Maps of star clusters

Question

Inside star clusters, there are regions where stars are within 1 light year of each other. I wish to create a sci-fi setting in one of these regions. To start, I have tried to find maps/charts of these star-dense regions, but I have not had much luck. Are there any resources that I could use that would show this information, including the distances between each star? I have been looking more at open clusters since those seem to be more viable for planets, but information on globular clusters could also be helpful.

Answer 1

Do you really need a spatial map of a real star cluster for your story? Stars tend to orbit and move around so the map would have to include hundreds of thousands of stars, each with a three dimensional spatial co-ordinates as well as velocities and accelerations in 3D as the picture would change over time.

This might not be exactly what you want but it's a start: https://stars.chromeexperiments.com/

Answer 2

It's certainly difficult to find information on the locations of individual stars in globular clusters, given that none are in our stellar neighborhood - which is expected, as they occupy the galactic halo. Messier 4 is believed to be the closest globular cluster to Earth, and while its distance is disputed, a value of ~7200 light-years seems to be commonly used. Its diameter is on the order of 75 light-years, so while determining whether a star along its line of sight is a member of the cluster isn't impossible (and might be made easier by looking at the star's motion), it's certainly not easy.

Open clusters are easier targets, as they lie in the galactic plane and are, in general, closer to Earth. The Hyades cluster is a mere 150 light-years away. This means distance estimates to individual stars might be possible, and errors on those values are likely to be smaller. In many cases, individual stars within the cluster may be studied as objects of interest in their own right. All the same, a complete map of the cluster is not easy to create. In observational astronomy, our instruments are biased towards detecting objects which are more luminous, so many lists of stars in individual clusters might well omit the dimmer objects.

With that in mind, I did some digging and came across the Milky Way Star Clusters Catalog, which contains data from approximately 3200 open clusters in our galaxy. There are three subcatalogs (1, 2, 3); the first contains the bulk of the original catalog (~3000 clusters), and the others contain about 200 additional sources. Each of these three subcatalogs in turn contains a number of files, including bulk properties of each cluster (e.g. mean radial velocity, age, proper motion, etc.). It also contains a directory (example for the original subcatalog) containing one .dat file per cluster detailing the properties of individual stars, including:

• Right ascension and declination (columns 1 and 2)
• Apparent magnitude in the bands B, V, J, H and K (columns 3-7) and measurement errors for the J, H and K (columns 8-10)
• Spectral type and luminosity class, if available (column 19)

These are detailed more in the README file for each subcatalog.

If you know the distance to each star, you can use its coordinates to find its location in space. Unfortunately, the distance to individual stars isn't listed, because it's quite difficult to determine en masse. You can do the calculations yourself if the star has a spectral type - look up the absolute magnitude of that spectral type in a given band (say, the V band) and then use the distance modulus equation: $$m_V-M_V=5\log_{10}(d)-5$$ where $m_V$ and $M_V$ are the apparent and absolute magnitudes in the band, and $d$ is the distance in parsecs to the star. If you know $m_V$ and $M_V$, you can compute $d$.

In theory, this is all well and good, but there are some prominent limitations:

• Magnitude uncertainties may lead to distance uncertainties greater than the typical separation between stars, particularly for clusters that are further away. To mitigate this, I'd recommend looking for clusters that are closer to Earth.
• Very few stars in the catalogs have spectral types (at least for the few datasets I've looked at). That's not surprising; determining a spectral type is also nontrivial. The lists may be more complete for well-studied clusters; again, look for ones closer to Earth.

The Coma Star Cluster, listed under the name Melotte 111, appears to have a couple hundred stars with spectral types in its .dat file, presumably including the brightest members, and it lies a comfortable 280 light-years away. It may be worth looking at as a test case.