University Lowbrow Astronomers

Concerning Clusters

by P. Walkowski
Printed in Reflections:  April 1999.

Second only to the planets as crowd pleasers at star parties, clusters have caught the eye of amateur and professional astronomers for centuries.  While Open and Globular Clusters have stars and the word cluster in common, they are quite distinct.  For my own edification, I started reading about these wonders of the heavens, and am recording the findings here to share with you.  In general a cluster of stars has enough bright stars grouped together to be visible over the background of the sky.  But to be a true cluster rather than as asterism, the stars must all be traveling together.

Open Clusters have been known since pre-biblical times, the most common names being bestowed by the ancients.  Hyades and the Pleiades are examples of these loose, irregular clumps of stars that are noted in the “Almagest” of Ptolemy (140 AD).  What makes them a cluster at all is that they are traveling together across the heavens.  In the 1930s Robert Trumpler’s system of classifying open clusters according to their size, magnitude, and richness came in to common use, such as I 3 r for the Pleiades.  His categories are:

Open Clusters were and are continuing to be born in the spiral arms and the central bulge of the Milky Way Galaxy.  It is as though these relatively new stars were being wrung out of the folds of the blanket of the heavens.  In fact radio telescope and infrared observation of intrastellar nurseries can detect these new stars millennia before they can be seen visually.  The spectacular Hubble Telescope stellar nursery photos of the pillars of gas birthing stars in the Eagle Nebula are examples of this phenomenon.  It is generally believed that all the stars of an open cluster are born of the same gas cloud, and velocity vector maps of the Hyades have been used as evidence of this common point of origin.  Their future as a cluster is less certain because few open clusters have sufficient mass to gravitationally bind the stars together, hence they continue to drift apart.  It is estimated that if a cluster is to remain together it should have a density of 10 stars/pc, while open clusters typically range from 0.1 to 10 stars/pc.  The stars of an open cluster can be thought of as the fragments of a bombshell, the center of mass is still following the same trajectory through space, but the fragments themselves are spreading apart with different velocities.

All of the open clusters about which we have knowledge (approximately 10,000 visible, 100,000 estimated) are in our near stellar neighborhood of the Milky Way and are sometimes referred to as galactic clusters (that is “within our galaxy”).  No observations of open clusters have been made outside of the Milky Way.  More distant clusters undoubtedly exist, but are obscured by interstellar dust.  From our vantage point on the inside edge of the Orion Arm of the Milky Way, we can observe open clusters not only all around us in the Orion arm, but inward to the Sagittarius and Centaurus arm, and Outward to the Presius arm.

This local proximity is convenient for taking spectra and photometric measurements of the individual stars, from which we are told the clusters are millions to nearly 12 billion years old, while the Milky Way and the entire Universe itself are said to be 1-14 billion years old.  From these near neighbors we continue to learn how stars are born, progress along the Hertzsprung-Russell diagram and die.  We have learned that open cluster stars generally follow the H-R main sequence, have a significant number of bright supergiants, and type I Cepheids.  While many different types of stars are present in open clusters, they are “common” types of galactic stars from the spiral arms they are called population type I stars.  Typical stars have 1-4% of their mass as heavy elements, like our sun.  It is noteworthy that extensive spectra studies of open clusters have found the H-R Horizontal branch missing, while all globular clusters have a strong horizontal branch.

Globular clusters are highly symmetric clusters of stars that form a globe shape which gradually brightens towards the center.  Many globulars have so many stars say 10 to 100 thousand) that it is impossible to count them or even see through the central portion of the cluster.  Their densities range from 10 to 1000 stars/pc.  They have a high orbital velocity around their own center of mass, and also around the Milky Way itself.  Since they have highly inclined orbits, a few globular clusters may be momentarily within the spiral arms, but most are above and below the plane of the galaxy.  There are estimates of 1-200 globulars in the Milky Way and numerous examples of globulars surrounding other galaxies as well.  The globular clusters orbit the nuclear bulge of the galaxy in what is called a “galactic halo” from 10,000 to about 50,000 light years from the center.  The Milky Way is generally a flattened disk 100,000 light years in diameter by 3,000 light years thick.  The globulars also do not participate the rotation about the galactic center that encompasses the spiral arms.  The Sun is 30 light years from the galactic center (located in Sagittarius) and almost all of the 150 known globulars are orbiting the Milky Way closer in than we are.

The orbits of Globulars around the Milky Way was part of the cutting edge astronomy by Harlow Shapley in 1917, and led to thinking less about the sun centered universe and more on the galactic centered universe.  Today this landmark research can be duplicated in minutes using computerized planetarium programs.

The earliest recorded galactic cluster was omega centauri, which could be seen with the naked eye in pre-biblical times.  Herschel expanded the list of globulars to a dozen using telescopes when he wrote the star catalogue “Uranometria” in 1603, and the list was expanded further in Charles Messier’s 1781 list of 103 nebulous appearing objects,  William Herschel’s “Catalogue of the Nebula” in 1864 and Dreyer’s “New General Catalogue” in 1888.

Stars in globular clusters are tightly bound to each other by their own gravity, and can withstand the tidal forces of the Milky Way without pulling apart.  There are high concentrations of binary stars in globular clusters, which help maintain the stability of the cluster, although occasional individual stars escape from the globulars into the galactic halo.

Scientists are fairly certain that the stars within globular clusters all have a common origin, albeit that origin is outside of the galaxy itself.  Globular Clusters are some of the most ancient objects observable, having ages from 12-20 billion years.  I have wrestled with this age dilemma, the globulars older than the Universe or the Milky Way and can only offer hypotheses, not answers.  Perhaps the globulars condensed out of the primordial soup earlier than the galaxies being smaller and of a simpler structure, and were gathered round the late forming galaxies by their gravity, much the way comets and asteroids are gathered in by the solar system from the Oort cloud.

Globular clusters have main sequence and have large populations of red and yellow giants as well as type II Cepheids, R. V. Tauri stars, and planetary nebulae as well.  Their stars are described as population type II stars and are noteworthy for the absence of heavy elements in the stars (0.001 to 0.1%).

The demise of stars in Globular clusters appears to be as a white dwarf star or as a planetary nebula, of which there is already ample evidence in the literature.  Since there are a considerable number of binary stars available, the exchange of stellar matter may result in novae and cataclysmic variables as well.

Fig 1 shows 2 equal area diagrams of the sky with the galactic plane as the horizontal line and the plus sign indicating the galactic center.  The top diagram is the distribution of open clusters, which lie in the spiral arms and completely surround the sun 5 position in the Milky Way.

--- FIGURE 1 ---
[Note, the graphics that appeared in the original article are not available at this time.  It might be added later].

The bottom diagram indicates that globular clusters are predominantly above and below the plane of the Milky Way clustered about the galactic center in the halo region.  Note that only a few globular clusters are out as far as the sun (30,000 l-yr) from the galactic center.

--- FIGURE 2 ---

Fig 2 is a H-R (color magnitude) diagram for Open Clusters.  While the open cluster H-R diagrams vary widely from each other, most stars are situated along or to the immediate right of the main sequence which stretches from lower right to upper left.  The dotted diagonal line from the center to the upper right represents the place where yellow and red giant stars should be, present on Open Clusters, which they rarely are.

--- FIGURE 3 ---

Fig 3 is a H-R diagram for globular clusters, which are extremely old objects.  Note the strong presence of the yellow and red giants and the horizontal branch which is always present in globular clusters but almost entirely absent from Open clusters.

[Note:  The following table which compares open clusters and globular clusters did not appear in the newsletter article but was used in a talk given April 1999.]


Globular Cluster   Open Cluster


20-200 pc (1 pc = 3.2 l-yr)   < 10 pc


5-60 Kpc (12 is typical)   0-2 Kpc

Number of Stars  

10,000-100,000   50-1,000

Absolute Magnitude  

-5 to -15   -5 to 20


Galactic Halo   Spiral Arms and Bulge

Quantity in MW   

150-200 (Est.)   10,000-100,000 (Est.)

Age in yrs  

16 Billion (vs MW at 12-14 B)   0.03-5 Billion

Orbit of MW  

Elliptical, Inclined to plane, eccentric   Near circular, in plane

Velocity in MW  

100-450 pc/sec   250 pc/sec

Rotational velocity   

Very high about center   Little noted


Highly symmetric globe, density increases towards center   Some to None (bomb shell analogy)


Gravity sufficient to overcome galactic tidal forces   Unstable; stars drifting apart

Density stars/pc^3  

1-1000   0.1-1.0 (>1 necessary for gravitational stability)

Distance from MW center l-yr  

5-35   0-50

Kinds of stars  

Some main sequence, red/yellow giants, horizontal branch; Population Type II; red cluster variables; Type II Cepheids   Varies widely; mostly main sequence, giants, no horizontal branch; Population Type I; Type I Cepheids

Heavy Elements in Stars  

0.001- 0.1%   1-4% (Earth’s Sun is 4%)

Binary Stars  

90%, apparently necessary for stability   55% binary, triplet, quadruplets


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