Starship Asterisk*

Palomar 12

Explanation: Palomar 12 was not born here. The stars of the globular cluster, first identified in the Palomar Sky Survey, are younger than those in other globular star clusters that roam the halo of our Milky Way Galaxy. Palomar 12's position in our galaxy and measured motion suggest its home was once the Sagittarius Dwarf Elliptical Galaxy, a small satellite of the Milky Way. Disrupted by gravitational tides during close encounters the satellite galaxy has lost its stars to the larger Milky Way. Now part of the Milky Way's halo, the tidal capture of Palomar 12 likely took place some 1.7 billion years ago. Seen behind spiky foreground stars in the sharp Hubble image, Palomar 12 spans nearly 60 light-years. It lies about 60,000 light-years away, toward the constellation Capricornus.

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Cool. I'm not surprised that Palomar 12 was attracted to the Milky Way over the Sagittarius Dwarf Elliptical Galaxy, even if just for the name. Since it has been 1.7 billion years, I say we grant Palomar 12 galactic citizenship.

This is an interesting cluster. Simbad Astronomical Database does not have a lot of information about it.

Looking at the picture, I have to wonder if this cluster has any blue horizontal stars at all. These evolved blue stars are always extremely metal-poor, and that is why we often find them in globular clusters, which are typically metal-poor. A good example is the great globular cluster in Hercules, M13, which is full of blue stars. But the second brightest globular in the Milky Way, 47 Tuc, lacks blue horizontal stars altogether. The reason is metallicity. M13 has a metallicity index, [Fe/H], of -1.65, which means that this is a typical metal-poor cluster. But the metallicity index of 47 Tuc is -0.71, still metal-poor compared with the Sun, but not metal-poor enough to permit the existence of blue horizontal stars.

Palomar 12, interestingly, has a metallicity index somewhere between M13 and 47 Tuc: -1.14. Is that metal-poor enough for blue stars? It is very hard to judge from the picture if any of the brighter stars in the cluster are blue, due to the filters used for the image. And surely most of the brighter stars belong to Palomar 12 itself? Only the very bright-looking star at bottom center looks like a foreground star to me.

Ann

The Palomar globulars are extreme observing challenges for visual observers. I think some of them have been discovered by many different astronomers including George Abell and I think Halton Arp discovered one. I would love to see Palomar 2 in Auriga imaged by Adam Block!

/snip

Ann wrote:And surely most of the brighter stars belong to Palomar 12 itself? Only the very bright-looking star at bottom center looks like a foreground star to me.

This is what I was thinking. Part of the description says:

Seen behind spiky foreground stars in the sharp Hubble image

So, each of the approximately 25 stars showing 'spiky-ness' are not part of Palomar 12?

Regardless, it's a very pretty image!

Palomar 12 is a most open looking globular. Does it really have enough stars now to be considered a true globular? I mean, look at it, you can see right through it to distant galaxies in the background, even at it's very center.

I don't doubt that this cluster once was a globular, but over time it must have lost a great many stars. Just as it has been tidally striped from its original galaxy, many stars must have been striped from it. It now is an open cluster heading toward complete dispersal.

Bruce

Would someone enlighten me as to how, over a span of a few years, there can be "measured motion"? I've been told before (here) that even close stars like those of Orion, would not have any apparent movt relative to one another in a single human's lifetime. So how can there be anything measured at 60,000 ly ?? Especially when the 'background' is not fixed, but both background and foreground are moving, and would need subtracted or taken into account?

Awesome, and the galaxies in the background. A nice, clear shot.

Oh, the gravity of it all....

:---[===] *

BDanielMayfield wrote:Palomar 12 is a most open looking globular. Does it really have enough stars now to be considered a true globular? I mean, look at it, you can see right through it to distant galaxies in the background, even at it's very center.

I don't doubt that this cluster once was a globular, but over time it must have lost a great many stars. Just as it has been tidally striped from its original galaxy, many stars must have been striped from it. It now is an open cluster heading toward complete dispersal.

It appears to have thousands of members, and has enough of a gravitational field to maintain a spherical structure with a strong radial density gradient. Its stars were not lost to evaporation (as occurs with all globulars) but to a (presumably) singular tidal disruption event. I expect what remains has the stars tightly bound and any further loss will be only by evaporation, which with a low density structure like this is very slow. This isn't an open cluster, or even close to being one, and should survive for billions of years yet, barring another close encounter with a galaxy.

tomatoherd wrote:Would someone enlighten me as to how, over a span of a few years, there can be "measured motion"? I've been told before (here) that even close stars like those of Orion, would not have any apparent movt relative to one another in a single human's lifetime. So how can there be anything measured at 60,000 ly ?? Especially when the 'background' is not fixed, but both background and foreground are moving, and would need subtracted or taken into account?

What is measured is probably the stellar velocities, by looking at Doppler shifts. This is relatively straightforward in globular clusters, because while Doppler measurements can only directly measure the axial velocity component, the structure of the cluster and the position of the star with respect to the center makes it possible to infer the radial component as well. Basically, spectroscopic measurements allow the orbits of individual stars in the cluster to be determined.

tomatoherd wrote:Would someone enlighten me as to how, over a span of a few years, there can be "measured motion"? I've been told before (here) that even close stars like those of Orion, would not have any apparent movt relative to one another in a single human's lifetime. So how can there be anything measured at 60,000 ly ?? Especially when the 'background' is not fixed, but both background and foreground are moving, and would need subtracted or taken into account?

Bright stars do not show noticeable motion over a few years, but "measurable" is a different thing. The paper linked under "tidal capture" derived the mean apparent motion of stars in Pal 12 with respect to distant background galaxies (so distant that they give a solid reference frame) to get its annual motion, averaged over all stars brighter than magnitude 22. (I thought that might have relied on Hubble data, but ground-based images spanning four decades did the job).

Can somebody furnish us the estimated age of this globular? The write-up mentions it's atypically young, but then doesn't give its age. I believe most Milky Way globulars are over 10 billion years and hence metal lean. The description said it was captured 1.7 billion years ago, so it's at least that old. There's a reason for my wondering (wandering?). I'm curious whether those stars are young enough to have high metal content, rocky planets, and (just maybe) life.

Amazing depth in this shot - foreground stars, distant globular cluster, background galaxies - a quite amazing sight!

Mulutunnel wrote:Can somebody furnish us the estimated age of this globular? The write-up mentions it's atypically young, but then doesn't give its age.

Rosenberg et al. gives the age as 68% ± 10% of the age of typical globulars (using M5 and 47 Tuc as references). I think there's a general reluctance to define an absolute age for any globulars (other than very old). Geisler et al. determine a fairly high metallicity (as globulars go).

I'm curious whether those stars are young enough to have high metal content, rocky planets, and (just maybe) life.

The problem with planets around stars in globulars is that they are unlikely to have stable orbits over long periods, given the fairly frequent gravitational perturbations that occur. Most planetary systems that might have formed were probably scattered quite early in the lives of the globulars.

what does " star seed " look like ?

ta152h0 wrote:what does " star seed " look like ?

In what context?

well, is Palomar 12 a collection of existing stars that just happened to be translating to occupy that particular volume or these stars were created in that particular volume ?

ta152h0 wrote:well, is Palomar 12 a collection of existing stars that just happened to be translating to occupy that particular volume or these stars were created in that particular volume ?

Globular cluster formation is poorly understood. But almost certainly, the stars are closely related, forming from the same pool of material over a short period of time. So your second suggestion is probably closer to the truth.

Chris Peterson wrote:

Mulutunnel wrote:Can somebody furnish us the estimated age of this globular? The write-up mentions it's atypically young, but then doesn't give its age.

Rosenberg et al. gives the age as 68% ± 10% of the age of typical globulars (using M5 and 47 Tuc as references). I think there's a general reluctance to define an absolute age for any globulars (other than very old). Geisler et al. determine a fairly high metallicity (as globulars go).

I'm curious whether those stars are young enough to have high metal content, rocky planets, and (just maybe) life.

The problem with planets around stars in globulars is that they are unlikely to have stable orbits over long periods, given the fairly frequent gravitational perturbations that occur. Most planetary systems that might have formed were probably scattered quite early in the lives of the globulars.

Thank you, Chris. I hadn't thought of that. It's amazing what kind of unexpected insights pop out from supercomputer long-range simulations of star systems. I'm guessing that's how they determined this.

This is a computer simulation of stellar motions and the general evolution of a globular cluster. Note that a lot of bright stars suddenly just pop out of existence.
Bear in mind that the passage of a dim red dwarf and its brown dwarf companion 0.8 light-years from the Earth some 70,000 years ago caused headlines when the close passage was discovered just recently. If the Earth had been situated inside a globular cluster, such close encounters - let alone much closer ones - would have happened very often.

Ann

Also I think there are some other globular clusters in the Milky Way that might have been captured from the Sagittarius Dwarf Galaxy.

BDanielMayfield wrote:Palomar 12 is a most open looking globular. Does it really have enough stars now to be considered a true globular? I mean, look at it, you can see right through it to distant galaxies in the background, even at it's very center.

I don't doubt that this cluster once was a globular, but over time it must have lost a great many stars. Just as it has been tidally striped from its original galaxy, many stars must have been striped from it. It now is an open cluster heading toward complete dispersal.

Bruce

Palomar 12 is very ancient and must have made multiple orbits around the Milky Way. I think as time has gone on, stars have been tidally stripped from it and added to the overall body of the Milky Way until a cluster completely "evaporates". A lesser known example of this is the southern globular cluster E3.

I wonder when the image was taken, as that is not given in the ESA/Hubble release brought up through the "Palomar 12 was not born here" link. Knowing the date an image was taken adds interest, but that information seems to be very rarely given in ESA/Hubble releases which is in sharp contrast to images released by such as the Hubble Heritage Project that often have a 'Fast Facts' section that gives such information. I wonder why the ESA seems so reluctant to give the date an image was taken? I have tried a search of the Hubble Heritage Project website but I did not find the image.

The fastest way to assess Hubble data is almost always the Hubble Legacy Archive. Entering Pal 12 as the target shows two series of images, 26 July 2003 and 21 May 2006. Both used the Advanced Camera for Surveys.

NGC3314 wrote:The fastest way to assess Hubble data is almost always the Hubble Legacy Archive. Entering Pal 12 as the target shows two series of images, 26 July 2003 and 21 May 2006. Both used the Advanced Camera for Surveys.

Thanks for that NGC3314.

I did not try the Hubble Legacy Archive (HLA) because when I have occasionally done for other objects I found there always seemed far too many results that made it difficult to find information. However, having just followed your recommendation I found that there were only 8 results for Palomar 12 and the images taken on May 21 2006 do seem to fit the ESA/Hubble released image. If that is the right date then it does seem odd that the ESA/Hubble have only just released an image taken almost 9 nine years back