Star Clusters

This is a lovely post for us to write because we get to all come together, but also slightly sad because this is the last of our posts on stellar evolution. Yes, we finally get to star clusters.  First question to ask any astronomer investigating clusters would be, “open or globular?”  Yes, just like any other astronomical object these have the same old tools we’ve used for many years, some types, and some very interesting problems.  Let’s see what clusters have to offer us astronomers.

Clusters in general are fantastic tools for any astronomer, with photometric, spectroscopic, and other studies done to find out what they are and why they’re important.  They are groups of stars that all formed from the same nebula.  This means they are all at the same distance, and there’s a higher than normal density of stars in that location that have the same age, the same chemical composition, and different masses.  The difference in mass is important, and it reflects how different stars can form in a region.  For all these reasons, clusters are perfect for studying stellar evolution, finding distances to phenomena, or better understanding properties of galaxies or certain stars.  Guess these clusters really show how “united we stand” can be quite the useful strategy (at least for us astronomy nerds anyway).

So let’s open up the first mystery present, open clusters.  These are famously filled with all sorts of massive, higher metallicity stars.  Yep, that means they can produce all sorts of fancy objects as per our understanding of stellar evolution.  Since they are massive, we also consider these clusters to be younger (millions of years old).  This logically follows into another observation.  Open clusters are defined by being in a galaxy’s disk.  This is the area of most star formation, and it makes sense younger star clusters would be forming there.  Last for these clusters is that they are usually sparser in member stars, having hundreds or thousands if a lot, because of the high mass stars.

See Explanation.  Clicking on the picture will download the highest resolution version available.

The Pleiades, or M45 because it is easier to pronounce, spell, and it gives us an excuse to bring up the Messier catalog again.

If we still have your attention then great.  If not, then perhaps we can look into the celestial sphere for a far more well-rounded cluster.  Alright, bad introductions to the next type aside, we have globular clusters.  These are older clusters up to gigayears old, in a galaxy’s halo (outside the disk that is normally seen) and typically having millions of stars.  Since they are older, they also have lower metallicity, and cluster members are not as massive stars.  As shown, clusters link together mass, evolution, age, and density of stars in different locations.  But don’t let these general properties fool you, globular clusters can still have blue stragglers.  Blue stragglers are unusually massive stars in globular clusters, which may be caused by some sort of binary star.

See Explanation.  Clicking on the picture will download the highest resolution version available.

This would be the brightest globular cluster we can see, Omega Centauri.  There are all sorts of fancy random-looking blue lights, and just look at that bright core…hopefully it doesn’t somehow cluster together against us.

So far we’ve defined what clusters are, and how they’re useful.  Let’s exemplify how useful they are.  The Color-Magnitude Diagram is essentially an H-R Diagram that plots up the color and magnitude of stars (determined using photometry, or amount of light, and spectroscopy, or the type of light we receive).  This may not sound useful, but by taking different clusters at different ages, astronomers can actually see how a whole entire cluster of stars evolves over time.  If you’re wondering how, we’ve mentioned it previously in this post because by seeing the different ages for clusters, then we can see the numbers of stars, the location of the stars, and qualities of those stars.  They in fact show exactly how population I (like stars in open clusters) and population II stars (like stars in globular clusters) are distinguished .

One last aspect of clusters to explore is how we classify them.  People have actually spent time trying to systematically distinguish certain clusters from one another.  One example is the Shapley-Sawyer morphology classification, which classifies different globular clusters by how dense they appear.  There is also a Trumpler classification of open clusters, which shows how nebulous and rich (concentrated with stars) the open cluster is.  Well, enough classes, that’s all for this lesson.

So yes, this is how they classify open clusters. As you can see, they all look fairly similar, so basically Trumpler developed really good skills at distinguishing clusters.


Sources and links for further reading (links to images are below):

Color-Mag Diagrams

Blue Stragglers, Stellar Evo with clusters



Both types of clusters

Cluster classification


Pleiades: NASA/APOD,

Omega Centauri: NASA/APOD,

Trumpler Classification:


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