Starship Asterisk*

Galaxy Group Hickson 31

Explanation: Will the result of these galactic collisions be one big elliptical galaxy? Quite possibly, but not for another billion years. Pictured above, several of the dwarf galaxies of in the Hickson Compact Group 31 are seen slowly merging. Two of the brighter galaxies are colliding on the far left, while an elongated galaxy above is connected to them by an unusual bridge of stars. Inspection of the above image further indicates that the bright duo trail a rope of stars pointing to the spiral galaxy on the far right. Most assuredly, the pictured galaxies of Hickson Compact Group 31 will pass through and destroy each other, millions of stars will form and explode, and thousands of nebula will form and dissipate before the dust settles and the final galaxy emerges about one billion years from now. The above image is a composite of images taken in infrared light by the Spitzer Space Telescope, ultraviolet light by the GALEX space telescope, and visible light by the Hubble Space Telescope. Hickson Compact Group 31 spans about 150 thousand light years and lies about 150 million light years away toward the constellation of Eridanus.

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It’s difficult for me to grasp a “billion” year galaxy merging process. In my work, I am usually on a much tighter schedule. Anyway, what this APOD has spawned in my head is the following question… Since we measure velocity of space objects based on other space objects… is it possible that the entire universe is also moving or rotating? I just had two cups of coffee, so my imagination is a bit pulsed. And this APOD also makes me wonder how galaxies can collide given the Big Bang Theory of origin? Does it have something to do with Dark Matter? I understand that we are on a collision course with Andromeda in a few billion years.

emc wrote:… is it possible that the entire universe is also moving or rotating?...

Compared to what? If it is, there's no way to know.

emc wrote:… this APOD also makes me wonder how galaxies can collide given the Big Bang Theory of origin? Does it have something to do with Dark Matter?

Although everything is moving away from everything else (more or less) because of the expansion of space, that doesn't mean it's all doing so evenly. There are clumps, and these clumps are interacting gravitationally. And yes, it seems as if DM is contributing to that.

(I hasten to add these are not my opinions. I am regurgitating, while waiting for more knowledgeable people to correct my statements.)

Rob

I have a simple question about today's picture. From the description of this picture, "Most assuredly, the pictured galaxies of Hickson Compact Group 31 will pass through and destroy each other, millions of stars will form and explode, and thousands of nebula will form and dissipate before the dust settles and the final galaxy emerges about one billion years from now." end quote. In many previous explanations about galaxies colliding APOD usually say it is unlikely that stars will collide. My question- What changed here? Why do the stars in the Hickson Group collide and in other galaxies they do not or rarely do?

APODfollower wrote:...In many previous explanations about galaxies colliding APOD usually say it is unlikely that stars will collide. My question- What changed here? Why do the stars in the Hickson Group collide and in other galaxies they do not or rarely do?

It's a bit unclear in its wording, but the explanation does not actually say that stars will collide, only that the galaxies will collide and destroy themselves. I wouldn't have put it that way, since I think of it more as a creative process, where a single larger galaxy will emerge from the combining (not the destruction) of several smaller ones. Yes, it says that stars will explode, but not because they're colliding, rather because they'll be new, large, short-lived stars, the sort that forms first when gas and dust get pushed into new clumps during the coming together of galaxies.

Otto Posterman seems to have a flare for the dramatic.

Rob

Rob's (rsteveson's) explanations are good. Since galaxies are mostly empty space, stars rarely impact each other when galaxies collide. A typical reason why a star would explode is that it was born so massive that it ended up producing nuclear explosions in its interior that tore it apart. For example, see: http://www.phys.ncku.edu.tw/~htsu/humor/msblues.html . - RJN

RJN wrote:Rob's (rsteveson's) explanations are good. Since galaxies are mostly empty space, stars rarely impact each other when galaxies collide. A typical reason why a star would explode is that it was born so massive that it ended up producing nuclear explosions in its interior that tore it apart. For example, see: http://www.phys.ncku.edu.tw/~htsu/humor/msblues.html . - RJN

I loved your story Robert; I promise never to do Hydrogen.

APOD Robot wrote: Galaxy Group Hickson 31

Explanation: Pictured above, several of the dwarf galaxies of in the Hickson Compact Group 31 are seen slowly merging. Two of the brighter galaxies are colliding on the far left, while an elongated galaxy above is connected to them by an unusual bridge of stars. Inspection of the above image further indicates that the bright duo trail a rope of stars pointing to the spiral galaxy on the far right. Hickson Compact Group 31 lies about 150 million light years away toward the constellation of Eridanus.

http://en.wikipedia.org/wiki/Eridanus_%28constellation%29 wrote:
<<The constellation name Eridanus refers to the Po River, the main river of northern Italy. In some star maps, Eridanus is depicted as a river flowing from the waters poured by Aquarius. It is connected to the myth of Phaëton, who took over the reins of his father Helios' sky chariot (i.e., the Sun), but didn't have the strength to control it and so veered wildly in different directions, scorching both earth and heaven. Zeus intervened by striking Phaëton dead with a thunderbolt and casting him to earth. The constellation Eridanus was supposed to be the path Phaëton drove along.>>

RJN wrote:Rob's (rsteveson's) explanations are good. Since galaxies are mostly empty space, stars rarely impact each other when galaxies collide. A typical reason why a star would explode is that it was born so massive that it ended up producing nuclear explosions in its interior that tore it apart. For example, see: http://www.phys.ncku.edu.tw/~htsu/humor/msblues.html . - RJN

Funny story! Humanizing a star strangely reminded me of my wife asking me not to drink too much coffee... Did you write that as a teaching tool or a class paper?... or just for fun?

RJN wrote:... For example, see: http://www.phys.ncku.edu.tw/~htsu/humor/msblues.html . - RJN

That's great!

Rob

emc wrote:Since we measure velocity of space objects based on other space objects… is it possible that the entire universe is also moving or rotating?

Everything in the Universe is moving and rotating with respect to everything else in the Universe. As to whether the Universe itself is moving, that is a difficult and unanswered question (and maybe an unaswerable one as well). The question assumes the existence of some sort of hyperuniverse that our Universe exists in- something that is not well supported by theory, and not at all by observation. However, since rotation creates inertial effects, some people have suggested that it might be possible to determine from within if the Universe has some sort of external rotation. But no such observation has been made.

Chris Peterson wrote:Since rotation creates inertial effects, some people have suggested that it might be possible to determine from within if the Universe has some sort of external rotation. But no such observation has been made.

It would seem to me that Big Bang theories would probably rule out rotation since one would expect centrifugal expansion forces to decrease with time whereas the observed Dark Matter expansion forces clearly increase with time.

neufer wrote:It would seem to me that Big Bang theories would probably rule out rotation since one would expect centrifugal expansion forces to decrease with time whereas the observed Dark Matter expansion forces clearly increase with time.

Do you mean dark energy expansion forces?

In any case, this assumes that the forces would be large enough for us to even detect their effects, and it begs the question of just how we would expect the Universe to respond to them, since the rotation would be in at least four dimensions. So this "force" might be acting on time as well as space. I can't visualize how that would affect what we can actually see.

Chris Peterson wrote:

neufer wrote:It would seem to me that Big Bang theories would probably rule out rotation since one would expect centrifugal expansion forces to decrease with time whereas the observed Dark Matter expansion forces clearly increase with time.

Do you mean dark energy expansion forces?

Indeed!

Chris Peterson wrote:In any case, this assumes that the forces would be large enough for us to even detect their effects, and it begs the question of just how we would expect the Universe to respond to them, since the rotation would be in at least four dimensions. So this "force" might be acting on time as well as space. I can't visualize how that would affect what we can actually see.

Isn't a rotation in time & space simply a Lorentz transformation which you have already pointed out as being probably undetectable?

Astrophysical Journal paper (2009): Is the Universe Rotating?
http://adsabs.harvard.edu/abs/2009ApJ...703..354S

Models of a rotating universe have been studied widely since the work of Gödel, who showed an example that is consistent with general relativity. ... By comparing the second-order Sachs-Wolfe effect due to rotation with the CMBA data, we constrain the angular speed of the rotation to be less than 10-9 rad yr-1 at the last scattering surface.

In other words, "Is the universe rotating" is a theoretically deep and observationally interesting question that can be answered simply just now: "No -- not that we can tell."

neufer wrote:Isn't a rotation in time & space simply a Lorentz transformation which you have already pointed out as being probably undetectable?

Is a rotation in time and space the same as a rotation of time and space? I don't know... it hurts my brain to think about it!

I do know that there are reputable cosmologists asking the question if the Universe is rotating, and trying to treat that question by both theory and observational methods. So at the least, Ed's question is a reasonable one.

RJN wrote:In other words, "Is the universe rotating" is a theoretically deep and observationally interesting question that can be answered simply just now: "No -- not that we can tell."

I'd probably rephrase that to just "not that we can detect with our current technology". I wouldn't want to categorically exclude the possibility that it is rotating but we don't have a well enough developed theory to predict what we should observe, or simply that it is rotating too slowly for us to detect any effects.

Isn't not that we can detect with our current technology just an unnecessarily verbose way of saying not that we can tell?

Chris Peterson wrote:

neufer wrote:Isn't a rotation in time & space simply a Lorentz transformation which you have already pointed out as being probably undetectable?

Is a rotation in time and space the same as a rotation of time and space? I don't know... it hurts my brain to think about it!

I guess you have a point somewhere in there.

A rotation in time and space would really correspond to a constantly changing rotation of time and space;
i.e., an acceleration in a certain direction (which would probably be undetectable as well).

The idea that galaxy mergers result in elliptical galaxies is challenged by this study:

http://www.sciencedaily.com/releases/20 ... 101719.htm

In this case there is so much gas and dust involved a merger would seem to guarantee a spiral.

bystander wrote:Isn't not that we can detect with our current technology just an unnecessarily verbose way of saying not that we can tell?

Well, the key adjustment I made was removing the explicit "no" that Bob placed at the beginning of his comment. Beyond that, "not that we can tell" could easily be interpreted as meaning unknowable, which is why I added the bit about technology. So far, the question isn't known to be unknowable, it is only the answer that is unknown.

Chris Peterson wrote: bystander wrote:Isn't not that we can detect with our current technology just an unnecessarily verbose way of saying not that we can tell?


Well, the key adjustment I made was removing the explicit "no" that Bob placed at the beginning of his comment. Beyond that, "not that we can tell" could easily be interpreted as meaning unknowable, which is why I added the bit about technology. So far, the question isn't known to be unknowable, it is only the answer that is unknown.

Did we not have a previous, on this Spinning universe Topic? If I could not see outside our galaxy, How could i tell it was spinning? Even the dark Matter spins with us. At 100,000 Light years across, its hardly noticeable. If the universe was spinning, And you could somehow see outside, I doubt you would notice any movement in a billion years.

But, Expansion and Angular Momentum, suggest to me, It could be spinning.

While it's true that there is no way to tell; it's fun to speculate. For instance; everything from the atom to the galaxy seems to be made of revolving pinwheels. So why not the universe? Like I said though; its just fin to speculate.

mark swain wrote:If I could not see outside our galaxy, How could i tell it was spinning? Even the dark Matter spins with us. At 100,000 Light years across, its hardly noticeable.

We do not need to see outside the galaxy to tell we are spinning. Since we are in a non-inertial frame, there are forces present that can be measured. It is true that the forces may be too small to detect with current technology, but that's a different issue. Consider that we can easily determine that the Earth is spinning (and how fast it is spinning) from inside an enclosed and isolated lab, using pretty simple equipment. It's harder with the galaxy, since the angular velocity is so small. But certainly not impossible.

If the universe was spinning, And you could somehow see outside, I doubt you would notice any movement in a billion years.

Again, you don't have to see outside (in fact, it is impossible to see outside the Universe). Although the effects of the Universe spinning in some higher dimension are going to be different from ordinary motion in three dimensions inside the Universe, we are still presumably talking about a kind of non-inertial frame that should produce effects that are, at least theoretically, observable. Theory predicts what those effects should be, and observations have failed to show them, which places an upper bound on the rotation rate- assuming the theories used are correct. But there are a lot of conflicting ideas about the geometry of the Universe, so I'd hesitate to say with any certainty whether or not it is rotating.

Chris Peterson wrote:

mark swain wrote:If I could not see outside our galaxy, How could i tell it was spinning? Even the dark Matter spins with us. At 100,000 Light years across, its hardly noticeable.

We do not need to see outside the galaxy to tell we are spinning. Since we are in a non-inertial frame, there are forces present that can be measured. It is true that the forces may be too small to detect with current technology, but that's a different issue. Consider that we can easily determine that the Earth is spinning (and how fast it is spinning) from inside an enclosed and isolated lab, using pretty simple equipment. It's harder with the galaxy, since the angular velocity is so small. But certainly not impossible.

One can even measure the Earth's spin from inside an enclosed & isolated lab with equipment using no moving parts:

A fellow grad physics student built a ring laser gyroscope which measured the rotation of the earth from a basement at the University of Maryland. (There also used to be a Foucault pendulum upstairs but the pendulum bob kept getting stolen by undergraduates.)

Attempts to measure frame dragging caused by nearby rotating celestial objects like the earth & sun could also be used to measure frame dragging of the universe at large.