Like Sitting Ducks — Time to Bag Some Open Clusters!

A neighbor of mine actually has a duck boat like the one pictured above and he often parks it outside his sleek, world-class Frank Lloyd Wright house — what a contrast! My youthful hunting and field sports teaching days in Indiana are long gone, but I often think about “bagging” deep sky objects the way I would have strategized my shooting in the past.

And this time of year the best “game” for us may well be the galactic or open clusters. They’re not maybe not as sexy as a diaphanous nebula or a swirling galaxies, but I assure you they’re not less interesting astronomically. One of my favorite two clusters flock high in the sky now, they’re the Double Cluster — h and chi Persei. Here’s a 10 minute exposure I recently took with my Vespera, which can catch both with one shot.

What’s also great about them is that there are so many and they are easy to image because of their high contrast. They only take minutes to get a successful shot. The newer smart scopes from Vaonis, ZWO, and Dwarflab are also perfect for open clusters because of their wide fields of view. These clusters will even hold up well even when the Moon tries to photo bomb us.

I sometimes find myself with an opportunity to take a picture with clouds threatening too. A scope that can be ready to go in ten minutes and make even a few nice exposures in half an hour can make an otherwise challenging night worth the trouble. An added bonus is that there is really little pressure to image process these things, either. What you see is what you get. They’re high contrast, don’t even really require a light pollution filter, and there’s just few benefits to tweaking them in image processing programs afterwards.

Finally, there are lots of them — almost 1/3 of the Messier Catalog is populated by open clusters, and there are many more to pick from…. some much better than the Messier ones, like my favorite, Carline’s Rose or the White Rose Cluster (NGC7789) in Cassiopeia.

Open Cluster “Talking Points”

Part of my motivation with the Smartstars blog is to fill in the gaps in our understanding of what we’re looking at. There are a few objects that are just astonishing — like M31 and M42 — and almost need no introductions. But, picture yourself at a large party and some of the guests are fascinating to talk with, yet they aren’t so conspicuous as the celebrities that are present. One way to arm oneself is with conversation topics. So, here are some general "conversation starters" when you are looking at open clusters.

1. Cluster Stats

Well over 1,000 open clusters have been identified in our Milky Way galaxy. Nearly all lie in or near the disk of our galaxy. They’re part of what’s called the disk population of stars or Pop I’s. Because of the absorption of starlight by interstellar dust in the galactic plane we can’t see most in visible light, but the total number is estimated to be around 10^5.

The Messier Catalog lists most of the brightest and conspicuous open clusters. By my count there are about 29 in the Catalog. The reason I say 29 is because some are identified with nebulae as well — like M16. Of course all open clusters started their life in molecular clouds that eventually became “torched” by some of the extreme UV emitting O and B stars in the cluster.

I did a reckoning of the Messier Catalog and threw in h and Chi Persei because the are such a fun twin cluster. Here’s what the averages come out to be.

The typical cluster …

… can boast easily 200 stars sibling stars all born in one go.

… is around 3,500 light years away. That’s about 13% of the way to Milky Way’s center.

… Measures 36 light years in diameter. Even by our measure they are generational.

2. Cluster Time Scales

So they’re born in molecular clouds and emerge as brilliant swarms of siblings. How old are they and how long will they live? Well, h and Chi Persei are just out of the nest at about 14 million years old, but the cluster at the heart of the Orion Nebula is only 3 million years old — it’s definitely still in the nest. One of my favorites is M67, which is quite old and could be more than ten times older at 5 billion years — close to our Solar System’s age.

Clusters are dated primarily by looking at the Herzsprung-Russell (HR) diagram of the stars. Since we assume they are a homogenous group born at the same time the only parameter that is different in these diagrams is the masses of the stars and their interior composition. The most massive stars use up their hydrogen the fastest, and indicate the age by noting the Main Sequence turn off point where they head to the upper right and power themselves on Helium. If you’ve taken astronomy 101, you’ve most likely seen this type of diagram. This one was from a historic paper by Harold Johnson and Allan Sandage from way back in 1955.

Those ages are interesting, but I also like to imagine how long the cluster stars take to circulate or swarm among themselves. The proper term for that is the “dynamical” age and can be gotten by taking the density of the cluster, multiplying it by the Newton’s constant of universal gravitation, then extract the square root and invert.

(If I ever wrote another book about astronomy, I’d probably call it “The Idler Astronomer.” In my mind life is too short, so I like easy to get answers that are at least an order of magnitude right. )

Here’s what I get with the Messier Catalog when I assume the average stars in a cluster are one solar mass and that they are uniformly distributed:

> The spunkiest cluster is M44, the Beehive or Praesepe, which readjusts itself on a timescale of just 13.5 million years.

> The pokiest (~least dense) is M46, which takes 144 million years, more than ten times longer than M44, to swarm once. That’s because it’s a full hundred times less dense than M44.

The important thing to note here is that these dynamical ages are still well short of the 200 to 250 million years they will take to migrate in their orbits around the galaxy once. So, picture them like a flock of ducks or geese shifting and re-shifting several times on their migratory route.

3. They Evaporate Starting with their Smallest Stars

To continue the bird flock analogy, imagine that occasionally the cluster ejects a star out of the group that goes its own way. I’m sure even in regular flocks some of the more individualistic ducks pick their own winter ponds. I’m not sure what ornithologists call that behavior, but stars that leave the flock do so in a process astronomers call cluster evaporation.

Evaporation happens because the clusters become uniform in how they parse out energy between the stars. It’s because they are “relaxed” gravitationally. I won’t explain that process, but it basically means they’ve rearranged themselves so many times that no one star has a kinetic energy (KE) advantage over another. They are, in a sense, isothermal.

But since kinetic energy depends on mass times speed squared, for two stars of the same KE and different masses, the least massive will be moving the fastest. For clusters like ours this means that often the littlest ones can reach escape velocity and head off into inter galactic space on their own. Maybe it’s more like birds kicking their little siblings out of the nest???

4. Clusters Outline and Weigh the Galaxy

Eventually all star clusters, except the most tightly bound, such as globular clusters, will evaporate or be pulled apart by tidal forces. The latter is because the force of gravity changes rapidly with distance and can stretch our clusters when a massive molecular cloud flys by. First, the molecular cloud will accelerate the cluster towards it, but secondly the cluster will experience a net stretching force because the gravity is slightly different on the front side versus the backside of the cluster.

We can use the threat of tidal disruption to actually use the existence and stability of open clusters in general to “weigh” or determine the mass of our galaxy. I Iike this calculation because it shows how powerful a little physics knowledge can be. Basically, one is equating the tidal force due to the mass of the galaxy to the gravitational force binding the cluster together. There are some assumptions and a slight factor or 3 because the cluster is in an orbit … But it works out like this.

(Mass of Galaxy/Mass of Cluster) ~ 1/3 x (Distance to Galactic Center/Radius of Cluster)^3

For an 18 light year radius cluster made up of 200 solar masses all we need is the 26,000 light years to the center of the Milky Way. This then basically sets a limit on the mass of the galaxy to be: 200 billion solar masses. This isn’t bad at all. (It’s actually the mass interior to the cluster’s orbit, but it’s close enough for me.)

So, what finally disrupts open clusters? It’s most likely the continually passing molecular clouds and evaporation. All flocks, like families eventually disperse I’m afraid.

A Couple of My Favorite Clusters

I know that my readers can all look up facts and details online about clusters and other celestial objects, so I’m usually going to focus on some of the idiosyncratic thoughts I have. Here are some of the strange things I think about with some of my faves.

1. M45 Pleiades -- The In our Face Cluster

This one is, of course so popular because you don’t even need a telescope and it always looks great. Here’s a quick snap I got of it recently with my new Seestar.

But there is one main idea I think about when I look at the Pleiades. It’s that the brightest stars are really young and hot. They’re B-stars and are around 13,500 degrees Kelvin. The peak of the light is then 215 nm, which is right inside the ultraviolet C band. This is the most dangerous type of UV for our skin cells. Luckily on Earth these wavelengths, though emitted in lesser relative amounts by our Sun, are absorbed by our ozone layer.

A number of years ago some UFO believers felt we were contacted by aliens from the Pleiades, I believe. This is one of the more absurd stories I’ve heard because the bright stars of the Pleiades would be a seriously unattractive neighborhoods for life.

The Pleiades also harbors one of the most fascinating stars known — Pleione. It’s the seventh brightest star in the cluster and just barely visible to the unaided eye. But it’s a zinger. At its equator, it’s rotating at 290 km/second! Our Sun only rotates at only 2.2 km/second — not even 1% the speed of Pleione. Pleione is lucky to be holding itself together.

Otto Struve, one of my astronomical heroes, discovered this rotational fact. He also showed that much slower rotating stars like our Sun had probably spun off their angular momentum while “making” planets. Pleione is not only too hot to handle, but probably not host to any planets, either — another good reason no to trust aliens who say they’re from the Pleiades.

(BTW… I am a hardy intelligent-ET skeptic. Maybe I’ll blog someday why the extinct paleontologist George Gaylord Simpson’s logic makes a lot of sense to me and is still relevant.)

Anyways, while I’m being skeptical and on the topic of the Pleiades, I might mention the Nebra Sky Disc (Himmelsscheibe von Nebra), ~1700 BCE. Some think it depicts the Pleiades along with the Sun and Moon. Hmmmm. Even if it does feature the seven sisters, why it’s on the disc would be anyone’s guess too. No one really knows without any written records. I’ll stick with science.

2. M67 -- The Oldest

I love M67 too. This image I’ve posted below is a relatively short exposure, but shows the richness of the cluster. It’s one of the oldest known — it’s a real survivor and about the age of our Solar System. This one is particularly useful because we can use it to gauge the magnitude limit of our images. I believe I wrote about that in an earlier blog.

Because of its age some have suggested that the Sun had been born with this cluster. But, alas, the chemical composition is not right. There is one candidate known for a solar sibling and it’s HD 162826. It’s in the constellation Hercules and has a magnitude 6.55. Should be easy to see for our smart scopes. You might add it to your observing list at: (17h 51m 14.02244s)

Open Clusters on the Horizon

Orion is starting to make its yearly evening appearance in our seasonal skies. The Trapezium, at the center, and environs in the Orion Molecular Cloud, will be tomorrow’s open cluster. It’s going to be a monster, with 2,800 stars within only 20 light years. Better catch the nebula within the next 100,000 years before a supernova takes it out.

I did a couple presentations recently on results from the JWST. This infrared telescope is ideal for seeing into the Orion Molecular Cloud region and imaging many more stars than we can see in visible light. Just above is a picture from JWST’s NIRCam (Near Infrared Camera). What’s amazing about this work is that the science team has discovered many forming solar systems and free floating planets here too. I wonder how many of these planets called JUMBOs are circulating in the clusters we observe and how many have been ejected into the galaxy?

Finally, while we’re thinking of clusters like flocks, just above is one of the laggards. It will not be seen until Sagattarius and Scutum are cruising across the sky next summer. It’s M11, the Wild Duck Cluster. It’s wonderful because of its size and richness and more than worth the effort to image. I’ll finish with one I got with my eVscope. I’m hoping to go after it with one of my other smart scopes. The duck “V” s apparently opening at about 5:30 in this image radiating from the bright star in the center.

Happy Holidays and Happy Hunting!

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Further Reading and References

This article about star clusters is nice: https://sites.astro.caltech.edu/~george/ay20/eaa-starclus.pdf

Johnson and Sandage article is in ApJ 121, pg. 616, 1955.

JWST Trapezium Image: Credit: NASA, ESA, CSA / Science leads and image processing: M. McCaughrean, S. Pearson.