magnitude larger than from grapefruit sized to the present size.
Hmmmmm, let me riff on that thought, to use a "beatism".

If by "its original size" you mean zero, well yes, you'd be right. Indeed you'd be right to say the universe's original size was an infinite number of orders of magnitude smaller than a grapefruit. But that gets us into math that involves dividing by infinity, and that way lies insanity, so let's not go there. Let's instead pick a size of about 1 picometre (1 trillionth of a metre) just to get started. (I pick that size only because it's a good deal smaller than any atom.) So, to restate your contention as a question, how close is a grapefruit to the midpoint of the range between 1 picometre and the current size of the universe?

The universe is thought to be about 156 Bly (billion light years) in size now. Let's keep the math simple and call it 100 Bly. A light year is about 10 trillion km. A (smallish) grapefruit is about 1/10th of a metre in size. Putting all that together we can say that the universe is about 1 x 1025 grapefruits in size (unless I've misplaced a zero somewhere, which is almost certain.)

So we need to see if a picometre is more or less than 1/(1 x 1025) of a grapefruit in size. Well, it's a trillionth of a metre and a grapefruit is a tenth of a metre, so a picometre is about one 100-billionth of a metre, or 1/(1 x 1011). (When you start writing out "one 100-billionth of a metre", you realize why we use scientific notation instead.)

So a picometre, my arbitrary starting point, is still 1 x 1014 times larger than it would be if a grapefruit were at the midpoint of the range. So we can go a lot smaller still for the starting size of the universe (as indeed we should, of course) before we even get to that equilibrium point, let alone get anywhere near "many, many orders of magnitude" smaller than a grapefruit.

I think you may have overstated your case Chris -- unless, as I mentioned above, you're starting with a size of zero, in which case any quantification is essentially meaningless.

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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Chris Peterson » Fri Oct 11, 2013 6:37 pm

rstevenson wrote:Hmmmmm, let me riff on that thought, to use a "beatism".

If by "its original size" you mean zero, well yes, you'd be right. Indeed you'd be right to say the universe's original size was an infinite number of orders of magnitude smaller than a grapefruit. But that gets us into math that involves dividing by infinity, and that way lies insanity, so let's not go there. Let's instead pick a size of about 1 picometre (1 trillionth of a metre) just to get started. (I pick that size only because it's a good deal smaller than any atom.) So, to restate your contention as a question, how close is a grapefruit to the midpoint of the range between 1 picometre and the current size of the universe?

The universe is thought to be about 156 Bly (billion light years) in size now. Let's keep the math simple and call it 100 Bly. A light year is about 10 trillion km. A (smallish) grapefruit is about 1/10th of a metre in size. Putting all that together we can say that the universe is about 1 x 1025 grapefruits in size (unless I've misplaced a zero somewhere, which is almost certain.)

So we need to see if a picometre is more or less than 1/(1 x 1025) of a grapefruit in size. Well, it's a trillionth of a metre and a grapefruit is a tenth of a metre, so a picometre is about one 100-billionth of a metre, or 1/(1 x 1011). (When you start writing out "one 100-billionth of a metre", you realize why we use scientific notation instead.)

So a picometre, my arbitrary starting point, is still 1 x 1014 times larger than it would be if a grapefruit were at the midpoint of the range. So we can go a lot smaller still for the starting size of the universe (as indeed we should, of course) before we even get to that equilibrium point, let alone get anywhere near "many, many orders of magnitude" smaller than a grapefruit.

I think you may have overstated your case Chris -- unless, as I mentioned above, you're starting with a size of zero, in which case any quantification is essentially meaningless.

Rob (who is avoiding work)
Your value of a picometer is just a guess, though. The actual theory describing the inflationary epoch places a minimum volumetric increase during that brief time of 1078, but it could be much more. Most variations on the BB theory don't assume that the Universe started as a dimensionless point, although they may allow for it.

So actually, the numbers are interesting. The volumetric growth during inflation was a minimum of 1078, and the volumetric growth from a grapefruit to the observable universe is about 1084. So it is possible that the growth in volume since the end of the inflationary period has been greater by a few orders of magnitude. In diameter, that would be no more than two orders of magnitude. So curiously, we have quite similar degrees of expansion for the inflationary period and for the subsequent 13.7 billion years.

Of course, there are still some important uncertainties: that the post-inflationary size was around 0.1 meters isn't known with any accuracy, and could differ by orders of magnitude in either direction. And the size of the entire Universe isn't known at all (which is why I used the observable universe in my calculation).

Anyway, thanks for making me go back and take a closer look at the numbers.
Chris

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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by rstevenson » Fri Oct 11, 2013 7:24 pm

That sounds reasonable, Chris.

I should have used volumetric comparisons, of course. And I can't now recall where I got that 156 Bly for the size of the universe, except that it wasn't from an obviously crackpot site.

With imponderables at both ends of the scale, I wonder if there can be any meaning attached to the fact that life-as-we-know-it is about in the middle of the size range. Nahhhh, not a chance.

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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by Beyond » Fri Oct 11, 2013 7:45 pm

haha, "not a chance", could mean "on purpose". No chance involved.
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by rstevenson » Fri Oct 11, 2013 8:21 pm

Beyond wrote:haha, "not a chance", could mean "on purpose". No chance involved.
You're thinking of not by chance, which is, of course, a banned subject. :mrgreen:

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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by geckzilla » Fri Oct 11, 2013 8:41 pm

:roll:
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Re: APOD: NGC 891 Edge On (2013 Oct 11)

Post by BDanielMayfield » Fri Oct 11, 2013 8:49 pm

geckzilla wrote:
BDanielMayfield wrote:So NGC 891 is about the same size as the Milky Way, but how about the galactic bulge? 891 looks like it barely has a bulge at all. Wouldn’t our galaxy, viewed from the same perspective, show much more of a bulge?
That's something we have a lot of pictures of. It doesn't look much different from NGC891 to me. http://en.wikipedia.org/wiki/File:Milkyway_pan1.jpg
Thanks for that picture Geckzilla, which, naturally and unavoidably, is taken from the inside the Milky Way, and not about 30 million lys away as was today’s subject. But I still suspect that our galaxy has a more pronounced bulge than NGC 891 does. I come at this opinion merely from the naked eye impression I’ve gotten by looking toward the center of our galaxy.

Happily my nighttime sky is still dark enough so that I can still see how wide the bulge looks from 30 thousand light years away.
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