APOD: Globular Cluster M15 from Hubble (2013 Nov 19)

[APOD Robot]

Globular Cluster M15 from Hubble

Explanation: Stars, like bees, swarm around the center of bright globular cluster M15. This ball of over 100,000 stars is a relic from the early years of our Galaxy, and continues to orbit the Milky Way's center. M15, one of about 170 globular clusters remaining, is noted for being easily visible with only binoculars, having at its center one of the densest concentrations of stars known, and containing a high abundance of variable stars and pulsars. Released only recently, this sharp image taken by the Earth-orbiting Hubble Space Telescope spans about 120 light years. It shows the dramatic increase in density of stars toward the cluster's center. M15 lies about 35,000 light years away toward the constellation of the Winged Horse (Pegasus).

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[starsurfer]

The other interesting thing about M15 is that it is one of four globular clusters in the Milky Way known to contain a planetary nebula. The one in M15 is called Pease 1 and would have been visible if exposures taken through an OIII filter had been included (that is if it lies in the field of view of this particular image).

[alter-ego]

The other interesting thing about M15 is that it is one of four globular clusters in the Milky Way known to contain a planetary nebula. The one in M15 is called Pease 1 and would have been visible if exposures taken through an OIII filter had been included (that is if it lies in the field of view of this particular image).

The nebula is visible in this field. The 2004 APOD (one of the links) shows it more clearly as a nebula, and I've overlapped the old APOD with the new. (The hover utility works funny now )

Assuming the larger image is 120 ly x 120 ly, the earlier Hubble M15 photo (inset) is ~68 ly x 47 ly

Click to view full size image 1 or image 2

[Nitpicker]

So, alter-ego, in today's APOD, Pease1, or Kuestner 648 appears as the bright blue smudge with diffraction spike, roughly midway between the centre and left edge of image? I am willing to take your word for it, as until you posted, I could not even find North. As Dave Bowman once said: "My God, it's full of stars!"

[Markus Schwarz]

Why are the diffraction spikes of the red stars different in size than the ones from the blue stars? Are there separate cameras mounted differently, or is it due to the different wavelengths?

[Juwie]

Seeing images like this I always wonder if there mustn't happen some star collisions. Or do they happen in a globular cluster - and if yes - how often?

[starsurfer]

The other interesting thing about M15 is that it is one of four globular clusters in the Milky Way known to contain a planetary nebula. The one in M15 is called Pease 1 and would have been visible if exposures taken through an OIII filter had been included (that is if it lies in the field of view of this particular image).

The nebula is visible in this field. The 2004 APOD (one of the links) shows it more clearly as a nebula, and I've overlapped the old APOD with the new. (The hover utility works funny now )

Assuming the larger image is 120 ly x 120 ly, the earlier Hubble M15 photo (inset) is ~68 ly x 47 ly

Click to view full size image 1 or image 2

I'm not sure if it is definitely visible in the new one. The details for the new image say it contains no narrowband exposures.

@Juwie: yes sometimes collisions can happen and might be the source of some x-ray binary systems found in some globular clusters.

[Boomer12k]

Gosh!!!! Where did I put my SUNGLASSES!?!?!?!?

WOW that is BRIGHT!!!!

Awesome picture of a Spectacular Globular Cluster....really grabs you!!!

:---[===] *

[Cousin Ricky]

Seeing images like this I always wonder if there mustn't happen some star collisions. Or do they happen in a globular cluster - and if yes - how often?

Look up “blue straggler.”

[rstevenson]

Seeing images like this I always wonder if there mustn't happen some star collisions. Or do they happen in a globular cluster - and if yes - how often?

The Wikipedia article Stellar collision says...

A stellar collision is the coming together of two stars, which through the force of gravity merge into one larger unit. Astronomers predict that events of this type occur in the globular clusters of our galaxy about once every 10,000 years.

There is some indication that "blue stragglers", an odd type of star for a globular cluster, are the result of collisions.

Rob

[neufer]

[url=http://www.universetoday.com/31592/messier-14/][size=150]Messier 14 by Tammy Plotner on Universe Today[/size][/url][b][color=#0000FF] (Note: M15 is just a modified M14 with a heavier barrel & stock, a hinged buttplate, a selector switch for fully automatic fire, and a bipod.)[/color][/b]

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That makes for:

[list]9 APODs on M15
1 APOD on M14[/list]

<<Messier 14 (also known as M14 or NGC 6402) is a globular cluster in the constellation Ophiuchus. It was discovered by Charles Messier in 1764. At a distance of about 30,000 light-years, M14 contains several hundreds of thousands of stars. At an apparent magnitude +7.6 it can be easily observed with binoculars. Medium-sized telescopes will show some hint of the individual stars of which the brightest is of magnitude +14. The total luminosity of M14 is in the order of 400,000 times that of the Sun corresponding to an absolute magnitude of -9.12. The shape of the cluster is decidedly elongated. M14 is about 100 light-years across. A respectable total of 70 variable stars is known in M14, many of the W Virginis variety common in globular clusters. In 1938, a nova appeared although this was not discovered until photographic plates from that time were studied in 1964. It is estimated that the nova reached a maximum brightness of magnitude +9.2, over five times brighter than the brightest 'normal' star in the cluster.>>

[tomatoherd]

I hate to beat a dead horse ( i've asked this before) but these clusters still seem to defy gravity to me. Since they have a spherical symmetry instead of an axial symmetry (like galaxies), they must not have an organiized rotation to amount to anything, and therefore lack much in way of centrifugal forces to counteract gravity. So I don't know, after billions of years especially, why they don't collapse together. They appear much denser than the universe, which until relatively recent times, was thought could eventually collapse under its own mass attractions.
They certainly MUST NOT contain any dark matter (which I don't believe in), or this conundrum would be even more flagrant. Pardon my use of lay-persons physics, etc. Thanks.

[Chris Peterson]

I hate to beat a dead horse ( i've asked this before) but these clusters still seem to defy gravity to me. Since they have a spherical symmetry instead of an axial symmetry (like galaxies), they must not have an organiized rotation to amount to anything, and therefore lack much in way of centrifugal forces to counteract gravity. So I don't know, after billions of years especially, why they don't collapse together. They appear much denser than the universe, which until relatively recent times, was thought could eventually collapse under its own mass attractions.
They certainly MUST NOT contain any dark matter (which I don't believe in), or this conundrum would be even more flagrant. Pardon my use of lay-persons physics, etc. Thanks.

There's really no such thing as "centrifugal force"; certainly, there is no such force that counteracts gravity. An orbiting body is always falling freely towards the mass it is orbiting around- it simply has a tangential velocity as well, so it never reaches the center. To a single body, it makes no difference whether other bodies around it are in similar orbits (as with a spiral galaxy) or in completely different orbits (as with an elliptical galaxy or a globular cluster). In every case, each body is simply orbiting the center of mass of the larger structure (ignoring perturbations).

There is no mechanism for collapse. Quite the opposite, as angular momentum is transferred during near misses, the cluster tends to break up, slowly losing its stars to intergalactic space.

Not "believing" in dark matter appears to be an irrational or religious viewpoint. The evidence strongly favors its existence; it isn't a matter of "belief". Dark matter does play a role in our understanding of the mechanics of globular clusters.

[neufer]

I hate to beat a dead horse ( i've asked this before) but these clusters still seem to defy gravity to me. Since they have a spherical symmetry instead of an axial symmetry (like galaxies), they must not have an organiized rotation to amount to anything, and therefore lack much in way of centrifugal forces to counteract gravity. So I don't know, after billions of years especially, why they don't collapse together. They appear much denser than the universe, which until relatively recent times, was thought could eventually collapse under its own mass attractions.
They certainly MUST NOT contain any dark matter (which I don't believe in), or this conundrum would be even more flagrant. Pardon my use of lay-persons physics, etc. Thanks.

Globular Clusters don't need organized rotation to avoid collapse
just so long as the density of stars is sparse enough to avoid collisions.

The Oort cloud avoids collapse just fine with disorganized rotation.

(The density of stars in globular clusters is, however, high enough that dark matter is not an issue.)

[FloridaMike]

"My God—it's full of stars!"

[Chris Peterson]

Why are the diffraction spikes of the red stars different in size than the ones from the blue stars? Are there separate cameras mounted differently, or is it due to the different wavelengths?

An interesting question. I think it is mainly just because more light in those bands reached the detector, both because the sources are brighter at longer wavelengths, and because the sensors are more responsive there. The orange stars correspond with visible and IR bands. Also, most of the signal we see there was captured with the ACS camera, not the WFC. Different sensors and optics (although the same primary optics, of course).

[dwainwilder]

I hate to beat a dead horse ( i've asked this before) but these clusters still seem to defy gravity to me. Since they have a spherical symmetry instead of an axial symmetry (like galaxies), they must not have an organiized rotation to amount to anything....

I've wondered something along a parallel line of thought: how could so many stars accumulate without any angular momentum? If there is angular momentum among the members of the cluster, would there not be an "aggregate effective center of rotation," to perhaps coin a term? And in that case, would the individual stars not begin to behave as a galaxy does, effectively generating whirlpools in the galactic maelstrom?

[geckzilla]

(The hover utility works funny now )

My bad, I just fixed it. I was messing with linked image display last week and neglected to realize my changes would affect the hover CSS.

[Chris Peterson]

I've wondered something along a parallel line of thought: how could so many stars accumulate without any angular momentum? If there is angular momentum among the members of the cluster, would there not be an "aggregate effective center of rotation," to perhaps coin a term? And in that case, would the individual stars not begin to behave as a galaxy does, effectively generating whirlpools in the galactic maelstrom?

Globular clusters do have a net angular momentum. They do demonstrate global rotation. There is evidence in some of differential rotation. I'm not sure what you mean by "behave like a galaxy". Do you mean develop a disc structure? That only applies to spiral galaxies (elliptical galaxies have dynamics quite similar to globular clusters in many respects). Spiral galaxies formed their disc structure when they were young, and there was more interaction among the stars, gas, and dust. Aggregates with net angular momentum will only form a disc if there is a mechanism to efficiently transfer angular momentum between components (dense gas and dust, for instance). No such mechanism exists in a globular cluster; gravitational perturbation is more efficient at keeping orbits random than any angular momentum transfer from near misses is in selecting for a common orbital axis.

[Chris Peterson]

(The hover utility works funny now :?: )

My bad, I just fixed it. I was messing with linked image display last week and neglected to realize my changes would affect the hover CSS.

Maybe trying to get the system to support hover images with attachments that don't need to be elsewhere in the post, as well? That's an annoyance.

[geckzilla]

Why are the diffraction spikes of the red stars different in size than the ones from the blue stars? Are there separate cameras mounted differently, or is it due to the different wavelengths?

An interesting question. I think it is mainly just because more light in those bands reached the detector, both because the sources are brighter at longer wavelengths, and because the sensors are more responsive there. The orange stars correspond with visible and IR bands. Also, most of the signal we see there was captured with the ACS camera, not the WFC. Different sensors and optics (although the same primary optics, of course).

After looking at a lot of HLA pictures this has been my experience. F814W tends to light up like a christmas (solstice?) tree and anything in the 400 range tends to be noisier and darker despite longer exposure times.

The blue channel of any given image can basically look like crap compared to the others and the overall image suffers minimally. I have a much harder time picking out cosmic rays and bad pixels when they are blue and often end up viewing it in black and white because of that. It's interesting because it's every bit as bright as the green or red channel if you go by the numbers but the blue is a lot more difficult to see. Green is definitely the easiest and red is somewhere in between.

Maybe trying to get the system to support hover images with attachments that don't need to be elsewhere in the post, as well? That's an annoyance.

Yeah, I think I can do something about that.

[tomatoherd]

Chris :
thanks, but yes, I know there is no such thing as centrifugal force. The earth is falling towards the sun in equal portion to its tangential inertia, etc etc. That's why i apologized for "layman's terms".
Nevertheless, they seem too stable. maybe it's a matter of gravity's extreme weakness compared to the other fundamental forces, I dunno.
No, my disbelief in dark matter is not 'religious'. But I think time and history will confirm that present day astrophysicists are just using dark matter as a gigantic fudge-factor for current deficiencies in the standard model. I nevertheless have a huge respect for these scientists.
But I am curious: do these same fudging a-physicists figure these clusters have or lack dark matter?

[Chris Peterson]

Nevertheless, they seem too stable. maybe it's a matter of gravity's extreme weakness compared to the other fundamental forces, I dunno.

Gravity is the only force that plays a significant role in globular cluster dynamics. Indeed, there are nice numerical simulations of globular clusters that align nicely with actual observation.

No, my disbelief in dark matter is not 'religious'. But I think time and history will confirm that present day astrophysicists are just using dark matter as a gigantic fudge-factor for current deficiencies in the standard model.

Sounds irrational to me. I'm not sure what the standard model has to do with this. Dark matter is solidly observed, and fits into our Big Bang cosmology. If there's any weakness in the standard model, it's the current uncertainty about where to place the particles that we call dark matter.

But I am curious: do these same fudging a-physicists figure these clusters have or lack dark matter?

There are lots of papers out there about how dark matter and dark matter halos interact with globular clusters. The biggest issue isn't with the presence of absence of dark matter, however, but with a general lack of understanding of how globular clusters form at all.

[tomatoherd]

Thanks, Chris. I do have lots of respect for you and everyone here. I have learned a lot from your replies over the years.
I may BE irrational. And I don't aim to be a troll. But to me, dark matter and energy is a case of an emperor without clothes, whom everyone genuflects to. If it is 90% of the universe, send me a piece of it.
And for anyone to imply that the standard model, current cosmology, and gravitation theory are neatly unified and things are 'almost figured out already' would be disingenuous.

[Chris Peterson]

Thanks, Chris. I do have lots of respect for you and everyone here. I have learned a lot from your replies over the years.
I may BE irrational. And I don't aim to be a troll. But to me, dark matter and energy is a case of an emperor without clothes, whom everyone genuflects to. If it is 90% of the universe, send me a piece of it.
And for anyone to imply that the standard model, current cosmology, and gravitation theory are neatly unified and things are 'almost figured out already' would be disingenuous.

For me, hearing "dark matter" and "dark energy" in the same sentence sends up big warning flags, since the two have nothing in common, and need to be addressed completely differently. Their existence is inferred from entirely different observations, their explanations depend on entirely different theory.

I believe it is a reasonable position (which may prove wrong, but probably will not) to say that the standard model (which applies to particles), the standard cosmological model, and gravitation are all mostly figured out (the latter, particularly). That doesn't mean that very important elements aren't still missing, but it does mean that the theories are substantially correct, and are much more likely to evolve slightly than to undergo any sort of radical revision.