APOD: Arp 94 (2013 Oct 09)

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APOD: Arp 94 (2013 Oct 09)

Post by APOD Robot » Wed Oct 09, 2013 4:05 am

Image Arp 94

Explanation: This telescopic snapshot records a cosmic moment in the tumultuous lives of large spiral galaxy NGC 3227 and smaller elliptical NGC 3226. Catching them in the middle of an ongoing gravitational dance, the sensitive imaging also follows faint tidal star streams flung from the galaxies in their repeated close encounters. Over 50 million light-years distant toward the constellation Leo, the pair's appearance has earned them the designation Arp 94 in the classic catalog of peculiar galaxies. But such galactic collisions and mergers are now thought to represent a normal course in the evolution of galaxies, including our own Milky Way. Spanning about 90,000 light-years, similar in size to the Milky Way, NGC 3227 is recognized as an active Seyfert galaxy with a central supermassive black hole.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by Beyond » Wed Oct 09, 2013 4:25 am

That's a relatively nice looking galaxy with all the pink and blue. Is it a boy or a girl?
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Re: APOD: Arp 94 (2013 Oct 09)

Post by Ann » Wed Oct 09, 2013 4:43 am

Nice picture!

As usual, I find myself wondering about the colors of the galaxies. The spiral and the elliptical look very alike, colorwise. I can think of two explanations for this. It could be that the color balance of the picture is calibrated for other purposes than for bringing out the color difference between the galaxies. Alternatively, the color difference of the galaxies really might really be very small. The big spiral might have a large yellow population, a comparatively modest amount of star formation and a significant amount of dust. In that case it will not be very blue at all. The smaller elliptical might have a halo population of relatively metal-poor stars, which may well be relatively neutral-colored overall.

Well, I find it interesting to wonder about such things. And indeed, the picture is very nice! :D

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Re: APOD: Arp 94 (2013 Oct 09)

Post by Ron_2 » Wed Oct 09, 2013 5:10 am

If galaxies are all being flung to the far-flung 'horizons' of the Universe, the outermost going fastest (?), how would they come to be on collision courses at all ?

Forgive my knowledge of such galactic behaviours, I have a short memory, and wasn't around when this all started...

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Re: APOD: Arp 94 (2013 Oct 09)

Post by Ann » Wed Oct 09, 2013 7:50 am

Ron_2 wrote:If galaxies are all being flung to the far-flung 'horizons' of the Universe, the outermost going fastest (?), how would they come to be on collision courses at all ?

Forgive my knowledge of such galactic behaviours, I have a short memory, and wasn't around when this all started...
Galaxies that are very close to one another interact strongly. Their mutual gravitational attraction is much stronger than the force imparted on them from the expansion of the universe.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by Markus Schwarz » Wed Oct 09, 2013 7:50 am

Ron_2 wrote:If galaxies are all being flung to the far-flung 'horizons' of the Universe, the outermost going fastest (?), how would they come to be on collision courses at all ?
The distance between galaxies is increasing, they are not "flung around". Though difficult to imagine this is something different than galaxies flying away from each other. The cosmic expansion makes itself felt on distances larger than about 400 Mpc, or about 1.3 Gly. The typical distance between galaxies is much smaller, say about 1 Mpc. Therefore, the interaction between galaxies is dominated by their mutual gravitational attraction.
Ron_2 wrote:Forgive my knowledge of such galactic behaviours
No problem! Asking questions about astronomy is what this forum is about.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by wonderboy » Wed Oct 09, 2013 12:27 pm

this reminds me of a question i posed before! if gravity pulls things in, or towards then what is the equal and opposing force? it must be somthing that pushes things out, or away from it. the question i asked (i think) was: "is gravity responsible for expansion?" sounds silly but if Einsteins theory is correct then gravity must have an equal and opposing force.
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Re: APOD: Arp 94 (2013 Oct 09)

Post by neufer » Wed Oct 09, 2013 1:01 pm

Markus Schwarz wrote:
Ron_2 wrote:
If galaxies are all being flung to the far-flung 'horizons' of the Universe, the outermost going fastest (?), how would they come to be on collision courses at all ?
The distance between galaxies is increasing, they are not "flung around". Though difficult to imagine this is something different than galaxies flying away from each other. The cosmic expansion makes itself felt on distances larger than about 400 Mpc, or about 1.3 Gly. The typical distance between galaxies is much smaller, say about 1 Mpc. Therefore, the interaction between galaxies is dominated by their mutual gravitational attraction.
Image
I like to think about the universe as a large expanding (e.g., perhaps because the sides suddenly disappeared) vat of milk containing Cheerios.

Cheerios that are close enough together will condense into a permanent fixed sized clusters of Cheerios which move away from other permanent fixed sized clusters of Cheerios. :arrow:
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Re: APOD: Arp 94 (2013 Oct 09)

Post by wonderboy » Wed Oct 09, 2013 1:16 pm

neufer wrote:
Markus Schwarz wrote:
Ron_2 wrote:
If galaxies are all being flung to the far-flung 'horizons' of the Universe, the outermost going fastest (?), how would they come to be on collision courses at all ?
The distance between galaxies is increasing, they are not "flung around". Though difficult to imagine this is something different than galaxies flying away from each other. The cosmic expansion makes itself felt on distances larger than about 400 Mpc, or about 1.3 Gly. The typical distance between galaxies is much smaller, say about 1 Mpc. Therefore, the interaction between galaxies is dominated by their mutual gravitational attraction.
Image
I like to think about the universe as a large expanding (e.g., perhaps because the sides suddenly disappeared) vat of milk containing Cheerios.



Cheerios that are close enough together will condense into a permanent fixed sized clusters of Cheerios which move away from other permanent fixed sized clusters of Cheerios. :arrow:

this being the case, do they infact say "cheerio" as they move away from each other?
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Re: APOD: Arp 94 (2013 Oct 09)

Post by neufer » Wed Oct 09, 2013 1:33 pm

wonderboy wrote:
this reminds me of a question i posed before! if gravity pulls things in, or towards then what is the equal and opposing force? somthing that pushes things out, or away from it.
If you are referencing Newton's third law:

3) For every action force there is an equal (in size) & opposite (in direction) reaction force.

The action force on NGC 3226 is an equal (in size) & opposite (in direction) to the reaction force on NGC 3227.
wonderboy wrote:
it must be the question i asked (i think) was: "is gravity responsible for expansion?" sounds silly but if Einsteins theory is correct then gravity must have an equal and opposing force.
If you are referencing Einstein's full field equation: Image

then the Einstein gravitational curvature tensor as defined by: Image

is curved one way by the matter/energy content of spacetime: Image on the right side of the equation but
an opposite (though possibly unequal) way by the Image cosmological constant on the left side of the equation.
Last edited by neufer on Wed Oct 09, 2013 2:03 pm, edited 1 time in total.
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Re: APOD: Arp 94 (2013 Oct 09)

Post by Nitpicker » Wed Oct 09, 2013 1:38 pm

wonderboy wrote:this reminds me of a question i posed before! if gravity pulls things in, or towards then what is the equal and opposing force? it must be somthing that pushes things out, or away from it. the question i asked (i think) was: "is gravity responsible for expansion?" sounds silly but if Einsteins theory is correct then gravity must have an equal and opposing force.
Gravity was Newton. Relativity was Einstein.

In the case of two massive bodies, they each exert a gravitational pulling force on the other, that is equal in magnitude and opposite in direction, regardless of any difference in mass between the bodies. This is according to Newton's Third Law of Motion, which is applicable to any kind of force, not just gravitational.

My simple understanding of the expanding universe is that the Big Bang caused it. Gravitational forces merely act on the resulting mass. The closer the masses are to each other, and the more massive they are, the greater the gravitational forces between them.

There are more complicated ways of thinking about it ... for example, I don't think I will ever understand neufer's Cheerio-milk-vat model.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by Nitpicker » Wed Oct 09, 2013 1:54 pm

Very impressive APOD by the way. Very sharp for a relatively narrow field of view observed through the atmosphere.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by neufer » Wed Oct 09, 2013 2:01 pm

wonderboy wrote:
this being the case, do they infact say "cheerio" as they move away from each other?
Is that what they say in Scotland :?:
Because it all depends upon the Bond number... James Bond
(ImageImageImage).
http://en.wikipedia.org/wiki/Cheerios_effect wrote:
<<In fluid mechanics, the Cheerios effect is the tendency for small wettable floating objects to attract one another. An example of the Cheerios effect is the phenomenon whereby breakfast cereal tends to clump together or cling to the sides of a bowl of milk. This clumping behavior applies to any small macroscopic object that floats or clings to the surface of a liquid. This can include a multitude of things, including hair particles in shaving cream and fizzy beer bubbles. The effect is not noticeable for boats and other large floating objects because the force of surface tension is relatively small at that scale. (The Casimir effect, with a similar result, occurs at nanoscopic scale, and boats and other large objects floating in a choppy sea are subject to its classical equivalent; both are caused by waves, not surface tension.)

Writing in the American Journal of Physics, Dominic Vella and L. Mahadevan of Harvard University discuss the cheerios effect and suggest that it may be useful in the study of self assembly of small structures. They calculate the force between two spheres of density Image and radius R floating distance Image apart in liquid of density Image as

Image

where Image is the surface tension, Image is a modified Bessel function of the first kind,

Image is the Bond number, and

Image

is a nondimensional factor in terms of the contact angle Image.>>
Last edited by neufer on Wed Oct 09, 2013 2:10 pm, edited 1 time in total.
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Re: APOD: Arp 94 (2013 Oct 09)

Post by Chris Peterson » Wed Oct 09, 2013 2:02 pm

Nitpicker wrote:My simple understanding of the expanding universe is that the Big Bang caused it. Gravitational forces merely act on the resulting mass. The closer the masses are to each other, and the more massive they are, the greater the gravitational forces between them.
Correct. Two bodies acted only upon by their mutual gravitational attraction will separate forever, in the absence of any other forces, as long as their relative velocity is above their mutual escape velocities. The rate they separate will get ever slower, but separate they will.

That idea was behind one of the great cosmological questions that came out of the development of the Big Bang theories. Since no other force appeared to be involved besides gravity, would the Universe expand forever or would it eventually reverse and collapse again? The question depended upon determining if the bits were flying apart at greater than escape velocity.

Of course, recent observations have demonstrated that the rate of expansion of the Universe is actually increasing- something that can only be explained classically by a force opposite that of gravity. We call that force "dark energy", although its actual nature remains very unclear.
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Re: APOD: Arp 94 (2013 Oct 09)

Post by Nitpicker » Wed Oct 09, 2013 2:12 pm

Neufer, sounds like your Cheerio-milk-vat model of the universe is closer to common-or-garden surface tension. And here I was thinking you'd developed a Grand Unified Theory.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by BDanielMayfield » Wed Oct 09, 2013 2:33 pm

Nitpicker wrote:My simple understanding of the expanding universe is that the Big Bang caused it. Gravitational forces merely act on the resulting mass. The closer the masses are to each other, and the more massive they are, the greater the gravitational forces between them.

There are more complicated ways of thinking about it ... for example, I don't think I will ever understand neufer's Cheerio-milk-vat model.
But the Big Bang doesn’t explain the more recent discovery that the rate of universal expansion is now accelerating. If it was only gravity at work here then the universal expansion would be slowing down.

“Neufer’s Cheerio-milk-vat model” is an excellent analogy, as long as it isn’t taken too far. In the bowl or vat of milk surface tension causes the observed clumps of cheerios, but it also pulls the clumps against the sides of the bowl. In the cosmos gravity causes galaxies to cluster and merge, but the force causing the outward expansion to accelerate is unknown, and is called Dark Energy.

What’s pulling the universe apart? This is one of the biggest mysteries of astrophysics.I could state what I strongly believe to be the cause, but doing so would be “frowned upon, in this establishment.” :wink:
Just as zero is not equal to infinity, everything coming from nothing is illogical.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by Chris Peterson » Wed Oct 09, 2013 2:41 pm

BDanielMayfield wrote:But the Big Bang doesn’t explain the more recent discovery that the rate of universal expansion is now accelerating. If it was only gravity at work here then the universal expansion would be slowing down.
However, dark energy is a fundamental part of the modern Big Bang theory, so in fact, the "Big Bang" does explain our observations.
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Re: APOD: Arp 94 (2013 Oct 09)

Post by geckzilla » Wed Oct 09, 2013 3:00 pm

BDanielMayfield wrote:What’s pulling the universe apart? This is one of the biggest mysteries of astrophysics.I could state what I strongly believe to be the cause, but doing so would be “frowned upon, in this establishment.” :wink:
You say it like it's a bad thing that we reject pseudoscience and fringe theories here. Chances are, if you have a strong emotional attachment to something, you aren't being objective, anyway.
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Re: APOD: Arp 94 (2013 Oct 09)

Post by neufer » Wed Oct 09, 2013 3:10 pm

Chris Peterson wrote:
BDanielMayfield wrote:
But the Big Bang doesn’t explain the more recent discovery that the rate of universal expansion is now accelerating. If it was only gravity at work here then the universal expansion would be slowing down.
However, dark energy is a fundamental part of the modern Big Bang theory, so in fact, the "Big Bang" does explain our observations.
Einstein's full field equation: Image is compatible with both the Big Bang
and the more recent discovery that the rate of universal expansion is now accelerating.

However, Einstein's full field equation was originally hypothesized on the assumption of a static universe with no Big Bang.
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Re: APOD: Arp 94 (2013 Oct 09)

Post by BDanielMayfield » Wed Oct 09, 2013 3:22 pm

Chris Peterson wrote:
BDanielMayfield wrote:But the Big Bang doesn’t explain the more recent discovery that the rate of universal expansion is now accelerating. If it was only gravity at work here then the universal expansion would be slowing down.
However, dark energy is a fundamental part of the modern Big Bang theory, so in fact, the "Big Bang" does explain our observations.
Then the modern Big Bang theory, as modified to account for accelerating universal expansion, is a theory that explains observations, but it does so with another major, fundamental unknown plugged in, which is dark energy. As you stated yourself Chris, dark energy’s “actual nature remains very unclear.”
Just as zero is not equal to infinity, everything coming from nothing is illogical.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by Nitpicker » Wed Oct 09, 2013 3:34 pm

The Big Bang presumably created matter, dark matter and dark energy. And thank goodness for that, as otherwise the recent observations of the professionals would be really hard to fathom. But the real beauty is that if, at some time in the future, they observe something else they can't explain, it will be apparent that its cause was created by the Big Bang too. Bloody brilliant! As theories go, they don't get much better than that!

On the other hand, the Cheerio-milk-vat model of the universe does not explain the angular momentum of galaxies, nor the acceleration of the expansion of the universe. Indeed, it cannot even explain the basic orbital motion of the Solar System. On the plus side, it might sustain one until lunch time.

Personally, as a rank amateur with a six-inch telescope, I have never made an observation which has required me to infer the existence of dark matter, nor dark energy. I am, however, quite comfortable with gravity, and I find it has a great many uses in my day to day life as well. Call me simple if you must.

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Re: APOD: Arp 94 (2013 Oct 09)

Post by BDanielMayfield » Wed Oct 09, 2013 3:49 pm

Nitpicker wrote:The Big Bang presumably created matter, dark matter and dark energy. And thank goodness for that, as otherwise the recent observations of the professionals would be really hard to fathom. But the real beauty is that if, at some time in the future, they observe something else they can't explain, it will be apparent that its cause was created by the Big Bang too. Bloody brilliant! As theories go, they don't get much better than that!
You got my point exactly Nitpicker. Goodonumate!
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Re: APOD: Arp 94 (2013 Oct 09)

Post by FloridaMike » Wed Oct 09, 2013 4:01 pm

Chris Peterson wrote: ..... the rate of expansion of the Universe is actually increasing- something that can only be explained classically by a force opposite that of gravity. We call that force "dark energy", although its actual nature remains very unclear.

Kudos Chris,
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Re: APOD: Arp 94 (2013 Oct 09)

Post by Chris Peterson » Wed Oct 09, 2013 4:09 pm

BDanielMayfield wrote:Then the modern Big Bang theory, as modified to account for accelerating universal expansion, is a theory that explains observations, but it does so with another major, fundamental unknown plugged in, which is dark energy. As you stated yourself Chris, dark energy’s “actual nature remains very unclear.”
I fear that even the scientifically literate tend to misunderstand what science is all about, sometimes.

Every theory is simply a mechanism designed to explain observations. Most non-trivial theories have undergone revisions with time as increasingly rich observations revealed previously unrecognized details.

Dark energy isn't a fundamental unknown. It is an observation, and by adjusting our cosmological model to account for it we are able to make other sorts of predictions that we couldn't before, and about more than simply the observation of increased universal expansion. That's one thing that results in increased confidence that our model is improving.

What does it even mean that some fundamental thing like dark energy is "unknown", or "unclear"? What isn't? We know nothing about what electric charge "really" is. We know nothing about what gravity "really" is. We know nothing about what any of the fundamental forces "really" are. Each of these things is just like dark energy: mysterious "forces" (even that word is applied as a simplification) which we can only describe or explain by using abstract theories that appear to describe the behavior we observe.

Fundamentally, dark energy is no more mysterious than electric charge or gravity. The only difference is that we don't yet have as richly developed a theory for it, we don't yet have as many independent observations of its effects.
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Re: APOD: Arp 94 (2013 Oct 09)

Post by Nitpicker » Wed Oct 09, 2013 4:11 pm

BDanielMayfield wrote:What’s pulling the universe apart? This is one of the biggest mysteries of astrophysics.I could state what I strongly believe to be the cause, but doing so would be “frowned upon, in this establishment.”
For the record, I accept that dark energy -- whatever it is -- creates the force that is accelerating the expansion of the universe, mainly because:

1) It has a robust mathematical form, however inferred.

2) I can't think of a more reasonable explanation.

3) It's not likely to affect anyone I care about in the near future.