Starship Asterisk*

Star Clusters M35 and NGC 2158

Explanation: Clusters of stars can be near or far, young or old, diffuse or compact. The featured image shows two quite contrasting open star clusters in the same field. M35, on the lower left, is relatively nearby at 2800 <a href= "http://starchild.gsfc.nasa.gov/docs/Sta ... questions/ question19.html">light years</a> distant, relatively young at 150 million years old, and relatively diffuse, with about 2500 stars spread out over a volume 30 light years across. Bright blue stars frequently distinguish younger open clusters like M35. Contrastingly, NGC 2158, on the upper right, is four times more distant than M35, over 10 times older, and much more compact. NGC 2158's bright blue stars have self-destructed, leaving cluster light to be dominated by older and yellower stars. In general, open star clusters are found in the plane of our Milky Way Galaxy, and contain anywhere from 100 to 10,000 stars -- all of which formed at nearly the same time. Both open clusters M35 and NGC 2158 can be found together with a small telescope toward the constellation of the Twins (Gemini).

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What is the most dense star cluster known?

What would be the average distance between stars at its most dense area?

How would that night sky appear to us relative to here?

and what is the dynamics here? How fast do the rogue planets and dwarf stars evaporate out of the cluster and larger stars and black holes sink to the center?

can GAIA see the motions and the differences between parallaxes within the M35's stars?
can we 3d map some of the M35's stars by radial Doppler's velocities?

VictorBorun wrote: ↑Mon May 10, 2021 4:58 am and what is the dynamics here? How fast do the rogue planets and dwarf stars evaporate out of the cluster and larger stars and black holes sink to the center?

Open clusters dissipate over millions of years. Globular clusters evaporate over tens to hundreds of billions of years.

NGC 2158 is a rich cluster, which is why it is still around after 2 billion years. But another important reason why it has not evaporated is the fact that it is located on the outskirts of the disk of the Milky Way.

Phys.org. wrote:

NGC 2158 (other designations: OCL 468, Lund 206 and Melotte 40) is an old, low-metallicity OC located at the Milky Way's periphery. The cluster is about 14,700 light years away, and its age is estimated to be some 2 billion years.

By comparison, M35 is nearby, young and somewhat sparse.

Ann

In the image by CFHT, Coelum, MegaCam, J.-C. Cuillandre (CFHT) & G. A. Anselmi (Coelum), the bright stars of M35 have large bright blue halos. The much fainter stars of NGC 2158 lack halos.

I think it is beautiful, but the presence and absence of halos makes the comparison between M35 and NGC 2158 a bit more difficult.

Ann

zendae1 wrote: ↑Mon May 10, 2021 4:44 am What is the most dense star cluster known?

What would be the average distance between stars at its most dense area?

How would that night sky appear to us relative to here?

https://en.wikipedia.org/wiki/Nightfall ... and_novel)

Isaac Asimov's story about that is one of the best short science fictions stories ever.

Beautiful is today's APOD!

Star Clusters M35 and NGC 2158

Both open clusters M35 and NGC 2158 can be found together with a small telescope toward the constellation of the Twins (Gemini).

Ala the cat & dog

zendae1 wrote: ↑Mon May 10, 2021 4:44 am What is the most dense star cluster known?

What would be the average distance between stars at its most dense area?

How would that night sky appear to us relative to here?

The most densest star cluster known is the Arches Cluster. NASA estimates that in a radius of 4 light years there are around 100,000 stars. source: https://www.nasa.gov/image-feature/godd ... -milky-way

NGC 2158 looks more like a globular cluster than an open cluster. Or is it due to the perspective of the APOD photo?

orin stepanek wrote: ↑Mon May 10, 2021 11:29 am Beautiful is today's APOD!

Click to view full size image

Star Clusters M35 and NGC 2158

Both open clusters M35 and NGC 2158 can be found together with a small telescope toward the constellation of the Twins (Gemini).

Ala the cat & dog

Click to view full size image

Thank you for always checking all the links in the caption and finding all the cutest pictures for us, Orin! I love the cat and the dog picture!

Ann

snuggs28 wrote: ↑Mon May 10, 2021 3:17 pm NGC 2158 looks more like a globular cluster than an open cluster. Or is it due to the perspective of the APOD photo?

Good question. The thing is that you can't always tell at a glance if a cluster is an open cluster or a globular.


As you can see, both open and globular clusters can be either rich of sparse. Admittedly, open clusters are usually puny indeed compared with globulars. But globular clusters can evaporate too. The globular clusters of M53 and NGC 5053 are strikingly different, even though they are not only close together on the sky, but also located at similar distances from us, so that they might indeed form a physical pair.


A color-magnitude diagram of a cluster plots the brightness of its stars versus the color of its stars. As stars age, they run out of fuel (hydrogen) in their cores, and when that happens, they swell and become brighter and redder in color. The more massive a star is, the bluer it is when it still has hydrogen in its core, but the sooner it runs ut of hydrogen in its core and turns into a red giant.

A color-magnitude diagram shows how evolved (i.e., old) a cluster is. The older it is, the shorter is its main sequence, because more and more of its stars have run out of hydrogen in their cores and turned into red giants. In the picture at left, you can see diagrams of four globular clusters. All have short main sequences (the thick black part at bottom). All have long red giant branches (the long branches curving away to the right from the main sequence at bottom and reaching up). All have horizontal branches curving left, although some globulars lack them.

The NGC 2158 color-magnitude diagram looks different. The main sequence is longer and reaches higher, because a much larger portion of the stars of NGC 2158 are still fusing hydrogen in their cores. The red giant branch does not reach all that high above the main sequence. And NGC 2158 appears to have a rather large population of blue stragglers, stars that are located above the main sequence, to the left. The blue stragglers are stars that have gained extra mass through some means, probably by interaction with other stars.

So NGC 2158 is not a globular cluster. It is a (very) rich open cluster. It is old as open clusters go (2 billion years old), but it is much younger than most globulars (some 12 billion years old).

Ann

I am surprised that AI struggles to darken the spikes and halo around a bright point.
Are they that unpredictable?

I understand there is a problem to mark low quality parts of the picture where the spikes and halo have been.
Well, can they be just pixelized with twice as large pixels?

I understand there is a problem to present the brightness and color of the bright point after its spikes and halo have been removed.
Well, can there be just a disk of right color and diameter, much smaller than the spikes and halo have been?

heehaw wrote: ↑Mon May 10, 2021 8:54 am

zendae1 wrote: ↑Mon May 10, 2021 4:44 am What is the most dense star cluster known?

What would be the average distance between stars at its most dense area?

How would that night sky appear to us relative to here?

https://en.wikipedia.org/wiki/Nightfall ... and_novel)

Isaac Asimov's story about that is one of the best short science fictions stories ever.

Thank you; I'll have a read.

De58te wrote: ↑Mon May 10, 2021 12:26 pm

zendae1 wrote: ↑Mon May 10, 2021 4:44 am What is the most dense star cluster known?

What would be the average distance between stars at its most dense area?

How would that night sky appear to us relative to here?

The most densest star cluster known is the Arches Cluster. NASA estimates that in a radius of 4 light years there are around 100,000 stars. source: https://www.nasa.gov/image-feature/godd ... -milky-way

Thanks for your reply. It must be quite a sight from the center.

VictorBorun wrote: ↑Mon May 10, 2021 5:03 am can GAIA see the motions and the differences between parallaxes within the M35's stars?
can we 3d map some of the M35's stars by radial Doppler's velocities?

The simple answer is no and no, at least in a meaningful way as I think you're asking. Stars cannot be located confidently within the ~30-ly optical diameter, and the doppler measurements lack the accuracy (and quantity). Mapping M35's stars based only on these parameters with any meaningful accuracy most likely can't be done.

At the simplest level, I've analyzed M35's best candidate stars having a parallax error ≤ 2%, and within an 0.6-degree circular FoV. There are 480 stars (out of thousands) that meet this criteria that range in magnitude from 7 to 16. The results are:
→ The typical 1-σ uncertainty in stellar distance is 55 light years (about 80% larger than the cluster's diameter), with a single minimum = 31 ly
→ Only 15 have Doppler / radial velocity measurements, and of those, only 7 to 10 have velocity uncertainties <50%. That means that it is possible to identify receding stars from approaching stars with some confidence, but that's about it.
Note: I used Gaia EDR3 data archive, but the radial velocity data were copied directly from Gaia DR2. I don't know what future updates to expect to the radial velocity fields. I'm doubtful of any significant improvements wrt M35.

Details
Star-count histogram showing significantly broader distance range than it's optical size ~30 ly.  
3D spatial distribution plot showing about 90% of the stars fall within 2840 ly ± 150 ly
The distance uncertainty of each star contributes significantly to the broad cluster distribution. However, the cluster is not a sphere, and possibly it is elongated towards Earth.

alter-ego wrote: ↑Mon May 17, 2021 2:50 am → Only 15 have Doppler / radial velocity measurements, and of those, only 7 to 10 have velocity uncertainties <50%. That means that it is possible to identify receding stars from approaching stars with some confidence, but that's about it.

wow, 15 is a crowd.
If they all rotate around the same axis then we can see a straight line-divided halves: one of approaching stars, the other of receding?

VictorBorun wrote: ↑Mon May 17, 2021 4:43 am

alter-ego wrote: ↑Mon May 17, 2021 2:50 am → Only 15 have Doppler / radial velocity measurements, and of those, only 7 to 10 have velocity uncertainties <50%. That means that it is possible to identify receding stars from approaching stars with some confidence, but that's about it.

wow, 15 is a crowd.
If they all rotate around the same axis then we can see a straight line-divided halves: one of approaching stars, the other of receding?

I don't think stars in open clusters rotate around any common axis. They are loosely bound collections that will retain the characteristics of the molecular cloud in which they formed. That means they are traveling as a group with other stars at their radial location in the galaxy, and are orbiting each other in fairly random inclinations.