APOD: The Antennae Galaxies in Collision (2010 Jul 18)

[APOD Robot]

The Antennae Galaxies in Collision

Explanation: Two galaxies are squaring off in Corvus and here are the latest pictures. But when two galaxies collide, the stars that compose them usually do not. That's because galaxies are mostly empty space and, however bright, stars only take up only a small amount of that space. During the slow, hundred million year collision, one galaxy can still rip the other apart gravitationally, and dust and gas common to both galaxies does collide. In this clash of the titans, dark dust pillars mark massive molecular clouds are being compressed during the galactic encounter, causing the rapid birth of millions of stars, some of which are gravitationally bound together in massive star clusters.

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[Ann]

Well, this is a golden oldie, but it is a lovely picture all the same and it looks great here.

In another thread, Chris said:

We may well merge with Andromeda, which is about the same size as the Milky Way, or slightly larger, in a few billion years. That's the soonest this could happen, and by that time the Earth will not be supporting human life. So we won't have the opportunity to enjoy the interesting new night sky.

Interesting new night sky, indeed! The night sky if you live in either NGC 4038 (the topmost galaxy in today's APOD) or in NGC 4039 (the bottom one) the night sky would look fabulous indeed. Unless, of course, you were actually sitting inside the thick dusty "bridge" joining the two galaxies, where you would be lucky to see much at all. On the other hand, there is so much star formation going on inside this huge dusty patch that if you were inside, you would be lucky not to be fried by all the intense ultraviolet light and all the energetic jets zapping the neighbourhood in there.

If you did have a clear view, however, how amazing it would be! Look at all the super-clusters in NGC 4039! Wowzers! And how interesting it is to compare NGC 4039 and 4039. Clearly most of the star formation goes on inside NGC 4038, which must have been a gas-rich spiral before it had its close encounter with 4039. The latter galaxy, by contrast, was probably a gas-poor disk galaxy before it ran into 4038. It may even have been a S0-type disk galaxy with no star formation at all. Compare the color and texture of the disks of 4038 and 4039 beyond the raging star formation and dramatic dust. The disk of 4038 is mottled and sprinkled with young blue star clusters, but the disk of 4039 is almost perfectly smooth and beige in color, because it is made up of uniformly distributed old yellow stars.

And to think that this fantastic cosmic fireworks will eventually produce... a featureless yellow ball of stars, an elliptical galaxy. How boring. Will there be anything to look at in the sky of a planet inside an elliptical galaxy, anything but a uniform sprinkling of stars in the sky? Will there be any recognizable patterns of constellations? I doubt it. There will be no Big Dippers inside elliptical galaxies, or that is what I think anyway. But there will be stars like Arcturus, bright-looking red giants, which may liven up the sky a bit. Also, of course, you would be able to see the your own galaxy in the sky, which for a planet inside an elliptical galaxy would look like a gradual circular or elliptical brightening in one direction of the sky, culminating in a brilliant core. In a dust-free environment you should be able to see the core of your own galaxy very well.

Ann

[owlice]

I'm pretty sure this is a new processing of the data.

If the night sky were different, people would still find patterns and visual relationships between stars, so there would still be asterisms and constellations. The heavens don't impose those on us; we create them out of our own need to find patterns and connect the dots.

[orin stepanek]

I'm pretty sure this is a new processing of the data.

If the night sky were different, people would still find patterns and visual relationships between stars, so there would still be asterisms and constellations. The heavens don't impose those on us; we create them out of our own need to find patterns and connect the dots.

After connecting the dots on Orion and the two dippers I have trouble finding the other constellations without a star map. It's hard to watch the sky from my yard as there are so many trees around.

[rstevenson]

And to think that this fantastic cosmic fireworks will eventually produce... a featureless yellow ball of stars, an elliptical galaxy. How boring. Will there be anything to look at in the sky of a planet inside an elliptical galaxy, anything but a uniform sprinkling of stars in the sky? Will there be any recognizable patterns of constellations?

You may be overly pessimistic there, Ann. We don't see nebulae or clusters or other interestingly detailed things with the naked eye -- at least not except under the most favourable viewing conditions, and then only a few of the most obvious. What we identify as constellations is just chance alignment of somewhat brighter or nearer stars. I don't think the night sky inside an elliptical galaxy would be all that different from what we see here. Until, that is, you started looking through a telescope.

Rob

[Chris Peterson]

Interesting new night sky, indeed! The night sky if you live in either NGC 4038 (the topmost galaxy in today's APOD) or in NGC 4039 (the bottom one) the night sky would look fabulous indeed.

I don't think so. There are still dust clouds that block the view in many directions. Nothing would be brighter than we see in our night sky (except maybe for some stars), and like our sky, almost everything except background structure would be fairly close. There would be no color, and little fine structural detail. Basically, I imagine the sky in a colliding galaxy pair to look something like ours, but instead of a linear Milky Way pattern you'd see a much more complex patchwork of gray over the sky. And certainly, with a lot of star forming regions, there could be a lot of young, bright stars.

[Chris Peterson]

If the night sky were different, people would still find patterns and visual relationships between stars, so there would still be asterisms and constellations. The heavens don't impose those on us; we create them out of our own need to find patterns and connect the dots.

Of course, a collision like this is glacially slow in terms of the lifetime of civilizations, or even species. It would take a remarkably stable intelligent species to even recognize that the sky was changing.

[biddie67]

I think (in my non-technical manner) that whatever cognitive entities might still be living there would be thinking "there go the old neighborhoods". So it would be out with the old star fields and in with the new!!

But what might be the consequences if two stars did happen to collide?

Could comet activity increase as gravity tugged in a different direction?

Could a star pass through a center of gravity "net" because of momentum and then get pulled backwards into a point of balance?

[Chris Peterson]

But what might be the consequences if two stars did happen to collide?

Nobody knows for certain, since stars are not completely understood and no star collision has ever been observed. Most likely you'd end up with a new, larger star. Any planets around either system would almost certainly be ejected from the combined system, or end up in radically different orbits.

Could comet activity increase as gravity tugged in a different direction?

This would almost certainly happen. It is likely that this happens in our own system periodically simply from stars passing fairly nearby (around a light-year or so). Bodies in the Oort cloud are perturbed and some are directed towards the inner system. There may be occasional periods of bombardment from this mechanism.

Could a star pass through a center of gravity "net" because of momentum and then get pulled backwards into a point of balance?

No. If the star has a relative velocity less than the escape velocity of the other, they will simply end up in orbit around each other.

[JoelN]

It would have been nice if the author of the blurb accompanying today's APOD had actually NAMED the two galaxies!!! A decent grasp of English grammar would help as well.

Thank you, Ann, above, for providing fascinating and much-needed follow-up information.

[JosephD]

It may be true most stars and planets do not collide but the black holes in each Galaxie must wreak havoc as they pass by and through and close to the stars and planets . There must be much destruction and rearanging of every thing .thereby creating a new order to everything.
some thing will pass and others a new genesis will begin. simply because of Gravity and the two black holes passing through the Galaxies will destroy a third of all matter in each galaxie while other planets and there systems suffer catastrophy.
Far more distruction is caused then any can realize.

Joseph

[rstevenson]

... . simply because of Gravity and the two black holes passing through the Galaxies will destroy a third of all matter in each galaxie while other planets and there systems suffer catastrophy.

"Matter can not be created nor destroyed." (Ignoring nuclear reactions of course. But that's not what you were referring to, I think.)

No such disruption as you describe would occur. There are vast distances between stars, and as Chris has said -- and as you can find out yourself with a very little on-line research -- the galaxies will pass through each other with just gravitic interaction. Disturbance on a galactic scale, yes. We see that in pictures of other galaxies engaged in such a "collision." But nothing like what you've described.

Far more distruction is caused then any can realize.

Any but you?

Rob

[neufer]

if you live in either NGC 4038 (the topmost galaxy in today's APOD) or in NGC 4039 (the bottom one)...

While NGC 4039 is indeed the skinny galaxy in today's APOD:


http://antwrp.gsfc.nasa.gov/apod/ap971022.html
http://antwrp.gsfc.nasa.gov/apod/ap970506.html
http://antwrp.gsfc.nasa.gov/apod/ap970602.html

it should be noted that the Wikipedia article mistakenly has NGC 4038 as the skinny galaxy.

[neufer]

Click to view full size image

<<NGC 4038/4039 are perhaps the most famous pair of nearby interacting galaxies known for their symmetrically placed tidal tails resembling insect antennae. N-body simulations of the formation of the Antennae's tidal tails allow us to set limits on the total amount of dark matter in the interacting galaxies. Shown below are two simulations using different amounts of dark matter in the galaxy models. The low mass model only contains 4 times as much mass in dark matter as stars while the high mass model contains 30 times as much dark matter. The low mass model easily produces long tidal tails like the real pair while the high mass model fails, producing only short, stubby features. Simulations using intermediate masses show that long tails only arise when the mass of dark matter is less than 10 times the mass of the stars. >>

[Ann]

If the night sky were different, people would still find patterns and visual relationships between stars, so there would still be asterisms and constellations. The heavens don't impose those on us; we create them out of our own need to find patterns and connect the dots.

Yes, of course people would still se patterns in the sky, even if they lived in an elliptical galaxy. But there would be nothing like the Pleiades in there, probably nothing like the Big Dipper in there, and definitely nothing like Orion in there.

And speaking about Orion, Chris objected to my suggestion that the night sky would look fabulous in NGC 4038 or 4039. He said:

I don't think so. There are still dust clouds that block the view in many directions. Nothing would be brighter than we see in our night sky (except maybe for some stars), and like our sky, almost everything except background structure would be fairly close. There would be no color, and little fine structural detail. Basically, I imagine the sky in a colliding galaxy pair to look something like ours

Yes, there would be dust clouds blocking the view in many directions, but not in all directions. But nothing would be brighter than we see in our night sky? Do we really have anything as bright in the Milky Way as the super star clusters of NGC 4038? If the brightest clusters of NGC 4038 are brighter than anything our galaxy can match, then it goes without saying that it would be possible to see brighter things inside NGC 4038 than we see inside the Milky Way. Also there are very many clusters of very bright stars in NGC 4038 - it's like having ten Orions in the sky instead of one! All right, no, not really, because the really huge clusters are far away from one another, but there are many parts of NGC 4038 that would offer a view of at least one moderately nearby super cluster that would beat Orion many times over.

And there would be no color in the sky inside NGC 4038? Sure there would be! There is color in the sky that we can see in the Milky Way, and there would be no less color in NGC 4038.

So I personally believe that there is a good chance that the night sky for a planet inside NGC 4038 would be quite spectacular, although it is also very possible that the planet would be wrapped in a thick cosmic fog of dust!

Ann

[Chris Peterson]

Do we really have anything as bright in the Milky Way as the super star clusters of NGC 4038?

Yes, we do. The Milky Way itself is about the same brightness as these clusters, as are most of the globular clusters we see around our own galaxy. From those galaxies, the local globulars will look pretty much the same as our own. If you were very close to them, they would look big, but no brighter than more distant globulars.

And there would be no color in the sky inside NGC 4038? Sure there would be! There is color in the sky that we can see in the Milky Way, and there would be no less color in NGC 4038.

I've never encountered anybody who reports seeing color in the Milky Way. It is far too dim to trigger photopic vision. Sure you can see color if you make a long exposure, just as you can in other galaxies. But no galaxy, including our own, shows any color (other than gray, of course) to the human eye.

[Ann]

Do we really have anything as bright in the Milky Way as the super star clusters of NGC 4038?

Yes, we do. The Milky Way itself is about the same brightness as these clusters, as are most of the globular clusters we see around our own galaxy. From those galaxies, the local globulars will look pretty much the same as our own. If you were very close to them, they would look big, but no brighter than more distant globulars.

And there would be no color in the sky inside NGC 4038? Sure there would be! There is color in the sky that we can see in the Milky Way, and there would be no less color in NGC 4038.

I've never encountered anybody who reports seeing color in the Milky Way. It is far too dim to trigger photopic vision. Sure you can see color if you make a long exposure, just as you can in other galaxies. But no galaxy, including our own, shows any color (other than gray, of course) to the human eye.

I'm amazed at what you are saying, Chris.

I don't quite get what you mean by saying that the Milky Way itself is about the same brightness as these clusters. Obviously our galaxy itself is brighter than any one of the clusters in NGC 4038. Are you saying that any one of the clusters of NGC 4038 would look as faint and diffuse to anyone inside NGC 4038 as the band of the Milky Way looks to us? Are you saying that the brightest clusters of NGC 4038 would be comparable in apparent brightness to the globular clusters that we see in our own galaxy, so that they would at best be faint-looking objects at about fourth magnitude?

I'm not buying that. All our own globular clusters are made of old, relatively faint stars, and they are all very far from us here on Earth. The super clusters of NGC 4038 are not as rich as our globular clusters, but they are made of young stars, some of which are bound to be O-type stars or B-type supergiants, which are very bright and conspicuous. And there may well be some M-type supergiants in there to provide color contrast. If an observer in NGC 4038 was much closer to one of the young super clusters in that galaxy than we are to any of our globular clusters, surely that cluster would look bright and awesome. What if an observer was as close to any of that super clusters as we are to the stars in Orion's Belt? The observer would have a magnificent view indeed, provided, of course, that there was not a lot of dust between the observer and the cluster to obscure the view.

Perhaps you are saying that the question is moot, since no human-like observer could survive being as close to one of the young super clusters of NGC 4038 as we are to the stars of Orion's Belt?

Ann

[neufer]

Do we really have anything as bright in the Milky Way as the super star clusters of NGC 4038?

Yes, we do. The Milky Way itself is about the same brightness as these clusters, as are most of the globular clusters we see around our own galaxy. From those galaxies, the local globulars will look pretty much the same as our own. If you were very close to them, they would look big, but no brighter than more distant globulars.

I don't quite get what you mean by saying that the Milky Way itself is about the same brightness as these clusters. Obviously our galaxy itself is brighter than any one of the clusters in NGC 4038. Are you saying that any one of the clusters of NGC 4038 would look as faint and diffuse to anyone inside NGC 4038 as the band of the Milky Way looks to us? Are you saying that the brightest clusters of NGC 4038 would be comparable in apparent brightness to the globular clusters that we see in our own galaxy, so that they would at best be faint-looking objects at about fourth magnitude?

I'm not buying that. All our own globular clusters are made of old, relatively faint stars, and they are all very far from us here on Earth. The super clusters of NGC 4038 are not as rich as our globular clusters, but they are made of young stars, some of which are bound to be O-type stars or B-type supergiants, which are very bright and conspicuous. And there may well be some M-type supergiants in there to provide color contrast. If an observer in NGC 4038 was much closer to one of the young super clusters in that galaxy than we are to any of our globular clusters, surely that cluster would look bright and awesome. What if an observer was as close to any of that super clusters as we are to the stars in Orion's Belt? The observer would have a magnificent view indeed, provided, of course, that there was not a lot of dust between the observer and the cluster to obscure the view.

This question is easily resolved by comparing the apparent sizes and
magnitudes of the Antennae Galaxies with say the Andromeda Galaxy:

Code: Select all

From Wikipedia:

_____________         Andromeda         NGC 4039        NGC 4038
---------------------------------------------------------------------
apparent size         190′ × 60′       3′.1 × 1′.6    5′.2 × 3′.1
apparent magnitude       3.44             11.1            11.2

NGC 4039 is indeed about twice as bright as Andromeda!

However, NGC 4038 is only a little more than half as bright
as Andromeda on average due to the dust clouds.

Note: Andromeda is about 3 times brighter than the Magellanic Clouds.

[Chris Peterson]

I don't quite get what you mean by saying that the Milky Way itself is about the same brightness as these clusters. Obviously our galaxy itself is brighter than any one of the clusters in NGC 4038. Are you saying that any one of the clusters of NGC 4038 would look as faint and diffuse to anyone inside NGC 4038 as the band of the Milky Way looks to us? Are you saying that the brightest clusters of NGC 4038 would be comparable in apparent brightness to the globular clusters that we see in our own galaxy, so that they would at best be faint-looking objects at about fourth magnitude?

Yes to all. The magnitude of extended objects is generally expressed as a brightness integrated over the area. That tells you how much light an object produces, but it doesn't tell you (directly) how bright it appears to the eye. In reality, things made up of tightly clustered stars tend to have similar surface brightnesses. This includes globular clusters and the cores of galaxies. Visually, these are all bright enough to be seen with the naked eye, but none are bright enough to stimulate color vision. Our own Milky Way, the cores of other galaxies, and globular clusters all look similar to our eyes. Sure, there is some variation because of stellar types, but it isn't great- probably not more than a few magnitudes at most. And distance makes no difference- getting closer doesn't make extended objects brighter, just bigger.

So if you were in one of these colliding galaxies, you'd still just see a sky covered with mottled gray marking regions of high stellar density, very similar to the way we see the Milky Way.

Of course, if you were close enough to a cluster to actually resolve individual stars with the eye, it would be impressive. But at that point you aren't seeing an extended object anymore, but just a lot of stars.

[ems57fcva]

I will repeat my major concern about this photo, namely the large dusty bridge connecting the two galaxies: It is not predicted by any of the current galaxy collision models! They predict only tidal tails. Yet the main tidal tails are heading out to the left and the top of this image, and are almost entirely out of the frame. At the same time, the secondary tidal tails can be seen. For the larger galaxy (NGC 4038) it heads off of the right side of and curves upward. For the smaller galaxy (NGC 4309) it heads downwards and curves to the left.

My suspiscion is that material is being pulled off of both galaxies and into the bridge between them. If this process is strong enough, it will create a new galactic center in what is now the bridge. I suspect that the bridge will then become a bar and that the distorted remnants of the two original galaxies will become spiral arms. The result will be something like NGC1300 (See http://antwrp.gsfc.nasa.gov/apod/ap060827.html).

On the other hand, if the process is not strong enough, it will only leave behind a slender bridge which will get stretch to and past the breaking point over time. A recent APOD (http://apod.nasa.gov/apod/ap100702.html) shows such a leftover bridge.

I am happy to note that my view of how this system may evolve is speculation. However, I repeat that the brdige itself is not predicted by the current models. IMO, that is a serious issue that needs to be dealt with if we are to truly understand how galaxies form, interact, and evolve.

Edward Schaefer

[bystander]

I will repeat my major concern about this photo, namely the large dusty bridge connecting the two galaxies: It is not predicted by any of the current galaxy collision models! ...

... A recent APOD (http://apod.nasa.gov/apod/ap100702.html) shows such a leftover bridge.

Simulations certainly show them. See the discussion for APOD 2010 July 02: http://asterisk.apod.com/vie ... 79#p126279
For ems57fcva's discussion on the Antennae Galaxies (APOD 2010 May 07), see: http://asterisk.apod.com/vie ... =9&t=19319

[Chris Peterson]

I will repeat my major concern about this photo, namely the large dusty bridge connecting the two galaxies: It is not predicted by any of the current galaxy collision models!

That is not strictly true. Collision models don't include dust, they only model the motion of stars. There is nothing in the current models that in any way excludes a dusty bridge, and certainly stellar motion not unlike this is seen in simulations. That said, I am not convinced that a dusty bridge between the two galaxy cores even exists. In the absence of three dimensional information, it is difficult or impossible to determine just where the dust lies in relation to the galactic centers.

[Don Lund]

"Matter can not be created nor destroyed." (Ignoring nuclear reactions of course.)

Matter is not destroyed in nuclear reactions, the same law applies.

[Chris Peterson]

"Matter can not be created nor destroyed." (Ignoring nuclear reactions of course.)

Matter is not destroyed in nuclear reactions, the same law applies.

While "matter" isn't strictly defined, I think most physicists are comfortable with the notion that matter may be destroyed in nuclear reactions. That is, "matter" as it is usually defined refers to atoms or subatomic particles that have mass. While matter is equivalent to energy, it is energy in a very specific form, and it can be converted to energy of a different form, essentially losing its "matter" quality. Energy is always conserved, but that doesn't mean matter can't cease to be matter (which is a pretty good definition of "destroyed").

[Don Lund]

"Matter can not be created nor destroyed." (Ignoring nuclear reactions of course.)

Matter is not destroyed in nuclear reactions, the same law applies.

While "matter" isn't strictly defined, I think most physicists are comfortable with the notion that matter may be destroyed in nuclear reactions. That is, "matter" as it is usually defined refers to atoms or subatomic particles that have mass. While matter is equivalent to energy, it is energy in a very specific form, and it can be converted to energy of a different form, essentially losing its "matter" quality. Energy is always conserved, but that doesn't mean matter can't cease to be matter (which is a pretty good definition of "destroyed").

What's lost or destroyed in nuclear reactions is not what I'd call matter, but rather nucear binding energy. Yes, there is a mass equivalent to this energy via E=mc^2, so I suppose it all boils down to how you look at it. Perhaps it just doesn't "matter"...