Difference in HR diagram clusters

Question

I was wondering what the difference in an HR-diagram would be between an open and globular cluster.

I've trying to compare both by looking up diagrams online, but can't find diagrams with same axes. I appreciate the help!

Answer 1

The main difference is due to the age of the cluster. In our Galaxy the globular clusters are old ($>10^{10}$ years) and as a result the main sequence turn-off is down at something like $0.9 M_{\odot}$. In contrast, the open cluster population is younger (in fact old open clusters are rather rare), this means that their main sequence turn-off is at higher masses, commensurate with their younger ages.

The older ages of globular clusters also means that a bigger fraction of their populations have lived their lives and become white dwarfs, sitting well-below the main sequence in the HR-diagram. For open clusters, the fraction of stars that have become white dwarfs will be age dependent, with almost no white dwarfs if the cluster is younger than about 50-100 million years.

Another difference would just be the number of stars. A typical globular cluster might have $>10^5$ stars, whereas a typical open cluster might have more like $\sim 10^3$. This means the various features in the HR diagram are much better delineated in a globular cluster - the turn-off, a red giant branch and a horizontal branch. The latter may be absent from an open cluster if it not old enough for its $<2 M_\odot$ stars to have evolved to core helium burning.

There are secondary, smaller differences associated with differing compositions. Galactic globular clusters tend to be metal-poor, though there is a dispersion. This means that a main sequence star of a given mass is bluer and more luminous. The net result is that the main sequence sits slightly lower in the H-R diagram. Further differences arise in galactic globular clusters with multiple populations. These produce multiple sequences in the HR-diagram (that are only seen in high quality data), that are associated with differences in the composition of the multiple populations within the globular cluster. Open clusters appear not to have these multiple populations, at least in our own Galaxy.

Answer 2

So, @ProfRob's answer is absolutely amazing and covers the bulk of star-cluster properties that I could think of.

Nevertheless, since the question specifically asked for HR diagrams of open vs. globular clusters, I am sharing some plots that I made myself using Gaia data exactly because, a couple of years ago, I stumbled upon the fact that I couldn't find nice didactic figures for a class I was a teaching assistant for. Here it goes!

Few caveats: (1) the axis are the same, but on slightly different scales. (2) I do not know where are my codes to reproduce all these figures. Sorry. (3) Titles on top of panels and legends are in Portuguese, because, well, I am Brazilian. In any case, you can use Google Translate.

Open clusters:

Pleiades

Jewel Box

Globular clusters:

NGC 3201

Omega Centauri / NGC 5139

About these plots.
Left panels: all stars in the field of the clusters. Do not remember exactly what I adopted at the time, but probably just took from catalogs linked below. Middle: same thing, but color-coded according to the probability of each actually belonging to given cluster based on proper motions and positions (evidently from Gaia). Right: clean HR diagrams with prob. > 99%.

Globular cluster catalog: Vasiliev & Baumgardt 2021

Open cluster catalog: Poggio+2021

Ah, another thing to keep in mind is that Omega Centauri is not a regular globular cluster, but rather the stripped nuclear star cluster of a dwarf galaxy accreted by the Milky Way in the past (see Limberg+2022).

Well, developing on the secondary properties @ProfRob started to list, I would like to add that, because open clusters are young, hence born within the Galactic disk, and globular clusters are old, and belong to the Galactic halo/bulge, their spatial distributions in the Galaxy are dramatically different! Open clusters are mostly contained within few 100 parsec from the Galactic plane even in the outermost regions of the disk. On the other hand, globular clusters can be found at 100s of kpc out in the halo. The refs. linked above account for these assertions. See this figure I prepared with the same data:

The instructions (and data) to reproduce this one are in this repository: https://github.com/guilhermelimberg/Star_cluster_distribution.

Finally, another consequence of this dichotomy in the properties of open and globular clusters is that their motions, i.e., orbits around the Galaxy, are completely different! Open clusters are (almost) always on quasi-circular orbits (low eccentricity). On the contrary, globular clusters can have all kinds of crazy orbits! See figure below that I made myself.

.

(To calculate these orbits, I used the gravitational potential of McMillan+2017 and line-of-sight velocities from APOGEE survey)

Ah, of course, open clusters dissolve quite quickly as was already mentioned, because they are not so massive and weakly bound gravitationally, so they actually do not complete these many full orbits around the Galactic center. Keep that in mind.

Hope this is useful to someone (: