APOD: Centaurus A (2015 Nov 19)

Comments and questions about the APOD on the main view screen.
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Glima49 » Thu Nov 19, 2015 11:13 pm

NGC 185 in the Local Group is a dwarf galaxy with a Seyfert 2 nucleus but it is also unsure if it should be considered a Seyfert. Nonetheless it is an active galaxy and a member of the M31 subgroup in the Local Group only 2.5 million ly away.

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Re: APOD: Centaurus A (2015 Nov 19)

Post by Pianosorplanets » Thu Nov 19, 2015 11:42 pm

1000 years ago, it would have seemed absurd to think that we could collect and make sense of data out of the lights in the sky, let alone light from the sky that is outside the ability of the human eye to detect via brightness or frequency. So is it absurd to suppose that there is decipherable data inside of event horizons which, one day, we will discover and learn from? Or perhaps, these black holes have actually been "broadcasting" data that we haven't the skill nor the insight to detect and decode today but one day... I always live in hope that the universe has made itself known and that all that is required is time, hard work, patience, and a lot of smarts to come into an understanding of valuable information that we deem locked away forever inside event horizons in our current understanding of "now." :?:
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Chris Peterson » Thu Nov 19, 2015 11:52 pm

Pianosorplanets wrote:1000 years ago, it would have seemed absurd to think that we could collect and make sense of data out of the lights in the sky, let alone light from the sky that is outside the ability of the human eye to detect via brightness or frequency. So is it absurd to suppose that there is decipherable data inside of event horizons which, one day, we will discover and learn from?
Absurd? No. Likely? Not at all. 1000 years ago we understood nearly nothing about the Universe. Now, our understanding is rich, and much of it is nearly complete.
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Ann » Thu Nov 19, 2015 11:54 pm

Boomer12k wrote:An Exquisite shot!

Are we viewing this Edge On?...or at a slight angle so we see the Top? The dust lanes make it look like Edge On...but if you look closely, there are star forming pink regions that look like they are..."on the other side of the galaxy"...
2 dimensional illusion???

Anybody know for sure???
Thanks.

:---[===] *
Centaurus A is the product of two colliding galaxies, a large elliptical galaxy and a smaller spiral galaxy. The dust lane belongs to the shredded spiral galaxy, and all the star formation is taking place in what remains of the spiral galaxy.

As for the elliptical galaxy, you can see it "poking its head up" over the the broad dust lane - or in today's APOD, it is poking its head "down below" the dust lane. In my opinion it is hard to say if Centaurus A has the sort of shape that makes the expression "edge on" meaningful when we talk about it. The elliptical component is, well, elliptical, and the former spiral is now so jumbled up that I don't know if we can talk about its disk any more.

But I agree with you that it looks as if the dust lane is actually encircling the elliptical component. There is a moderately bright bluish star just below the brightest part of the of the elliptical component of the galaxy (at 6 o'clock), and behind it is what looks like a long line of nebulas and young clusters stretching from the left edge of the dust lane at about 7.30 to, possibly, all the way to the right edge of the dust lane. Interestingly, this line of nebulas and clusters does not seem to be associated with any dark dust at all. But if the dust is behind the elliptical component, we don't expect it to look very dark - in fact, we may not necessarily expect it to look dark at all.
Take a look at this Hubble image of M104. The dust lane passing in front of the bright elliptical component of M104 looks very dark, but the dust lane that is behind the elliptical component does not look dark at all.

Another way of saying this is that the dust that is behind the elliptical component is illuminated by the light of this vast "bulge" of stars, while the dust that is in front of the elliptical component blocks most or all the light from it, making it appear very dark.

That is why I think today's APOD is providing evidence, or at least a suggestion, that there is a star forming dust lane is running all the way around the elliptical component of Centaurus A. It would be very interesting if that is indeed the case!

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Re: APOD: Centaurus A (2015 Nov 19)

Post by ta152h0 » Thu Nov 19, 2015 11:57 pm

1`000 years ago a church had an iron grip on what you know and not know. took them a while to realise the Earth was not the center of anything
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Re: APOD: Centaurus A (2015 Nov 19)

Post by MarkBour » Thu Nov 19, 2015 11:59 pm

Ann wrote: ... So the objects that fall into black holes typically begin their journey of death by circling around the black hole, in tighter and tighter orbits. As the orbits get smaller, the objects move faster. Before they get so close that they actually fall in, they typically swirl around close to the speed of light.
What causes their orbits to keep shrinking? Don't you need some resistance in the orbit to have a "death spiral" ?
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Chris Peterson » Fri Nov 20, 2015 12:03 am

MarkBour wrote:
Ann wrote: ... So the objects that fall into black holes typically begin their journey of death by circling around the black hole, in tighter and tighter orbits. As the orbits get smaller, the objects move faster. Before they get so close that they actually fall in, they typically swirl around close to the speed of light.
What causes their orbits to keep shrinking? Don't you need some resistance in the orbit to have a "death spiral" ?
For the most part, they don't spiral in. This only happens if they are part of a fairly dense zone of matter, so that you have collisions and angular momentum is lost. Otherwise, they simply orbit as they would any other massive body, unchanged for billions of years. Of course, in a galaxy, you have many bodies, and that means you have interactions and what is actually a chaotic system. So you're not likely to find bodies in highly stable orbits around massive black holes for billions of years. But where you don't have dust and gas (just stars), objects are much more likely to be ejected than to fall into the black hole.
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Ann » Fri Nov 20, 2015 12:29 am

Chris Peterson wrote:
Pianosorplanets wrote:1000 years ago, it would have seemed absurd to think that we could collect and make sense of data out of the lights in the sky, let alone light from the sky that is outside the ability of the human eye to detect via brightness or frequency. So is it absurd to suppose that there is decipherable data inside of event horizons which, one day, we will discover and learn from?
Absurd? No. Likely? Not at all. 1000 years ago we understood nearly nothing about the Universe. Now, our understanding is rich, and much of it is nearly complete.
I hesitate to make this post, for the following reasons:

1) I am not a scientist,

2) my understanding of math is simply lousy,

3) my self-taught understanding of astronomy and astrophysics tells me that nothing can move faster than light, and if an object has crossed the event horizon of a black hole, it has to move faster than light to "get back out" again; ergo, it can't do it, and

4) I'm playing a losing game if I want to get into a real argument with Chris! :wink:

Nevertheless, it seems a little risky to me to say that much of our understanding of the universe is nearly complete. I get what Chris is saying, and I agree, too. But let's not forget this quote by Lord Kelvin, which is sobering even if it should turn out to be embellished or misquoted by the page where I found it:
"There is nothing new to be discovered in physics now, All that remains is more and more precise measurement."
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Ann » Fri Nov 20, 2015 12:31 am

MarkBour wrote:
Ann wrote: ... So the objects that fall into black holes typically begin their journey of death by circling around the black hole, in tighter and tighter orbits. As the orbits get smaller, the objects move faster. Before they get so close that they actually fall in, they typically swirl around close to the speed of light.
What causes their orbits to keep shrinking? Don't you need some resistance in the orbit to have a "death spiral" ?
I think the accretion disk is what causes the orbits to shrink. There is a lot of resistance there.

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Re: APOD: Centaurus A (2015 Nov 19)

Post by Chris Peterson » Fri Nov 20, 2015 12:55 am

Ann wrote:
Chris Peterson wrote:Absurd? No. Likely? Not at all. 1000 years ago we understood nearly nothing about the Universe. Now, our understanding is rich, and much of it is nearly complete.
Nevertheless, it seems a little risky to me to say that much of our understanding of the universe is nearly complete. I get what Chris is saying, and I agree, too. But let's not forget this quote by Lord Kelvin, which is sobering even if it should turn out to be embellished or misquoted by the page where I found it:
"There is nothing new to be discovered in physics now, All that remains is more and more precise measurement."
Well, there's no evidence that Kelvin actually said this. But there was a sense of this running through late 19th century physics. For myself, I do not find it sobering at all, given how radically different science was 150 years ago. I am quite comfortable making the argument that much of our physical understanding is nearly complete. Despite the fact that our rate of knowledge growth remains exponential, it remains true that our most fundamental theory- gravity, quantum mechanics, particle physics- is hardly changing in substance, and has been stable for decades, approaching a century. Nothing like that has ever been the case before. I think it is a pretty safe assumption that our understanding of these things will continue to improve, but that the basic theory will not change again. At this point we're looking at refinement, and in a few cases large holes that still need to be filled in. But probably not the complete revision of major theories.
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Big_Lew » Fri Nov 20, 2015 1:04 am

+100 For the Frank Zappa reference! "Look here, brother... Who you jivin' with that Cosmic Debris?" (Creationists: "But - I got the Crystal Ball!")

Disappointed the answer to the first half of the security question isn't "Sol". I get it, but it says a lot about us.

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Re: APOD: Centaurus A (2015 Nov 19)

Post by Big_Lew » Fri Nov 20, 2015 1:28 am

Chris Peterson wrote:
Ann wrote:
Chris Peterson wrote:Absurd? No. Likely? Not at all. 1000 years ago we understood nearly nothing about the Universe. Now, our understanding is rich, and much of it is nearly complete.
Nevertheless, it seems a little risky to me to say that much of our understanding of the universe is nearly complete. I get what Chris is saying, and I agree, too. But let's not forget this quote by Lord Kelvin, which is sobering even if it should turn out to be embellished or misquoted by the page where I found it:
"There is nothing new to be discovered in physics now, All that remains is more and more precise measurement."
Well, there's no evidence that Kelvin actually said this. But there was a sense of this running through late 19th century physics. For myself, I do not find it sobering at all, given how radically different science was 150 years ago. I am quite comfortable making the argument that much of our physical understanding is nearly complete. Despite the fact that our rate of knowledge growth remains exponential, it remains true that our most fundamental theory- gravity, quantum mechanics, particle physics- is hardly changing in substance, and has been stable for decades, approaching a century. Nothing like that has ever been the case before. I think it is a pretty safe assumption that our understanding of these things will continue to improve, but that the basic theory will not change again. At this point we're looking at refinement, and in a few cases large holes that still need to be filled in. But probably not the complete revision of major theories.
This is true, but I quibble with your semantics here. Chris: If you look at the progress in physics from the 50,000' level, what has happened is that we have been surprised time after time by probing deeper into nature (relativity, quantum mechanics, etc.). And, most importantly, none of those refinements invalidated the prior theories, rather they expanded them into regimes of energy and measurement that we had been unable to explore and revealed that the correct (at the time) theories were only approximations in the limited regimes of energy and measurement available at the time they were formulated. As our technology improves (LHC, anyone?), I posit that we can surely expect more stunning surprises of similar impact like Relativity and Quantum Mechanics. For example, the Double-Slit Experiment (in all its incarnations - straight, quantum eraser, delayed choice, etc.) richly demonstrates our profound ignorance of the true underlying nature of the universe.

String Theory and Quantum Loop Gravity both come readily to mind...

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Re: APOD: Centaurus A (2015 Nov 19)

Post by Chris Peterson » Fri Nov 20, 2015 2:15 am

Big_Lew wrote:This is true, but I quibble with your semantics here.
Well, I think you're right that it is semantics. I don't think we're saying different things, or have substantially different views. I'm certainly not suggesting there's nothing new under the Sun... only that the new things we're going to discover are not likely to overturn what we already know. So in the context of what I was originally responding to, it's very unlikely that we're going to discover new physics that demonstrates black holes are shedding information across their event horizons- because we already have very solid theory telling us why that isn't so. Theory that is likely to stand the test of time.
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Big_Lew » Fri Nov 20, 2015 2:49 am

Chris Peterson wrote:
Big_Lew wrote:This is true, but I quibble with your semantics here.
Well, I think you're right that it is semantics. I don't think we're saying different things, or have substantially different views. I'm certainly not suggesting there's nothing new under the Sun... only that the new things we're going to discover are not likely to overturn what we already know. So in the context of what I was originally responding to, it's very unlikely that we're going to discover new physics that demonstrates black holes are shedding information across their event horizons- because we already have very solid theory telling us why that isn't so. Theory that is likely to stand the test of time.
Hasn't Leonard Susskind in "Black Hole War" (ISBN 978-0316016407) laid out a compelling case for information trapped in a black hole eventually returning to the non-relativistic universe we can see? Hawking paid off the bet he lost... And Hawking described a possible mechanism by which information within an event horizon can return to this universe with his rigorous mathematical formalization of Hawking Radiation.

I get that we mostly agree. My point is that future discoveries may well be as earth-shaking as Relativity and Quantum Mechanics, while at the same time not invalidating current theories. No, future discoveries will not invalidate what we currently know - for example, the Standard Model has been experimentally verified to a fantastic degree - but future discoveries may demonstrate that our current understanding is woefully inadequate.

Of course, the current conflict between classical physics and quantum mechanics (both unerringly accurate within their domain) is only the most prominent manifestation of this.

It's not going to turn out that the "Theory of Gravity" is wrong, but it may very well turn out that gravity is caused by agents that we can't even CONCEIVE of today. Thanks to Newton and Einstein, we're experts at describing the EFFECTS of gravity, but as yet we have no viable, valid answer describing the AGENT of gravity.

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Re: APOD: Centaurus A (2015 Nov 19)

Post by neufer » Fri Nov 20, 2015 3:46 am

MarkBour wrote:
Ann wrote:
... So the objects that fall into black holes typically begin their journey of death by circling around the black hole, in tighter and tighter orbits. As the orbits get smaller, the objects move faster. Before they get so close that they actually fall in, they typically swirl around close to the speed of light.
What causes their orbits to keep shrinking? Don't you need some resistance in the orbit to have a "death spiral" ?
https://en.wikipedia.org/wiki/Accretion_disk#Accretion_disk_physics wrote:
<<In the 1940s, accretion disk models were first derived from basic physical principles. In order to agree with observations, those models had to invoke a yet unknown mechanism for angular momentum redistribution. If matter is to fall inwards it must lose not only gravitational energy but also lose angular momentum. Since the total angular momentum of the disk is conserved, the angular momentum loss of the mass falling into the center has to be compensated by an angular momentum gain of the mass far from the center. In other words, angular momentum should be transported outwards for matter to accrete. According to the Rayleigh stability criterion,
  • Image
where Image represents the angular velocity of a fluid element and R its distance to the rotation center, an accretion disk is expected to be a laminar flow. This prevents the existence of a hydrodynamic mechanism for angular momentum transport.

On one hand, it was clear that viscous stresses would eventually cause the matter towards the center to heat up and radiate away some of its gravitational energy. On the other hand, viscosity itself was not enough to explain the transport of angular momentum to the exterior parts of the disk. Turbulence-enhanced viscosity was the mechanism thought to be responsible for such angular-momentum redistribution, although the origin of the turbulence itself was not well understood. The conventional Image-model introduces an adjustable parameter Image describing the effective increase of viscosity due to turbulent eddies within the disk. In 1991, with the rediscovery of the magnetorotational instability (MRI), S. A. Balbus and J. F. Hawley established that a weakly magnetized disk accreting around a heavy, compact central object would be highly unstable, providing a direct mechanism for angular-momentum redistribution.>>
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Chris Peterson » Fri Nov 20, 2015 5:24 am

Big_Lew wrote:It's not going to turn out that the "Theory of Gravity" is wrong, but it may very well turn out that gravity is caused by agents that we can't even CONCEIVE of today. Thanks to Newton and Einstein, we're experts at describing the EFFECTS of gravity, but as yet we have no viable, valid answer describing the AGENT of gravity.
I disagree. We have a very good handle on the agent of gravity: it is how we observe the curvature of spacetime. GR doesn't just describe the effects, it describes the cause.
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Re: APOD: Centaurus A (2015 Nov 19)

Post by Pianosorplanets » Sun Nov 29, 2015 5:14 am

Chris Peterson wrote:
Big_Lew wrote:This is true, but I quibble with your semantics here.
Well, I think you're right that it is semantics. I don't think we're saying different things, or have substantially different views. I'm certainly not suggesting there's nothing new under the Sun... only that the new things we're going to discover are not likely to overturn what we already know. So in the context of what I was originally responding to, it's very unlikely that we're going to discover new physics that demonstrates black holes are shedding information across their event horizons- because we already have very solid theory telling us why that isn't so. Theory that is likely to stand the test of time.
Before I proceed to place my foot into my mouth, let me preface that I have the greatest respect for you, Chris, and your knowledge. I am not even about to actually quibble with you. But here goes:

My comment concerning gaining data from black holes was obviously a radical example of my belief that man will, in the fullness of time, figure out ways to deduce information currently lost forever to us (or we think it's lost forever.) Would you agree that we tend to find several new questions we hadn't thought to ask each time we come close to answering one question that we had thought to ask?

I'm not going to stretch my neck out further than that but I have "faith" that man can and will discover new ways to extract knowledge from the universe that we don't even suspect is out there at this point in history.

The belief that we have completed our reckoning of the universe and that all that remains is fine tuning is a position that I respect but not one I can share. I suspect that the pool of data accumulated by the scientific community very likely only touches upon a small fraction of concepts (a word I'm finding vastly understated but I lack a better at the moment) which define the universe as a whole.

My personal belief that I would not ask anyone else to share: Earth based human life will be snuffed out (meaning by our sun going splat or some other similar occurrence) before we get anywhere close to a respectable percentage of actual collectable data that is out there and collectable (if you're clever enough.) And I'm not meaning the picky naming of every star that can be directly or indirectly identified or every crater, volcano, valley and plain we could define and name. I'm referring to important concepts equal to that of Relativity.

As I read and listen to scientists in the field of Astronomy from, say, Tycho Brahe to today; I am amused at the amount of chutzpah it takes (I'm just as guilty more often than I care to admit) to talk as if you have a really good line on how something this complex "goes." I'm not accusing Chris personally of having too much chutzpah, really, I'm not. I'm saying that overconfidence in our complete/accurate knowledge base is a very obvious recurring theme; one I find fault with. I think it's safer for such as I to take a more humble tack so I continue to believe that we've barely scratched the surface.

But I don't expect anyone else reading this to agree with me in the slightest. We're all entitled to our own world view.
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