Starship Asterisk*

The Antennae Galaxies in Collision

Explanation: Two galaxies are squaring off in Corvus and here are the latest pictures. 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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This galactic pair is a golden oldie, but it is nice to see it - them - again!

As a color commentator, I must say something about the colors of this particular version of the well-known Hubble image. The Antennae galaxies are two colliding galaxies, similar in size, but in other respects they were quite different before they collided. One of them, NGC 4038, was rich in gas, possibly similar to a galaxy like NGC 3184, with a yellow center and a gas-rich disk. The other one member of the Antennae, NGC 4039, was decidedly gas-poor before it collided. It may well have been a lenticular galaxy, possibly similar to NGC 524, but smaller.

As the gas-poor and the gas-rich galaxies collided, fascinating things happened to the gas. Parts of it formed a brilliant starforming "arc", turning NGC 4038 into "half a Cartwheel galaxy". Much of the rest of the gas ended up "at the impact site", forming a tremendous, dark dust cloud bursting with new stars. In today's APOD, this truly gigantic star forming region is the enormous central brown dust area full of yellow, dark orange, red and magenta light.

Just possibly the only area in NGC 4039 that had a gas supply of its own is the central area. Today's APOD shows a small blue arc "hanging down" from the center of NGC 4039. Fascinatingly, the blue arc seems to be connected to the nuclear area by a thin bright "stalk".

NGC 4038, by contrast, is full of bright new stars. It has many extremely bright emission nebulae (likely enhanced in visibility in this picture), and young blue stars are streaming out from these star forming areas in a tangential direction. Note one such "stellar fountain" at about 9 o'clock, another one at about 11 o'clock and a third one at about 12 o'clock.

One of the links in the caption of today's APOD is this one, which shows the full disk of NGC 4039. Note how large, "undisturbed" and yellow it is. Before its collision, this must indeed have been a galaxy with a yellow, non-starforming disk. Note, nevertheless, the dark dust features seen in silhouette against the yellow disk. They are somewhat similar to the brown dust features in the otherwise all yellow galaxy NGC 1316, except that the dust features in NGC 1316 are much larger and darker. NGC 1316 is a post-collisional galaxy, one where all the starforming material has been spent and only the ashes, the dark dust, remain.

Ann

Antennae ???? I think it looks more like Africa. We have celestial bodies named after North America, California, etc. why not rename this one to Africa ?

AWESOME!!!

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PhilT wrote:Antennae ???? I think it looks more like Africa. We have celestial bodies named after North America, California, etc. why not rename this one to Africa ?

Because this is only a CLOSE UP....you need to back out further....do a websearch for more images of it....

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It's always said that even in a galactic collision the stars stay apart. But what about the gas and dust that are there too?
Star forming areas will develop, with all the radiation that will engender. Will there be a higher rate of nova formation, or other events that would raise the radiation level in the vicinity?
For those reasons, would a colliding galaxy be less favourable for life than one in a quieter neighborhood?

I understand that the Sun is presently in a low density area of the Milky way, and will enter one with a higher density soon (galactic soon - hundreds of thousands of years). What can we expect from that?

JOhn

APOD Robot wrote: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.

When playing golf, one's opponents often claim that trees (eucalypt, say) are 90% air. This is probably true, but when trying to hit a golf ball through a gum tree, it is the free cross-sectional area that is relevant, and this is much smaller. Much like astronomy, golf is very humbling.

Get out your red and blue glasses. Oh wait...

JohnD wrote:It's always said that even in a galactic collision the stars stay apart. But what about the gas and dust that are there too?
Star forming areas will develop, with all the radiation that will engender. Will there be a higher rate of nova formation, or other events that would raise the radiation level in the vicinity?
For those reasons, would a colliding galaxy be less favourable for life than one in a quieter neighborhood?

Indeed, galactic collisions result in changed conditions for billions of star systems. In regions of star formation you have high radiation. Stars pass close enough together to perturb planetary systems.

I understand that the Sun is presently in a low density area of the Milky way, and will enter one with a higher density soon (galactic soon - hundreds of thousands of years). What can we expect from that?

We're far enough from the galactic center that even the higher density regions of arms are still pretty low density. So we shouldn't expect much, but maybe a more interesting night sky. The fact that our planetary system is intact and substantially unchanged after more than a dozen complete orbits argues that densities at this orbital radius are not typically high enough to pose problems.

Thank you, Chris!

In which case, doesn't the Drake Equation need another expression, perhaps Ffp for "Fraction fatally perturbed"? I couldn't put a value on it.

John

Wow you can see more of the faint outer parts surrounding the main central part! Also this image reminds me of a certain song:

JohnD wrote:In which case, doesn't the Drake Equation need another expression, perhaps Ffp for "Fraction fatally perturbed"? I couldn't put a value on it.

I think that falls under the n_e term. Stars that have planets in regions of the galaxy where perturbations are too great to allow for stable orbits over hundreds of millions or billions of years are probably not good candidates for life. Or maybe the f_l term is better, since any life that forms on such planets would have little opportunity to evolve significant complexity.

I keep my red/blue glasses next to this laptop, just in case the APOD guys decide to ujpload another anaglyph. Off topic- recent studies indicated, and reported one black hole spun at half the speed of light. I thought spins were measured in radians/second or more commonly, revolutions per unit of time .

ta152h0 wrote:I keep my red/blue glasses next to this laptop, just in case the APOD guys decide to ujpload another anaglyph. Off topic- recent studies indicated, and reported one black hole spun at half the speed of light. I thought spins were measured in radians/second or more commonly, revolutions per unit of time .

I assume the reference was to the equatorial surface speed at the event horizon. Just like the Earth rotates at two pi radians per day, but has an equatorial surface speed of 1670 km/h.

Nitpicker wrote:

APOD Robot wrote: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.

When playing golf, one's opponents often claim that trees (eucalypt, say) are 90% air. This is probably true, but when trying to hit a golf ball through a gum tree, it is the free cross-sectional area that is relevant, and this is much smaller. Much like astronomy, golf is very humbling.

If the space between the leaves is 10 million times their diameters (i.e, the tree is 99.999999999999% air), and the golf ball is no bigger than the leaves, then the golf ball will probably sail right through.

ta152h0 wrote:
I keep my red/blue glasses next to this laptop, just in case the APOD guys decide to ujpload another anaglyph.

• But his brain was so loaded it nearly ujploaded
  The poor girl would shake with alarm;
  He'd ne'er leave the girl with the strawberry curls
  And the band played on.

ta152h0 wrote:
Off topic- recent studies indicated, and reported one black hole spun at half the speed of light. I thought spins were measured in radians/second or more commonly, revolutions per unit of time .

All particles have either integer spin or half-integer spin
(in units of the reduced Planck constant ħ)
but occasionally the equatorial speed is more relevant.

Wait - the previous versions of this object appear to be reversed left to right. Which is correct?

Think of the Worlds...possibly being created....

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booky1@earthlink.net wrote:Wait - the previous versions of this object appear to be reversed left to right. Which is correct?

It is flipped the wrong way in this APOD.

neufer wrote:All particles have either integer spin or half-integer spin
(in units of the reduced Planck constant ħ)
but occasionally the equatorial speed is more relevant.

You do like to add to confusion sometimes, don't you?

"Spin" is not the same as "rotation". In quantum mechanics (which this question did not address), spin is a measure of quantized angular momentum. It does not mean that a particle has a physical rotation, with some "equatorial" speed. Black holes demonstrate physical rotation, and their angular momentum is not quantized.

Chris Peterson wrote:

neufer wrote:
All particles have either integer spin or half-integer spin
(in units of the reduced Planck constant ħ)
but occasionally the equatorial speed is more relevant.

You do like to add to confusion sometimes, don't you?

• You're expecting a confusion confession

Chris Peterson wrote:
"Spin" is not the same as "rotation". In quantum mechanics (which this question did not address), spin is a measure of quantized angular momentum. It does not mean that a particle has a physical rotation, with some "equatorial" speed. Black holes demonstrate physical rotation, and their angular momentum is not quantized.

I'm pretty sure that all angular momentum is quantized.

http://en.wikipedia.org/wiki/Azimuthal_quantum_number wrote:
<<The azimuthal quantum number is a quantum number for an atomic orbital that determines its orbital angular momentum and describes the shape of the orbital. The azimuthal quantum number is the second of a set of quantum numbers which describe the unique quantum state of an electron (the others being the principal quantum number, following spectroscopic notation, the magnetic quantum number, and the spin quantum number). It is also known as the orbital angular momentum quantum number, orbital quantum number or second quantum number, and is symbolized as ℓ

The letters after the f sub-shell just follow f in alphabetical order except those already used. One mnemonic to remember the sequence S. P. D. F. G. H. ... is:

"Sober Physicists Don't Find Giraffes Hiding In Kitchens Like My Nephew".

A few other mnemonics are:

[list]Smart People Don't Fail,
Silly People Drive Fast,
Silly Professors Dance Funny,
Scott Picks Dead Flowers,
Some Poor Dumb Fool! etc.>>[/list]

neufer wrote:I'm pretty sure that all angular momentum is quantized.

Maybe. There are theories that suggest there could be a minimum angular momentum value. But that's far from certain, and certainly not verified by observation.

A bit confusing at first (comparing to older APODs of the Antennae Galaxies) but didn't take long to figure out. What an amazing image!

one more, considering this is Sunday. Are black holes still considered singularities acting as giant magnets ?

ta152h0 wrote:one more, considering this is Sunday. Are black holes still considered singularities acting as giant magnets ?

Giant magnets? They were never considered to act like magnets.

Mathematically, the center of a black hole is a singularity. But what actually happens beyond the event horizon, and especially very near the center, isn't certain.